Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (Review Article)
During the three decades after the discovery of superconductivity at high temperatures in copper oxides, intense research activities generated a tremendous progress in both, mastering the scientific challenges underpinning the understanding of the properties of these chemically and structurally comp...
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nasplib_isofts_kiev_ua-123456789-1293062025-02-09T21:45:20Z Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (Review Article) Habermeier, H.-U. К 30-летию открытия высокотемпературной сверхпроводимости During the three decades after the discovery of superconductivity at high temperatures in copper oxides, intense research activities generated a tremendous progress in both, mastering the scientific challenges underpinning the understanding of the properties of these chemically and structurally complex materials as well as achieving a mature technology in preparing single phase bulk specimens—including single crystals—and epitaxially grown single crystalline thin films. This review covers in addition to more basic physics oriented developments mainly technological aspects of complex oxide thin film deposition as an enabling technology to explore the physics of these materials. It consists of two parts: after a brief introduction to the materials development prior to the discovery of superconducting copper oxides, a description of the relevant properties of copper oxide superconductors with focus on YBa₂Cu₃O₇−δ is given, followed by the coverage of essentials of complex oxide thin film deposition technology with the copper oxides at its core. Here, the major physical vapor deposition technologies (evaporation and oxide molecular beam technology, sputtering and pulsed laser deposition) are described followed by an overview of substrate requirements to deposit high quality thin films. Opportunities by choosing special substrates with unique properties far beyond the usual mechanical support for a film are introduced with examples aside from usual lattice mismatch induced strain effects. One is the continuous modification of the strain state by poling ferroelectric oxide substrates linked to a piezoelectric effect, the other is the nanoscale tailoring of substrate step-and-terrace structures resulting in a controllable generation of planar defects in complex oxides, thus contributing to the physics of flux-line pinning in cuprate superconductors. In the second part of this review, first some highlights of single layer thin film research are given such as to tailor thin film orientation, generating well defined antiphase boundaries in YBa₂Cu₃O₇−δ thin films as flux-line pinning centers as well as contributions to understand fluctuation conductivity in relation to the pseudogap state. In the last section new developments in high Tc cuprate based heterostructures and superlattices are reviewed with a special focus on the opportunities offered by interface-induced electronic interactions. The author is highly indebted to the current and previous members of the Technology Group of the Max-PlanckInstitute for Solid State Research, Stuttgart, for their enthusiasm in the thin film work for complex oxides. He benefitted much from in-depth discussions with M. Cardona, B. Keimer and O.K. Andersen and the members of their departments. The financial support of the Max-PlanckSociety, the German Science Foundation (DFG) as well as the European Union within the frame of several EU-NMP projects is greatly appreciated. 2016 Article Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (Review Article) / H.-U. Habermeier // Физика низких температур. — 2016. — Т. 42, № 10. — С. 1075-1102. — Бібліогр.: 127 назв. — англ. 0132-6414 PACS: 74.72.–h, 74.25.–q, 74.78.–w https://nasplib.isofts.kiev.ua/handle/123456789/129306 en Физика низких температур application/pdf Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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К 30-летию открытия высокотемпературной сверхпроводимости К 30-летию открытия высокотемпературной сверхпроводимости |
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К 30-летию открытия высокотемпературной сверхпроводимости К 30-летию открытия высокотемпературной сверхпроводимости Habermeier, H.-U. Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (Review Article) Физика низких температур |
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During the three decades after the discovery of superconductivity at high temperatures in copper oxides, intense research activities generated a tremendous progress in both, mastering the scientific challenges underpinning the understanding of the properties of these chemically and structurally complex materials as well as achieving a mature technology in preparing single phase bulk specimens—including single crystals—and epitaxially grown single crystalline thin films. This review covers in addition to more basic physics oriented developments mainly technological aspects of complex oxide thin film deposition as an enabling technology to explore the physics of these materials. It consists of two parts: after a brief introduction to the materials development prior to the discovery of superconducting copper oxides, a description of the relevant properties of copper oxide superconductors with focus on YBa₂Cu₃O₇−δ is given, followed by the coverage of essentials of complex oxide thin film deposition technology with the copper oxides at its core. Here, the major physical vapor deposition technologies (evaporation and oxide molecular beam technology, sputtering and pulsed laser deposition) are described followed by an overview of substrate requirements to deposit high quality thin films. Opportunities by choosing special substrates with unique properties far beyond the usual mechanical support for a film are introduced with examples aside from usual lattice mismatch induced strain effects. One is the continuous modification of the strain state by poling ferroelectric oxide substrates linked to a piezoelectric effect, the other is the nanoscale tailoring of substrate step-and-terrace structures resulting in a controllable generation of planar defects in complex oxides, thus contributing to the physics of flux-line pinning in cuprate superconductors. In the second part of this review, first some highlights of single layer thin film research are given such as to tailor thin film orientation, generating well defined antiphase boundaries in YBa₂Cu₃O₇−δ thin films as flux-line pinning centers as well as contributions to understand fluctuation conductivity in relation to the pseudogap state. In the last section new developments in high Tc cuprate based heterostructures and superlattices are reviewed with a special focus on the opportunities offered by interface-induced electronic interactions. |
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Habermeier, H.-U. |
| author_facet |
Habermeier, H.-U. |
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Habermeier, H.-U. |
| title |
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (Review Article) |
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Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (Review Article) |
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Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (Review Article) |
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Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (Review Article) |
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Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (Review Article) |
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science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (review article) |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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2016 |
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К 30-летию открытия высокотемпературной сверхпроводимости |
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https://nasplib.isofts.kiev.ua/handle/123456789/129306 |
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Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices — the first 30 years (Review Article) / H.-U. Habermeier // Физика низких температур. — 2016. — Т. 42, № 10. — С. 1075-1102. — Бібліогр.: 127 назв. — англ. |
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Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10, pp. 1075–1102
Science and technology of cuprate-based high
temperature superconductor thin films, heterostructures
and superlattices — the first 30 years
(Review Article)
H.-U. Habermeier
Science Consulting International, Niersteinerstr, 28, Stuttgart D 70499, Germany
Max-Planck-Institute for Solid State Research, Heisenbergstr, 1, Stuttgart D 70569, Germany
E-mail: huh@fkf.mpg.de
Received May 30, 2016, published online August 25, 2016
During the three decades after the discovery of superconductivity at high temperatures in copper oxides, in-
tense research activities generated a tremendous progress in both, mastering the scientific challenges underpin-
ning the understanding of the properties of these chemically and structurally complex materials as well as
achieving a mature technology in preparing single phase bulk specimens — including single crystals — and
epitaxially grown single crystalline thin films. This review covers in addition to more basic physics oriented de-
velopments mainly technological aspects of complex oxide thin film deposition as an enabling technology to ex-
plore the physics of these materials. It consists of two parts: after a brief introduction to the materials develop-
ment prior to the discovery of superconducting copper oxides, a description of the relevant properties of copper
oxide superconductors with focus on YBa2Cu3O7–δ is given, followed by the coverage of essentials of complex
oxide thin film deposition technology with the copper oxides at its core. Here, the major physical vapor deposi-
tion technologies (evaporation and oxide molecular beam technology, sputtering and pulsed laser deposition) are
described followed by an overview of substrate requirements to deposit high quality thin films. Opportunities by
choosing special substrates with unique properties far beyond the usual mechanical support for a film are intro-
duced with examples aside from usual lattice mismatch induced strain effects. One is the continuous modifica-
tion of the strain state by poling ferroelectric oxide substrates linked to a piezoelectric effect, the other is the
nanoscale tailoring of substrate step-and-terrace structures resulting in a controllable generation of planar defects
in complex oxides, thus contributing to the physics of flux-line pinning in cuprate superconductors. In the second
part of this review, first some highlights of single layer thin film research are given such as to tailor thin film ori-
entation, generating well defined antiphase boundaries in YBa2Cu3O7–δ thin films as flux-line pinning centers as
well as contributions to understand fluctuation conductivity in relation to the pseudogap state. In the last section
new developments in high Tc cuprate based heterostructures and superlattices are reviewed with a special focus
on the opportunities offered by interface-induced electronic interactions.
PACS: 74.72.–h Cuprate superconductors;
74.25.–q Properties of superconductors;
74.78.–w Superconducting films and low-dimensional structures.
Keywords: high temperature superconductor, high temperature superconducing thin films, multilayers and
superlattices, complex oxide thin film preparation.
Contents
1. Introduction ........................................................................................................................................ 1076
1.1. The road to copper-oxide based high temperature superconductors ........................................... 1076
1.2. Basic properties of YBa2Cu3O7–δ .............................................................................................. 1078
1.2.1. Crystal structure ............................................................................................................... 1078
1.2.2. Electronic phase diagram ................................................................................................. 1078
1.2.3. Superconducting properties ............................................................................................. 1079
© H.-U. Habermeier, 2016
H.-U. Habermeier
1.2.4. Current challenges in copper-oxide science and technology ............................................ 1079
2. Physics and technology aspects of high temperature superconductor thin film deposition ................ 1080
2.1. General aspects........................................................................................................................... 1080
2.2. Substrate requirements and opportunities ................................................................................... 1081
2.3. Physical vapor deposition techniques for cuprates and other complex oxides ........................... 1084
2.3.1. General remarks ............................................................................................................... 1084
2.3.2. Reactive evaporation........................................................................................................ 1084
2.3.3. Oxide molecular beam epitaxy ........................................................................................ 1084
2.3.4. Sputtering......................................................................................................................... 1085
2.3.5. Pulsed laser deposition ..................................................................................................... 1086
2.3.6. Comparison of the different deposition techniques .......................................................... 1088
3. Physical properties of single layer high temperature superconductor thin films–some current trends1088
3.1. General remarks ......................................................................................................................... 1088
3.2. Tailoring cuprate thin film orientation ....................................................................................... 1088
3.3. Controlled generation of antiphase boundaries .......................................................................... 1090
3.4. Fluctuation conductivity. ............................................................................................................ 1091
4. Complex oxide heterostructures and superlattices based on superconducting cuprates ..................... 1092
4.1. General remarks ......................................................................................................................... 1092
4.2. Ferromagnet-superconducting bilayers and superlattices ........................................................... 1093
4.2.1. Early experiments ............................................................................................................ 1093
4.2.2. Determination of the spin diffusion length ...................................................................... 1094
4.2.3. (110)-oriented YBCO/LCMO heterostructures and trilayers ........................................... 1095
4.3. Ferromagnet–Superconductor interfaces .................................................................................... 1097
4.3.1. General remarks ............................................................................................................... 1097
4.3.2. YBCO–LCMO interfaces ................................................................................................ 1098
5. Summary and future perspectives ...................................................................................................... 1099
References .............................................................................................................................................. 1099
1. Introduction
1.1. The road to copper-oxide based high temperature
superconductors
In the decades after the discovery of superconductivity
(SC) by Kammerlingh–Onnes in Hg with a critical tempe-
rature, Tc, of 4.2 K, — excellently described in the Physics
Today article by van Delft and Kess [1] — SC has been a
challenging central topic in condensed matter physics. Su-
perconductivity shows the fundamental property of the
occurrence of vanishing dc-resistivity and the expulsion of
weak magnetic fields (Meissner–Ochsenfeld effect). The
field expulsion holds up to the thermodynamic critical
field, Hc, (type-I superconductors) and for type-II super-
conductors with a gradual decrease up to the upper critical
field, Hc2. The vanishing resistivity holds up to a certain
current density, called critical current, jc. SC has applica-
tion perspectives in a wide field of active and passive de-
vices, cables, resonators, current fault limiters, just to name
a few. The roadblock for a widespread application of SC
was seen in the relatively low Tc’s (< 10 K) and low criti-
cal fields (< 1 T, e.g., for Nb and V). Consequently, the
quest for robust materials with higher Tc’s and jc’s has
been a driving force for materials scientists consisting in
the empirical search for appropriate metal-based alloys.
B. Matthias compiled the results of this research in his fa-
mous 4 rules for finding new superconductors as described
in the monograph by Blundell [2]:
1. Transition metals are better than simple metals.
2. Peaks of density of states at the Fermi level are helpful.
3. High crystallographic symmetry is good: Cubic is best.
4. Stay away from oxygen, magnetism, and insulating
phases.
Albeit these empirical rules had been very helpful in
designing conventional superconducting materials, they
failed in paving the way to the discovery of new families
of superconductors more appropriate for practical applica-
tions. Additionally, during the time span until the early
50’s, a theoretical guideline for a systematic search for
better materials was missing completely. Deviating from
his rules, however, Matthias found a new class of super-
conductors by combining ferromagnetic and semiconduct-
ing elements in CoSi2 with a Tc of 1.33 K [3] indicating the
openness required in the search for a new domain of super-
conducting materials. SC has been seen till the early 60’s
last century more as a scientific curiosity with a bright ap-
plication potential rather than being useful for applications.
This situation changed 1962, when Berlincourt and Hake
discovered high critical currents in Nb–Ti alloys with a Tc
of 10 K and a Hc2 of 10 T [4], materials suitable for mag-
netic coil fabrication. 1952 Hulm and Hardy [5] discovered
superconductivity in the “A-15” (β–W structure) family
where in the A3B structure the A site is typically occupied
by a group VA transition metal (e.g., V, Nb, Ta) and the B
site is occupied by a group IVB element (e.g. Sn, Ge, Si ).
Matthias and coworkers produced ~ 30 A-15 compounds
by bulk preparation techniques mostly in thermodynamic
1076 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
equilibrium, all of them with Tc’s below 20 K. A break-
through achieved Gavaler by synthesizing stoichiometric A-
15 materials by thin film sputtering techniques and found in
the thermodynamically metastable Nb3Ge a record Tc of 23
K and an upper critical field of 37 T [6]. It is interesting to
note that the highest Tc of all A-15 compounds is the ther-
modynamically metastable Nb3Ge with pronounced peaks
in the density of states (DOS) at the Fermi level, EF. Sys-
tematic variations of the preparation conditions for Nb3Ge
thin film growth, e.g., revealed a distinct correlation of
lattice defects (mainly in the Nb-chains of the structure)
and the physical properties of the films [7]. Attempts to
enhance Tc in A-15 compounds, e.g. by substitution of Ge
by Si with an expected reduction of the lattice parameter
and thus a Tc > 30 K failed.
In the early 1950’s, the Tc, of the superconductors was
shown to be sensitive to the isotopic mass of the constitu-
ent elements, pointing towards a role of phonons in super-
conductivity [8]. This observation led to the major theoret-
ical breakthrough in 1957 when a microscopic theory of
superconductivity was introduced by Bardeen, Cooper and
Schrieffer (BCS theory), which described quite well the
essentials of SC in metallic elements and alloys [9]. The
BCS theory has two main conceptual ingredients, first, the
pairing of electrons into Cooper pairs consisting of two
electrons near the Fermi surface with opposite spins and
momenta, glued together by lattice vibration of ions and
second, their phase coherence. This theory served as a
guideline for the search for higher Tc’s highlighting the
essential role of electron–phonon coupling and the high
DOS at EF. In the following decades progress in SC was
made in several directions. One was the investigation of
magnetic properties of superconductors by introducing the
concept of flux-lines [10] and flux-line pinning to enhance
the current-carrying capability, thus leading to large scale
applications such as wires, cables and superconducting
magnets, the other one in developing passive (microwave)
and active (digital) devices based on thin film technolo-
gies. The third one was the further development of the
BCS concept of electron–phonon coupling by Eliashberg
[11] especially for the strong coupling case. Efforts to fur-
ther enhance Tc have not been successful, on the contrary,
is was believed that the maximum Tc is confined to values
< 30 K mainly due to lattice instabilities.
In 1986, J.G. Bednorz and K.A. Müller discovered su-
perconductivity in the ceramic La–Ba–Cu–O copper oxide
with a Tc of about 35 K [12], which is considered to be a
landmark event and represents the beginning of the field of
high temperature superconductivity [HTS]. Their search
for new superconductors was stimulated by the ideas of
Jahn–Teller based polaronic effects associated with the
polarizability of oxygen in the perovskite structure. Müller
regarded that to be of major importance for the formation
of Cooper pairs and thus superconductivity. It turned out
that this concept of polarons (or bipolarons) being relevant
to the origin of HTS experienced a strong opposition from
the community relating HTS to strong electron correlation
mechanisms [13]. The discovery of La2–xBaxCuO4 was a
breakthrough in the field of superconductivity and was
followed by avalanche-like research activities culminating
in a “Woodstock”-type meeting of the American Physical
Society, 1987, and the excitement to publish research re-
sults in newspapers prior to serious peer-reviewed scien-
tific journals. Whereas in the La2–xBaxCuO4 system one
CuO2 plane is embedded in between of (La,Ba)-O blocks
regarded as a charge reservoir, Wu et al. found 1987 a
multiphase Y–Ba–Cu–O cuprate system to be supercon-
ducting at 93 K (i.e., well above 77 K the boiling tempera-
ture of liquid nitrogen) [14]. The superconducting phase in
the Y–Ba–Cu–O system was identified as YBa2Cu3O7–δ
(YBCO), a layered structure with 2 CuO2 planes and Cu–O
chains as the dominant structural elements. (c.f. Fig. 1).
Since then, different structural and physical properties of
cuprates have been studied in detail, and to raise the Tc for
these materials, cation substitution has been extensively in-
vestigated. Since cuprate SC’s with one CuO2 plane show
Tc’s of ~ 35 K those with 2 CuO2 planes ~ 92 K and with
three planes ~ 134 K attempts to synthesize materials with
more than 3 CuO2 planes have been made, with no success,
however. At present the highest Tc of 134 K is found in the
three layer mercury-cuprate system (HgBa2Ca2Cu3O8+x)
with an increase to 150 K under high pressure conditions
Fig. 1. (Color online) Unit cell of YBCO indicating the CuO2
planes (with Cu (2), O (2) and O (3) atoms) and CuO chains (with
Cu (1) and O (1) atoms) (after Wikipedia).
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1077
H.-U. Habermeier
[15]. The cuprate high Tc superconductors identified so far
have CuO2 square planes as structural elements in com-
mon, outside this plane the structural elements serving as
charge reservoir can be quite different thus causing the
wide spread of Tc’s in the copper oxides. After the discov-
ery of superconductivity in La–Ba–Cu–O and especially
YBCO, the research activities followed two main direc-
tions. The first is more materials oriented with the focus оn
achieving higher Tc’s, investigating the flux-line properties
and understanding the mechanisms of flux-line pinning as
well as technological work in fabricating materials as bulk
samples, tapes, wires and thin films for practical applica-
tions. Here, large scale applications such as cables for
power transmission and magnetic coil technology are en-
visaged as well as developing a reliable thin film technolo-
gy for passive (filters, antennae) and active devices (sen-
sors, Josephson junctions for digital applications). The
proceedings of the Applied Superconductivity Conferences
(ASC) as well as the European Conference on Applied
Superconductivity (EUCAS) may serve as reference source
in this area [16,17]. The other direction is dedicated to the
understanding of the origin of superconductivity in the
cuprate system with emphasis on research on the pairing
mechanism. In the nearly three decades since the discovery
of superconductivity in the copper oxides much knowledge
has been accumulated about novelties of quantum matter,
but despite of all the efforts up to now no generally accept-
ed theoretical model to explain the origin of superconduc-
tivity in copper oxides emerged. In a recent review Keimer
et al. [18] stated that a qualitative understanding of the
nature of the superconducting state itself has been
achieved. Unresolved issues include the astonishing com-
plexity of the phase diagram, the unprecedented promi-
nence of various forms of collective fluctuations, and the
simplicity and insensitivity to material details of the “nor-
mal” state at elevated temperatures.
1.2. Basic properties of YBa2Cu3O7–δ
Yttrium–Barium–Copper oxide, YBa2Cu3O7–δ is the
most frequently investigated member of the family of cop-
per-oxide superconductors and serves as a prototype mate-
rial for this class of materials. Its key properties are de-
scribed in this section.
1.2.1. Crystal structure
As shown in Fig. 1, YBCO has a layered structure which
can be described as a distorted, oxygen deficient perovskite
of the form ABO3, tripled along the c-axis, where Y and
Ba are central cations which are stacked in Ba–Y–Ba se-
quence in the c-axis and Cu is the corner cation B. Yttrium
is coordinated to 8 oxygen atoms forming a square prism
while Barium is coordinated to 10 oxygen atoms. The va-
lence of Y and Ba are +3 and +2, respectively, while Cu
has mixed valence of +2 or +3 to ensure charge neutrality.
The structure of stoichiometric superconducting YBCO is
orthorhombic belonging to Pmmm symmetry and characte-
rized by two CuO2 planes ((a), (b) planes) and Cu–O
chains. Highly oxygen deficient non-superconducting
YBa2Cu3O7–δ with δ > 0.6 has a tetragonal structure with
P4/mmm symmetry due to the interrupted or missing Cu-O
chains, in YBa2Cu3O6 the chains are missing completely.
Cu (1) is at the center of a rectangle formed by its coordi-
nation with two O(1) and two O(4) atoms, these rectangles
are connected by the vertices and give rise to CuO chains
along b-axis. Cu (2) is coordinated in the form of a square
pyramid to two O (2) and two O (3) atoms in the (a), (b)
plane and one O (4) atom along the c-axis [19]. CuO2
planes formed by Cu (2), O (2), and O (3) atoms are ex-
tended infinitely in the specimen and are crucial to deter-
mine the transport and superconducting properties of the
material. For δ = 0, i.e., YBa2Cu3O7 the oxygen sites along
a-axis in the basal CuO chain plane are completely empty.
With increasing δ, the one dimensional chains in the ortho-
rhombic phase are disrupted by an increasing number of
oxygen vacancies at the O (1) site along b-axis. All oxygen
sites in the basal plane become empty for δ = 1, i.e.,
YBa2Cu3O6 [20].The lattice parameters for this material are
a = 0.3822 nm, b = 0.3891 nm and c = 1.1677 nm making
unit cell volume 3 times the standard perovskite cell [21].
1.2.2. Electronic phase diagram
Hole doped cuprates have a generic phase diagram of
their phase evolution in terms of temperature vs the Cu
valency (hole doping) as depicted in Fig. 2. The undoped
parent compound with Cu2+ is an antiferromagnetic Mott
insulator [22] which can be doped either by substituting
trivalent cations with divalent ones as in the case of
La2CuO4 or by oxygen filling the Cu–O chains as in
YBCO. With increasing doping (corresponding to an in-
crease of the oxygen valency), the antiferromagnetic tran-
Fig. 2. (Color online) Schematic electronic phase diagram of
cuprates [23].
1078 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
sition temperature is rapidly suppressed and becomes zero;
a further increase causes the superconducting state to
emerge with low Tc at the beginning and gradually in-
creases with the doping level (underdoped region), reach-
ing a maximum at ~ 0.16 and then falls down gradually to
zero (overdoped region) [23].
In the underdoped region above Tc, the normal state of
cuprates is characterized by an intriguing feature called
pseudogap which results in a depletion of the density of
states. It was observed first in the NMR and inelastic neu-
tron studies and was interpreted as a spin gap. Later
ARPES, heat capacity and tunneling studies revealed that it
is rather a gap in the spectrum of the quasiparticles. The
exact origin of the pseudogap in cuprates is not yet clear,
however the general scenario is that it arises from pairing
fluctuations above Tc with no phase coherence, while in an
alternative view pseudogap is a competing correlation that
takes away the spectral weight that would be otherwise
available for superconductivity. The pseudogap tempera-
ture specified in the phase diagram is not a sharp transition
temperature but marks a gradual crossover into pseudogap
region. In the overdoped region above Tc the conductivity
of the copper oxides is smaller by nearly two orders of
magnitude compared to usual metals and show a frequency
and temperature dependence incompatible with the con-
ventional transport theory of metals.
1.2.3. Superconducting properties
YBCO with a Tc = 92 K, is a highly anisotropic materi-
al based on the anisotropic crystal structure. It belongs to
the Type II class of superconductors characterized by a
large (> 2–1/2) Ginzburg–Landau parameter κ = λ/ξ with λ
being the magnetic penetration depth and ξ the supercon-
ducting coherence length. Type II superconductors differ
from type I superconductors, because in the latter the su-
perconductivity is lost above a critical magnetic field Hc,
whereas the former exhibits a complete Meissner state only
below a lower critical field Hc1, above Hc1 it shows a
mixed state in which the magnetic field penetrates in the
specimen in the form of flux filaments called vortices, ar-
ranged in a triangular lattice whose lattice constant de-
crease with field like a ~ B–1/2 up to the upper critical field
Hc2. For YBCO the upper critical field is about 150 T for
magnetic fields applied parallel to the (a), (b)-planes and
40 T parallel to the c-axis. The superconducting gap in
YBCO has a large value of about 30 meV which is much
higher than any of conventional BCS superconductors with
typical values below 1 meV. The superconducting gap in
conventional metallic superconductors has s-wave sym-
metry around the Fermi surface, in the case of YBCO it
has d-wave symmetry with nodes in different directions.
The gap is zero in Cu–Cu direction whereas it maximizes
along the Cu–O direction. The distance between two elec-
trons of a Cooper pair is describe by BCS coherence length
given by ξ ~ vF/kBTc and in conventional metal supercon-
ductors it is quite large, e.g. ξ = 37 nm for lead. Since Tc is
92 K in YBCO we can expect a small coherence length,
however, due to the low density of carriers in cuprates, the
Fermi velocity is also very small which makes coherence
length even smaller such that ξab = 1.5 nm and ξc = 0.2 nm
[24]. YBCO is characterized by an electronic mass anisot-
ropy, γ, causing ξc shorter than that of the basal plane by
ξc = ξab/γ. The extremely short coherence length poses
some conceptual problem about pairing mechanism and
determines the length scale for homogeneity requirements
for the material. The penetration depth λ of a superconduc-
tor, which measures the distance to which magnetic field
penetrates through a superconductor, is related to the su-
perfluid density ns by the relation ns ∝ 1/ λ2. For YBCO
the penetration depth is very large with λab (0) = 150 nm
and λc (0) = 600 nm, consequently the superfluid density is
relatively small. Due to the presence of Cu–O chains along
b-axis, the coherence length is smaller compared to that
along a-axis, the value λab is calculated as (λa λb)1/2.
The current carrying capacity, expressed as the critical
current density, jc, is an important figure of merit for a
superconductor from potential applications’ point of view.
High values of jc require the blocking of the motion of the
Abrikosov vortices — nanoscale tubes of magnetic flux
that form spontaneously inside a superconductor when
exposed to a magnetic field larger than Hc1 in the form of a
triangular lattice. Their dimensions are determined by the
coherence length, the corresponding supercurrents decay at
a distance of λ. In general, jc is a measure of the volume
pinning force exerted to the vortex lattice. For layered
cuprates jc is very anisotropic. In case of YBCO, the value
of jc depends upon the nature of the specimen itself. For
bulk specimens, e.g. single crystals, the jc is pretty low rang-
ing from 10–103 A/cm2 , highest values are however ob-
tained for epitaxial thin films of YBCO with jc ~ 107 A/cm2
at 77 K. Lower jc values for bulk specimen arise mainly
from intergrain weak links [25].
1.2.4. Current challenges in copper oxide science and
technology
To improve and potentially surpass the superconducting
properties of the existing copper oxide materials a funda-
mental comprehension of the essential principles underly-
ing superconductivity in this class of materials is required.
To achieve this goal several research challenges have to be
met. One is seen in the understanding of the pairing mech-
anism — may be the interplay of several of them — that
causes the formation of Cooper pairs, the other is the phys-
ics of the competing electronic phases especially in the
pseudogap region. The investigation of the coupling of
superconductivity with other competing electronic phases
and their potential manipulation to improve their properties
(Tc, jc) is another big challenge. Currently, the theoretical
methods available seem not to be adequate to describe the
rich physics of the pseudogap phase and the region of the
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1079
H.-U. Habermeier
strange metal behavior in the electronic phase diagram and
much efforts are required to improve this situation.
Another cross-cutting challenge is more technology ori-
ented. It is the further development and refinement of tech-
niques to grow samples (single crystals, thin films,
heterostructures and superlattices) with atomic precision and
to improve existing diagnostic tools for the analysis down to
the atomic level. These requirements arise from the fact, that
the essential ingredients for superconductivity — Cooper
pairs forming the superconducting state and Abrikosov vor-
tices mediating the current-carrying capacity — have in the
copper oxide superconductors dimensions in the nm range.
The nanoscale interactions between themselves and crys-
tallographic structures of comparable size determine their
superconducting properties. Consequently, both, prepara-
tion methods and analytical tools have to take this basic
feature into account and must meet the corresponding pre-
cision requirements.
2. Physics and technology aspects of high temperature
superconductor thin film deposition
2.1. General aspects
Fabricating electronic devices and integrated circuits
using superconductors requires a reliable thin film technol-
ogy. In the case of low temperature metallic superconduc-
tors (LTS) this has been accomplished to a high degree of
perfection by conventional techniques such as evaporation
and sputtering and led, e.g., to optical and magnetic detec-
tors with very high sensitivity [26]. Additionally, many
efforts have been undertaken to fabricate digital integrated
circuits; they have found so far niche applications, only
[27]. Soon after the discovery of superconductivity in the
copper oxides, the challenging task to develop a technolo-
gy to prepare textured and single-crystal type thin films has
been accomplished. The success in mastering the challeng-
es has opened the door for research activities directed to-
wards the fabrication of thin films, heterostructures and
superlattices of other complex oxides such as manganites,
ferroelectrics, and piezoelectrics. The availability of re-
search-grade complex oxide thin film samples will further
promote physics experiments directed to answer the basic
questions of pairing mechanisms and current-carrying
properties. By mastering the thin film technology for com-
plex oxides, a new research field is arising using the oppor-
tunity to combine materials with different functionalities.
An emerging topic is seen in the corresponding interface
physics where contributions from the defect chemistry are
mandatory to understand the interface properties. There is
a long list of perovskite-type complex oxides with attrac-
tive physical properties which can be prepared as single-
crystal type single layer thin films on lattice matched oxide
substrates. Their application potential ranges from elec-
trodes for solid oxide fuel cells to magnetic films for data
recording/reading devices as well as superconducting thin
film devices either of an active (SQUID basis) or passive
(filters and antennae) type. The complex oxide thin film
deposition technology is already covered by several review
papers, conference proceedings and books [28–32]. This
chapter does not aim to provide an exhaustive discussion
of the physical processes behind film growth and deposi-
tion technologies, but focuses on the most relevant ones
and the background for their relevance.
The key requirements for a mature technology suitable
for applications are the ability to deposit single-crystal type
layers and multilayer structures with the desired homoge-
neous chemical composition and nearly atomically flat
surfaces. The length scale for inhomogeneities either struc-
turally or morphologically is given by the short interaction
lengths for the cooperative phenomena such as supercon-
ductivity and ferromagnetism in those materials. For su-
perconducting cuprates, e.g., the coherence lengths along
the c-axis is in the range of less than 1 nm [22]. The dif-
ficulties achieving such perfect complex oxide thin films
(COTF’s) are not only caused by the necessity to have the
correct particle flux ratio of the constituents at the sub-
strate site forming the correct crystal structure but also by
the complex parameter space for the deposition of films
with the desired crystallographic orientation and oxygen-
ation state. This includes the deposition rate, kinetic en-
ergy of the particles impinging the substrate surface, sub-
strate flatness and termination, and the thermodynamic
requirements for phase stability as well. As an example the
oxygen-pressure/temperature phase stability diagram for
YBa2Cu3O7–x is given in Fig. 3. There are stability lines
for the formation of the desired oxygenation state of Cu
Fig. 3. (Color online) Phase stability diagram 2Op vs 1/T for
YBa2Cu3O7 thin film deposition [31].
1080 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
and the different levels of oxygen incorporation. Control-
ling the temperature and oxygen partial pressure inde-
pendently during the cooling process it is possible to adjust
the oxygen content of a film in a well-defined way. Within
the parameter range set by the phase stability criteria,
thermodynamic and kinetic factors determine the growth.
The growth process of the films starts typically by the for-
mation of a 2D nucleus with radius, r, and an edge free
energy, γ, and is governed by the Gibbs free energy
2 2( )G r r h r h= − ∆µπ + π γ . (1)
With ∆µ being the excess chemical potential and h the
height of the nucleus. For the formation of a hetero-
epitaxial nucleus a surface energy term for the interface
has to be added. The nucleation and growth mode is gov-
erned by the relative supersaturation, p/peq — with p being
the vapor pressure and peq the equilibrium vapor pressure
at a certain temperature — as driving force for the film
growth related to the excess chemical potential ∆µ by
eqln /( )B sk T p p∆µ = . (2)
The supersaturation has a strong influence on the
growth mode of the films — either dislocation-controlled
spiral growth (small supersaturation) or two-dimensional
nucleation [island formation] up to a critical super-
saturation (p/peq)*. Further increase of (p/peq) causes a
transition to unstable growth, uncontrolled nucleation of
growth centers on top of each others lead to a dendritic
growth type [34].
In addition to supersaturation the deposition tempera-
ture, Ts, plays an important role in determining the growth
kinetics and the surface morphology of the films. The rele-
vant quantity is the normalized bonding energy, ( )i
bE , for
the different species,
( ) ( ) 4 /2i i
ss B sbE k T= Φ (3)
( )( i
ssΦ denote the potential energy of a solid-solid nearest
neighbor pair of atoms in the substrate unit cell). It is obvi-
ous that increasing deposition temperature implies a small-
er Eb causing a higher density of kink sites on the surface
and a more rapid growth. For the spontaneous nucleation
of a unit cell a critical volume of the deposited material is
necessary. Since single atoms are impinging the substrate,
diffusion is required to ensure that a nucleus has the ap-
propriate constituents of the complex material to grow. The
surface mobility of the particles impinging the substrate
surface is one of the most important growth parameters.
The adatom mean diffusion length, λ, is given by
1/2( )sDλ = τ where sD is the surface diffusion constant
and τ the surface residence time. The diffusion coeffi-
cients for the different cations, however, can differ drasti-
cally; for YBCO e.g. they vary by 4 orders of magnitude
from Y (10–13 m2/s) to Cu (10–9 m2/s) at a deposition tem-
perature, sT , of 800 °C. Consequently, the formation of
nanoprecipitates and compositional phase separation is
facilitated [35,36].
STM studies of YBCO, e.g. grown by pulsed laser dep-
osition on SrTiO3 (STO), showed that the growth generally
proceeds layer-by-layer followed by the formation of is-
lands (multilevel 2-dimensional growth) [37]. In cases
where epitaxial strain plays a role Stranski–Krastanov
growth (islands on a wetting layer) occurs. Fig. 4 shows an
example of an ultrathin (7.5 nm) La0.9Sr0.1MnO3 film de-
posited onto a STO (001) substrate where dot-like features
(diameter ~ 20 nm height ~ 7 nm) are formed on a 2 unit cell
thick wetting layer [38]. Terashima et al. [39] directly
proved the Stranski–Krastanov growth of YBCO by observ-
ing oscillations in the intensity of reflection high energy
diffraction [RHEED] signals during film formation using
reactive evaporation techniques. The oscillations in reflec-
tivity are inferred to be the complete unit cell of the oxide.
Compared to the layer-by-layer growth of semiconductor
heterointerfaces, controlled by the highly directional cova-
lent bond of the constituents, the growth of COTF’s and
consequently their film properties are governed by the con-
straint of preserving charge neutrality and stoichiometry
imposed by the ionic character of the bonds. The minimum
growth unit is thus a complete unit cell [40]. This opens the
opportunity to grow heterostructures and superlattices of
complex oxides with atomically sharp interfaces and basical-
ly no interdiffusion and antisite defects across the interface.
2.2. Substrate requirements and opportunities
For the development of a reliable deposition technology
for high-quality COTF’s, the choice of the substrate mate-
rial is of primary importance. A review of substrates suita-
ble for complex oxide thin film deposition with emphasis
Fig. 4. (Color online) Nanodot-type surface of a 7.5 nm
La0.9Sr0.1MnO3 film deposited on a (100) SrTiO3 substrate (taken
from [31]).
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1081
H.-U. Habermeier
on substrates for high temperature superconductors for
microwave circuit applications is given by Hollmann et al.
[41]. Following Wördenweber [28], the optimal substrate
has to meet the following conditions:
a) crystallographic lattice match between film and
substrate to avoid strain-induced defect generation,
b) similar thermal expansion coefficients of film and
substrate,
c) no chemical interaction at the interface between
film and substrate,
d) atomically flat polished surfaces, structurally as
well as chemically stable.
Generally we can distinguish between two classes of sub-
strates: (i) those compatible with the oxide thin film deposi-
tion technique without additional buffer layers and (ii) those
where a buffer layer is required to accommodate large lattice
mismatch or to prevent chemical interaction between substrate
and oxide thin film. Typical candidates of the first category
are LaAlO3, SrTiO3 and MgO for cuprate and manganite thin
film deposition. In Table 1 some of the most frequently used
substrates and their properties are listed.
Substrates are much more than merely a chemically inert
mechanical support for thin films; they are functional ele-
ments in thin film technology [42]. On the one side, they
can be prepared as chemically well terminated atomically
flat surfaces at microscopic dimensions [43,44] on the oth-
er, intentional surface miscut and subsequent recrystalliza-
tion (vicinal cut substrates) enables a nanoscale tailoring of
step and terrace structures of the surface [45]. Figure 5
represents a TiO2 terminated STO (001) substrate with
large area atomically flat terraces interrupted by 1 unit cell
high steps. The terrace width is determined by the una-
voidable miscut of the substrate, which is determined in
case of the Fig. 5 to be ~ 0.0740.
Figure 6 shows the rather regular step- terrace structure
of a STO (001) substrate with an intentional miscut of 1.20
towards the [010] direction. Depending on the terrace
width, the growth mechanism of films using such sub-
strates usually changes from the Stranski–Krastanov mode
to the step-flow mode generating antiphase boundaries at
the step edges if there is a mismatch of substrate and film
unit cell height [45]. In the case of YBCO this causes ani-
sotropic flux-line pinning and thus flux penetration [46] in
case of ferromagnetic manganites a uniaxial anisotropy of
the magnetic domain arrangements and an anisotropy of
the hysteresis curves [47].
Furthermore, controlled lattice mismatch can be used to
tailor the biaxial in-plane strain states in the films and the rela-
tion to their physical properties. In the case of optimally
doped bilayer copper oxides (e.g. YBCO) the primary super-
conducting properties (Tc, Hc2, jc,) are found to be weakly
strain dependent. Pahlke et al. determined the shift of the su-
perconducting transition temperature to about 0.75 K per 1%
compressive biaxial strain for optimally doped YBCO,
whereas a significantly higher strain sensitivity of 4.20 K per
1% strain was found for underdoped YBCO [48]. For single
layer cuprates (e.g. La1.9Sr0.1CuO4) Locquet et al. could
achieve a doubling of Tc by compressive strain [49]. It is
believed, that the lattice deformations associated with the
strain fundamentally modify the energy scales, leading to
the formation and condensation of the Cooper pairs with
consequences for the properties of the normal state. In other
complex oxides, too, the strain state can substantially affect
phase transitions and transport properties. Razavi et al. [50]
Table 1. Properties of substrates suitable for thin film deposition of complex oxides
Material Thermal expansion
coefficient (10–6)
Melting temperature, °C Misfit to YBCO, % Twinning Chemical stability
NdGaO3 9–11 1670 0.04 no good
LaSrGaO3 10 1520 0.26 no good
SrTiO3 9.4 2090 1.4 no good
YSZ 11.4 2550 6 no good
MgO 14 2825 9 no good
LaAlO3 10 2100 –2 yes good
LaSrAlO3 7.55 1650 –2.6 no good
Fig. 5. (Color online) TiO2 terminated SrTiO3 surface showing
large atomically flat surfaces (Courtesy of H.J. Kim, Max-
Planck-Institute for Solid State Research Stuttgart, 2007).
1082 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
could convert the antiferromagnetic insulating state of
La0.1Sr0.1MnO3 into a ferromagnetic metallic one by com-
pressive in-plane strain. Here, epitaxial strain compression
of the lattice by the substrate enhances the lattice stiffness,
and consequently the Debye temperature, ΘD. The en-
hancement of ΘD causes a reduction of the static Jahn–
Teller distortion and an increase of the double exchange
interaction Mn3+–O–Mn4+. The group around D. Schlom
explored the possibility of adjusting the ferroelectric (FE)
transition temperature, TFE, by strain [51]. They could
show that for heteroepitaxially grown SrTiO3 films depos-
ited on DyScO3, tensile epitaxial strain causes an increase
of TFE by hundreds of degrees and produce room-
temperature ferroelectricity in strontium titanate, a material
that normally is not ferroelectric at any temperature. This
strain-induced enhancement in an ordering temperature is
the largest ever reported.
A further sophistication of the opportunities offered by
dedicated substrates is seen in piezoelectric substrates where
the in plane lattice constant of the substrate can be continu-
ously changed by ferroelectric poling and the inverse piezoe-
lectric effect linked with it. To study strain effects in complex
oxide thin films, conventionally films of nominally identical
thicknesses are prepared using identical deposition conditions
on substrates with different lattice parameters. This approach
implies that extrinsic variables such as substrate surface qual-
ity, film microstructure and oxygen stoichiometry are identi-
cal. An elegant way to circumvent these problems and focus
on the strain state only, is the application of the converse
piezoelectric effect by poling ferroelectric substrates. This
is accomplished by reversibly compressing the isotropic in-
plane lattice parameter of the rhombohedral piezoelectric
(0.67[Pb(Mg1/3Nb2/3)O3]-0.33[PbTiO3]) (001) substrates by
application of an electric field E < 12 kV·cm–1 yielding an
in-plane change of the lattice parameter < 0.12%. The ap-
plication of PMN–PT single crystals has shown to be
promising [52,53]. Depending on the polarity of the ap-
plied voltage, compressive or tensile strain can be
achieved. Applying ac-voltage dynamical processes can be
realized (c.f. Fig. 7).
In a practical example Zheng et al. [54] tested these
concepts using PMN–PT single crystals as ferroelectric
substrates and LaMnO3 as well as CaMnO3 thin films and
observed a strong coupling of the Jahn–Teller distortion to
the charge carriers. In Fig. 8 the principle of the experi-
mental set-up is shown and in Fig. 9 the results for sput-
tered LaMnO3–δ films are given. The data clearly show that
poling of the ferroelectric substrate curtail a reduction of the
in-plane tensile strain in LaMnO3–δ films, thus weakening
the electron-lattice coupling and generating a decrease of the
resistivity and increase of the Curie temperature.
In general, using substrates as functional elements in
the thin film technology of complex oxides, a rich oppor-
tunity is offered to tune their properties. They represent a
possibility to investigate details of the structure-property
relationship in complex oxide thin films. Strain generation
and relaxation research can contribute to shed light on the
intrinsic properties of these materials. Controlled modifica-
tions of the substrate surfaces are seen as a tool to study
details of the film growth mechanisms and generate defect
structures manageably.
Fig. 6. Step-and-terrace structure of a TiO2 terminated SrTiO3
substrate with an intentional miscut of 1.20 vs [010] (taken from
Haage et al. [46].
35 nm
Fig. 7. Principle of the poling experiment.
Fig. 8. (Color online) Sketch of the ferroelectric poling measure-
ment [54].
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1083
H.-U. Habermeier
2.3. Physical vapor deposition techniques for cuprates and
other complex oxides
2.3.1. General remarks
Nearly all commonly used thin film deposition tech-
niques have been more or less successfully employed for
COTF thin film and multilayer fabrication, e.g. thermal
and e-beam evaporation or co-evaporation, thermal or laser
molecular beam epitaxy (MBE), on-axis and off-axis high-
pressure sputtering, on-axis and off-axis pulsed laser depo-
sition. While the in-situ deposition route (deposition and
subsequent annealing/oxygenation in one chamber) is
commonly used for most of the complex oxides, the ex situ
routes (deposition of a precursor with subsequent treatment
outside of the growth chamber) are applied, if one compo-
nent forms volatile oxides at deposition conditions (e.g.
TlBaCaCuO and HgBaCaCuO [55] film deposition).
In-situ deposition techniques usually use two steps, i.e.
the deposition at high temperature (substrate temperature
and oxygen partial pressure during deposition determined
by the phase stability diagram) followed by a post-depo-
sition oxygen treatment for the desired oxygenation and
improved crystallization. For all deposition techniques, e.g.
for copper oxides, three criteria must be fulfilled — at least
when it comes to epitaxial films. (i) Since 3 or more cati-
ons are present, a precise tuning of their stoichiometry
must be accomplished to avoid undesired foreign phases,
(ii) a strong oxidizing environment must be ensured to get
the Cu into the desired oxidation state and (iii) off-
stoichiometry of the oxygen has to be avoided to prevent a
degradation of the superconducting properties.
The description of the deposition techniques in this part
is confined to the superconducting complex oxides of the
rare earth (RE) RE–Ba2Cu3O7–x type which are considered
to be the blueprint for other complex oxide materials.
2.3.2. Reactive evaporation
The most straight forward approach for YBCO thin film
deposition is the application of co-deposition techniques
using resistively heated boats or electron beam guns for
high melting point metallic constituents combined with a
rate control by quartz-crystal microbalances and computer-
controlled tuning of the flux of evaporants. Reactive evap-
oration is an excellent technique especially to finely tune
the composition of the individual layers and to grow large
area films. One fundamental problem is the appropriate
amount of reactive oxygen in conjunction with the re-
quirement of the operation conditions for the e–-guns and
the long mean free path of the evaporant. A very effective
implementation of the evaporation technique has been real-
ized by Utz et al. [56] utilizing a black-body-type rotating
disk heater which had a deposition section and a oxidation
pocket with a narrow opening so that in the oxidation sec-
tion a relatively high oxygen pressure of 1 Pa can be main-
tained whereas in the chamber the background pressure is
lower by a factor of 100 (see Fig. 10). Evaporation tech-
niques require an accurate method of controlling the rela-
tive deposition rates of the constituents involved. This can
be done by quartz crystal monitors, quadrupole mass-
spectrometers or optical absorption measurements.
2.3.3. Oxide molecular beam epitaxy
Molecular-beam epitaxy (MBE) uses well-defined
thermal beams of atoms or molecules which react at a crys-
talline surface to produce an epitaxial film. Originally de-
veloped for the growth of GaAs and (Ga,Al)As it is re-
garded now as a well matured growth technology for
semiconductor Si and GaAs-type thin films, heterostruc-
Fig. 9. Magnetic field dependence of the ferroelectric poling in-
duced reduction in resistance of a LaMnO3–δ thin film [54].
Fig. 10. Schematic sketch of a thermal co-evaporation system
introduced by Utz et al. [56] with an oxidation pocket enclosing a
part of the heater. The substrates are mounted on a rotatable disk
moving continuously through the pocket.
1084 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
tures and superlattices [57,58]. One reason for the big suc-
cess of GaAs semiconductor MBE is seen in the adsorption
controlled growth. Here, if sufficient As is supplied to sat-
urate each Ga monolayer stoichiometric films grow, and
the excess As desorbes from the surface and reenters the
vapor phase. There is a wide growth window that eases the
precision of the flux and temperature control requirement.
MBE in general is a low kinetic energy deposition tech-
nique (< 10–1 eV) thus avoiding a large concentration of
defects (especially point defects) usually arising from high-
energy species impinging the growing films. Epitaxial
growth in a clean ultra-high vacuum deposition environ-
ment combined with in situ characterization during growth
and the absence of highly energetic species are characteris-
tic, distinguishing MBE from other deposition techniques.
The transfer of this technology to complex oxides is a tre-
mendous challenge. On the one hand this is due to the
change from purely covalently bonded atoms in the semi-
conductors to the more ionically bonded charge reservoirs
in the superconducting cuprates, on the other hand it is due
the requirements of the appropriate oxygen supply to the
growing film under MBE UHV conditions. The big ad-
vantage of techniques, reactive evaporation and oxide-
MBE is the possibility to fine-tune the cation composition
by an active composition control.
Due to the short length scale for inhomogeneities either
structurally or morphologically, for all heterostructure and
superlattice work one needs to deposit ultra-thin layers
with interfaces as perfect as possible. Pioneered by
Bozovic [59] and Schlom [60], reactive MBE has been
demonstrated to be one of the most successful techniques
to accomplish complex oxide film preparation with atomic
precision. Reactive MBE being similar to reactive evapora-
tion with the exception of a pressure range during deposi-
tion low enough to ensure a mean free path of the species
emitted from the sources much longer than the source —
substrate distance. Typical working pressures for reactive
MBE are below 10–3 Pa. An advanced MBE system for
complex oxide thin film work is depicted in Fig. 11. To
provide molecular beams of the elemental constituents
radiative heated effusion cells are used and the molecular
beams are chopped by computer controlled mechanical
shutters. The advantage of effusion cells over resistively
heated boats is the more homogeneous momentum distri-
bution of the evaporant impinging the substrate and the
easier evaporant flux control. For successful atomic layer
epitaxy it is necessary not only to monitor the relative rati-
os of the molecular beams of the different constituents but
also the relative fluxes of the beams. Spectroscopic tech-
niques such as atomic absorption spectroscopy are applied
in addition to conventional quartz-crystal monitors. Mod-
ern MBE systems for complex oxide thin film research
combine flux-controlled deposition from up to ten effusion
cells with sophisticated in-situ analytical tools such as low
energy electron microscopy [LEEM]/photo emission elec-
tron microscopy [PEEM], low energy electron diffraction
[LEED] and time of flight ion scattering and recoil spectros-
copy [TOF–ISARS]. The reactive MBE technique repre-
sents the most elegant and clean — however most costly —
technique for COTF fabrication. Oxide MBE allows to
further explore the layer-by-layer deposition process to
prove the principles of the atomic layer engineering (ALE)
of new metastable layered oxide compounds with in-situ
RHEED control.
2.3.4. Sputtering
Sputtering — often described as playing billiard with
atoms — is the ejection of target atoms as a result of bom-
bardment by ions from a persistent glow discharge. It is a
routine technique for numerous industrial applications and
offers a possibility to quickly produce large area thin films
even on curved substrates. The process relies on the produc-
tion of plasma via an electrical discharge and the accelera-
tion of ions (typically Ar+) towards a target of the material
to be deposited used as a cathode. The kinetic energy of the
accelerated Ar+ ions is usually in the range of 10–1000 eV.
By a knock-on process on the target its material is ejected. In
Fig. 12 the scheme of a sputtering system is depicted.
Reviews covering the basics of the physical processes
in sputtering are given by Rossnagel [61] and Willmott
[62]. It is a conventional widely used method where the
film composition is normally expected to be equal to the
target composition. As sputtering gas usually Ar is used,
desired chemical reactions of the sputtered atoms/ions in
the plasma can be achieved by adding reactive gases to the
sputtering gas. In the case of multicomponent oxide sys-
tems tremendous problems arise from the formation of
negatively charged particles at the target [cathode] side.
Fig. 11. (Color online) Sketch of a state-of the-art oxide MBE
system as installed at the MPI-FKF Stuttgart. (Courtesy of
G. Logvenov).
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1085
H.-U. Habermeier
Elements with a large electronegativity difference such as
oxygen and the alkaline earth metals form negative ions
which are accelerated away from the target by the potential
difference of the cathode dark space. This energetic parti-
cle flux is directed to the substrate in conventional sputter-
ing geometries leading to selective resputtering of the
growing film which modifies the film composition or gives
rise — in extreme cases — to etching of the substrate ra-
ther than film deposition. To overcome these problems at
least four different routes are used: (i) to work at gas
pressures high enough to reduce the kinetic energy of the
ions striking the substrate below the binding energy [63],
(ii) to place the substrate off-axis so it will not face the
cathode. A very convincing realization of this idea is the
inverted hollow cathode arrangement by Geerk [64], (iii)
to design the system for a minimum discharge voltage
[65] and (iv) to adjust the target composition for a com-
pensation of the resputtering effects [66]. Practically, a
huge variety of different arrangements of target — sub-
strate position and target configurations have been ap-
plied or tried for in situ processes.
2.3.5. Pulsed laser deposition
Using pulsed laser deposition techniques short pulses –
(typically 10–30 ns) of UV light are focused onto a target
of desired composition. The light is absorbed according to
its penetration depth and a thin layer of superheated mate-
rial is formed which expands into the chamber in the form
of a forward directed luminous laser plume. Figure 13
shows the principle of a pulsed laser deposition (PLD) sys-
tem, schematically.
This technique, originally introduced by Dijkamp et al.
[67] for the deposition of YBCO single layers has experi-
enced a wide spread use for all kinds of oxide thin film
deposition. A general description of the application poten-
tial of PLD and the chemical processes taking place during
the ablation is given in the book of Bäuerle and references
herein [68]. A review paper by Jackson and Palmer [69]
discusses the operation of a laser deposition system and the
influence of the operating parameters on the process. In
brief, the PLD process consists of three parts: (i) the pho-
ton-target interaction, (ii) the plume formation and expan-
sion and (iii) the film forming process at the substrate side.
The main features of the photon-target interaction, i.e.
the high ablation rate, stoichiometric material transfer and
the formation of a forward directed plume are usually de-
scribed in a one-dimensional heat-flow equation.
( ) ( ) ( ) ( ) ( ),
, p
T z t TT c T I z t T
t t z
∂ ∂ ∂ ρ = α + κ ∂ ∂ ∂
.
Here, c, κ and ρ are the heat capacity, thermal conductiv-
ity and resistivity of the target material, α is the absorp-
tion coefficient for the impinging photons. The power den-
sity of the laser light at the target surface is 0 (1 )I I R= −
with 0I being the incident power density and R the reflec-
tivity causes a depth profile I(z,t) giving rise to a tempera-
ture profile T(z,t) inside the target. Reaching the tempera-
ture threshold for vaporization, Tth, at the target surface a
superheated sheath is formed and the vaporization process
begins. Using a laser fluence of 0.4 J/cm2 from a laser with
12 ns pulse duration Otsubo et al. [70] calculated the depth
profile of T(z) and found at a depth of 0.07 μm the maxi-
mum temperature of 2000 °C is reached in a YBCO target
(absorption coefficient 2⋅105 cm–1). When Tth is reached in
the subsurface region the material will explode out of the
target; the rapid speed of this process explains the nature of
the congruent ablation of multicomponent materials and
the feature of the highly directed plume of material above
the target. The short time span for the material ejection and
plume formation is the essential feature for congruent abla-
tion due to the lack of time for segregation of the compo-
nents of the plum according to their equilibrium vapor
pressures which occurs by continuous heating, e.g. using
electron guns.
The plume formation and expansion has been regarded
already at early times of oxide-PLD as an essential part of
Fig. 12. Scheme of a sputtering system (after Wördenweber [28]). Fig. 13. Sketch of a typical PLD system.
1086 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
the deposition process including chemical reactions with the
processing gas, pressure dependent particle velocity distribu-
tion, pressure gradients and material homogeneity within the
plume. Its main features are the direction perpendicular to
the target surface and the brightly colored material specific
appearance. Dyer et al. [71] investigated the principal lumi-
nescent species, expansion velocities, and the extent of ioni-
zation using visible UV spectroscopy and ion probes. Their
results have identified the major luminescent species in the
plume of KrF laser ablated Y–Ba–Cu–O superconductor
samples as arising from neutral and ionized atoms and
suboxides. Relatively high temperatures appear to accom-
pany ablation, resulting in high velocity emission (104 m/s)
and corresponding particle energies in the 25–50 eV range.
The extent of ionization in the plume is low (S < 4%) for
fluences up to ~ 4 J·cm–2, i.e., neutral rather than ionized
species will predominate in the deposition process under
these conditions. Aruta et al. [72] performed extensive
studies of plume properties and plume expansion using
KrF laser photons during the ablation process of LaAlO3
and LaGaO3 by fast photography and space-resolved opti-
cal emission spectroscopy in an oxygen background pres-
sure up to 10−1 mbar oxygen. They observed a strong de-
pendence of the kinetic energies of the ablated species on
the oxygen background pressure. For
2Op ~ 10–1 Pa the
kinetic energy of La was determined to be 58 eV whereas
in the case of pO2 ~ 10 Pa this value drops to 0.7 eV. This
result indicates that the oxygen background pressure dur-
ing ablation is a crucial parameter for the film forming
process since the oxygen bond, e.g. to SrTiO3, are in the
order of 10 eV. This implies that at low oxygen pressures
during ablation defect formation and oxygen vacancy gen-
eration is a complication to be considered in optimizing the
PLD process.
Using typical deposition conditions for YBCO [TD =
= 760 °C,
2Op = 20 Pa] growth rates of ~ 0.25 nm/s are
achieved using a 5 Hz pulse repetition rate. Due to the stoi-
chiometric ablation from the target it is quite simple to
achieve chemical composition identity of target and film —
provided no volatile species are formed. Heterostructures
and superlattices can easily be prepared using this method.
Computer-controlled target exchange systems are used and
the individual layer thickness is defined by a separate
growth rate determination. This is possible because the nom-
inal thickness of the ablated material per pulse is much
smaller as compared to the individual building blocks of the
relevant oxide materials, simple pulse counting is sufficient
for the thickness monitoring. This procedure however re-
quires constant laser fluency per pulse which is dependent
on the quality of the laser used. Systematic improvements of
growth control can be achieved by the implementation of a
differentially pumped Reflection High-Energy Electron Dif-
fraction (RHEED) system suitable for operating pressures in
the range of around 100 Pa [39]. The advantage of the PLD
approach is its simplicity, flexibility and the possibility to
switch from one material to the other just by replacing tar-
gets of typical sizes ~ 15 mm diameter and 5 mm thick.
They can be fabricated in house usually, so new material
combinations can be assessed quite quickly. The disad-
vantage of the PLD technique is seen in the restriction to
small substrate areas of ~ 2 cm2 and in the homogeneity of
thickness and composition. Additionally, the high kinetic
energies of the particles impinging the substrate [10–60 eV]
can cause re-sputtering effects if the incident laser beam
fluency is too high. The PLD approach is seen as an ideal
low cost tool for exploratory research and development. In
Fig. 14(a) (complete system) and 14(b) (inner part) a stand-
ard PLD system (DCA Finland) designed for heterostructure
and superlattice thin film is shown, including the facility for
a RHEED control of the growth process.
Fig. 14. (Color online) View of a UHV PLD system (DCA Finland) (left) and the inner part showing the target exchanger, sample heater
assembly and RHEED screen (right).
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1087
H.-U. Habermeier
2.3.6. Comparison of the different deposition techniques
Each of the in situ deposition techniques have their own
advantages and disadvantages and different ultimate goals.
Whereas PLD techniques is the most flexible one and is
capable to produce excellent single layer films, multilayers
and superlattices, detailed parameter adjustments are nec-
essary to avoid and/or minimize the droplet problem aris-
ing from the explosive character of the impact of the short
pulse photons. One drawback, however, is seen in the
thickness and compositional uniformity of the films limit-
ing the useful size of a substrate to ~ 1×1 cm. Sputtering
results in films of excellent quality suffers however from
very slow deposition rates in the range of 20–100 nm/h.
Evaporation and MBE techniques are scientifically the
most rewarding ones due to the cleanliness of the process,
the possibility of fine tuning the composition and the in-
corporation of in situ analytical tools. In Table 2 the ad-
vantages and disadvantages of the techniques are given
schematically (after H. Yamamoto et al. in Ref. 32)
3. Physical properties of single layer high temperature
superconductor thin films – some current trends
3.1. General remarks
In case of a successful accomplishment of a mature thin
film technology to deposit single layer copper oxide thin
films a plethora of opportunities to explore the fundamen-
tal properties of these materials is opened and the transfer
of concepts for electronic devices developed for classical
metal superconductors to their copper oxide counterparts is
facilitated. In Fig. 15(a)–(c) typical results for a single lay-
er YBCO film of 40 nm thickness are represented (x-ray
diffraction pattern showing a purely c-axis oriented film
(a) temperature dependence of resistance (b), and high res-
olution TEM (c) demonstrating the state-of-the-art quality
of YBCO thin films. The technology has been developed
to a high degree of perfection to address questions associ-
ated with the basics of high temperature superconductivity.
From the application point of view the materials issues
are solved to the extent that Josephson-effect based devices
(superconducting quantum interference devices–SQUIDS)
for digital applications, passive devices such as microwave
components, resonators etc. can be fabricated [16,17]. The
main hurdle in accomplishing the technology transfer from
low-Tc to high-Tc materials is seen in the extremely short
coherence lengths of the cuprate superconductors (a few
nm only) compared to the classical metallic ones (10s to
100s nm). Additionally, the d-wave symmetry of the order
parameter as compared to the s-wave symmetry of the
metals generates a further difficulty.
A comprehensive coverage of the field of active and
passive electronic devices are beyond the scope of this
paper, reviews can be found in the proceedings of the Ap-
plied Superconductivity Conferences (IEEE Transactions
on Superconductivity), relevant reference books, e.g. [73],
and review papers by Koelle et al. [74] and Newman et al.
[75]. Here, only 2 more materials science related and one
physics related examples are briefly described. One deals
with the deliberate tailoring of the orientation of copper
oxide thin films, with the c-axis either perpendicular or
parallel to the substrate plane or tilted by 45° with respect
to it. The second one describes the controlled generation of
antiphase boundaries by nanoscale regular substrate sur-
face patterning using the technique of substrates with a
vicinal cut and to study details of flux pinning mechanisms
by planar defects, and a third one covers properties of the
potential of fluctuation conductivity measurements to shed
some light on the physics of the pseudogap state.
3.2. Tailoring cuprate thin film orientation
Generally, superconducting copper oxide thin films
grow with the crystallographic c-axis perpendicular to the
substrate plane, i.e. the short coherence length is pointing
perpendicular to the film plane. The driving force for this
growth direction is seen in the different surface energies of
the (100), (010) and (001) surface planes and the lattice
match of, e.g., YBCO on SrTiO3. Films of both types, c-
axis aligned either perpendicular or parallel to the film
plane are important for fundamental studies of the anisot-
ropy transport properties of them as well as for potential
applications, e.g., as planar Josephson junctions. Devices
Table 2. Comparison of the deposition techniqes
Method Molecular beam epitaxy Pulsed laser deposition Sputtering
Superconducting properties
(Tc, Hc2, jc)
Excellent Excellent Fair
Composition control QCM, AAS, EIES Limited by process Limited by target
Kinetic energy of particles
at substrate, eV
10–2–10–1 1–60
Depending on the background pres-
sure
2–10
Depending on the input power
Thickness control Excellent Excellent Fair
Surface flatness Good (if stoichiometric) Good ( but droplet problem) Good
Ultra-thin film Possible Possible in optimized conditions Poor
Large-area growth Possible Not achieved to satisfaction Possible
1088 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
based on planar Josephson junctions should have the (a),
(b) plane perpendicular to the substrate plane which is the
case of (100) and (110) oriented films. In the late 80’s sev-
eral authors described the deposition of (110)-oriented
YBCO thin films grown on (110) SrTiO3 substrates
[76,77]. Care must be taken that a dedicated film analysis
is capable to distinguish between the (110) orientation and
the competing (103) / (103) orientation with identical peak
positions in the x-ray Θ–2Θ diffraction patterns. In Fig. 16
the two competing film orientations are displayed.
In addition to x-ray pole figures, Raman spectroscopy
has been proven to be an elegant and fast tool to character-
ize high temperature superconductors as reviewed by
Thomsen and Cardona [78]. Since the relative magnitudes
and signs of the components of the Raman tensor ( )ijα for
a given Raman mode are rather different for light polarized
parallel or perpendicular to the CuO2 planes, Raman scat-
tering is an easy and fast method to determine the orienta-
tion and oxygenation of a single crystal or thin film.
Habermeier et al. [79] applied the PrBa2Cu3O7–x (PBCO)
hetero-templating technique, where a PrBa2Cu3O7–x template
of ~ 60 nm is deposited at a substrate temperature of
660 °C at an oxygen partial pressure of 1mbar followed by
the YBCO deposition with continuously increasing the
substrate temperature to 720 °C. The films show values for
Tc between 83 K and 87 K. The corresponding x-ray dif-
fraction pattern revealed almost single phase (110)-orien-
ted films with the dominant diffraction peaks ascribed to
the (110) and (220) reflections adjacent to the substrate
(110) and (220) peaks. A weak indication of some c-axis
grains could be identified, whose volume fraction was es-
timated to be less than 1/1000. Raman spectroscopy clearly
revealed the (110) orientation as shown in Fig. 17.
(103) / (103) films can be grown using standard deposi-
tion conditions (Ts = 750 °C,
2Op = 50 Pa) using (110)-
oriented SrTiO3 substrates. The (103) phase can be com-
pletely suppressed, if the substrate has an intentional
miscut of 3.5° vs the [100] direction [80]. These films
Fig. 15. (Color online) X-ray diffractogram of a 40 nm YBCO
film deposited on a SrTiO3 (100)-oriented substrate (a) corre-
sponding resistance vs temperature plot (b). High resolution TEM
of an YBCO film deposited on a SrTiO3 (100)-oriented substrate
showing the perfection of the film structure and the interface (c).
Fig. 16. Schematic representation of YBCO growth in (103)/(103)
orientation (a) and (110)-orientation (b).
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1089
H.-U. Habermeier
show a slightly higher Tc and lower resistivity along the
(a),(b)-plane compared to the (110) YBCO films indicating
a lower defect density of the films.
The conclusion is that from the technological point of
view detailed investigations of the anisotropy of the optical
and transport properties are possible and the basis for fab-
ricating planar Josephson junctions is accomplished.
3.3. Controlled generation of antiphase boundaries
As already mentioned, a current in a superconductor
causes the Abrikosov vortices to move in a dissipative way
unless they are pinned by defects. In general, the critical
current, jc, is a measure of the volume pinning force exert-
ed to the flux-line lattice by extrinsic defects. The largest
jc’s in YBCO of 3⋅1011 A/m2 at 5 K have been observed in
epitaxial films with a high density of different lattice de-
fects related to the Stranski–Krastaov growth of the films
and the subsequent formation of grain boundaries. Since
the microstructure is rather complex, much effort has been
spent to identify the most effective pinning sites.
The concepts developed to treat flux pinning in classi-
cal superconductors are based on the analysis of the ele-
mentary pinning force, fp, exerted to a single flux-line
caused by a defect in the crystal lattice and the statistical
summation of elementary pinning forces to an average
volume pinning force, Fp. As one of the fundamental
mechanisms leading to flux line pinning, suprastrictive
interactions have been discussed. They are associated with
stress fields around defects in an interaction volume de-
termined by the superconductor coherence length and the
dimension of the defect. The theoretical base of this con-
cept is the phenomenological Ginzburg–Landau theory
which gives a good description of the reality — at least in
the vicinity of Tc, if the superconducting order parameter is
only slowly varying within distances of the superconduct-
ing coherence length. This approach, however, cannot
simply be transferred to the high temperature superconduc-
tors [HTS] mainly due to the fact of the extremely short
anisotropic coherence length, ξ in the HTS materials with
ξab = 1.5 nm in YBa2Cu3O7 in the CuO2 plane and ξc =
= 0.15 nm along the c-axis. Superimposed to the problems
arising from the small values in ξ is the difficulty due to
the poly-elemental composition of the material with a vari-
ety of possible lattice defects giving rise to a plethora of
pinning mechanisms with — probably — quite different
efficiency. In YBCO single crystals randomly distributed
point defects such as oxygen vacancies lead to a weak col-
lective pinning [81,82], correlated pinning has been
demonstrated to arise from twin boundaries [83] and artifi-
cial columnar defects generated by heavy particle irradia-
tion [84]. They allow the appropriate alignment of the vor-
tex system with the defect structure resulting in higher
pinning forces compared to collective pinning induced by
point defects. In epitaxial thin films of YBCO the critical
current, jc, is generally at least one order of magnitude
larger compared to twinned single crystals [85,86]. One
dimensional defects such as screw dislocations [87], two
dimensional defects like grain boundaries [88], precipitates
[89] and surface roughness [90] had accounted for the
strong pinning. Up to now it is not clear which of the dif-
ferent types of defects causes the higher jc.
The growth controlled generation of an array of
antiphase boundaries [APB’s] leading to a planar defect
structure and its effect on the transport properties of the
films is one way to contribute to disentangle this complex
situation. It is caused by the modification of thin film
structure via growth on vicinal cut SrTiO3 single crystal
substrates. In Fig. 18(a) the formation of an APB at a sub-
strate step is shown schematically.
Domains nucleating at adjacent terraces are shifted ver-
tically by a fraction of a unit cell. Alternatively, a change
of the stacking sequence on adjacent terraces enables a
smooth and structurally defect free overgrowth of a sub-
strate step. Figure 18(b) shows the antiphase boundary
emerging from a STO surface step in an YBCO thin film.
Combining UHV–STM investigations of the substrate in
situ and film surface analysis after deposition in conjunc-
tion with transport and magneto-optical investigations, a
more detailed analysis of these growth-induced pinning
centers could be achieved [91]. In order to gain a more
thorough understanding of the thin film growth and the
Fig. 17. Raman spectra of a (110) YBCO film. Lower spectrum:
the laser beam propagating perpendicular to the film plane along
[110], the incident and scattered light polarized along the c-axis
lying in the plane. Upper spectrum: same direction of the laser
beam but incident and scattered light polarized perpendicular to
the c-direction along a (110) direction lying in the plane. [79].
1090 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
defect generation, the substrates and films were studied by
UHV scanning tunneling microscopy (STM) in situ. Fur-
thermore, this experiment may be regarded as a case study
for the controlled generation of defects by intentional sub-
strate modifications which is general in nature. It could be
demonstrated, that pinning at growth induced planar
APB’s in YBCO thin films is dominated by quasiparticle
scattering induced variation of the condensation energy.
Crucial for this study of jc in YBCO films is that the mi-
crostructure is dominated by only one kind of pinning site
exhibiting strong correlated and anisotropic pinning. The
combined study of magnitude, temperature and angular
dependence of jc with respect to the APB’s plane by means
of magneto-optics allows us to identify the pinning mecha-
nisms. Microscopic theory [92] suggests two fundamental-
ly different mechanisms giving rise to high pinning forces:
(i) disorder induced spatial variation in Tc (δTc pinning)
and (ii) local variation of the quasiparticle mean free path l
(δl pinning). In contrast to δTc pinning, where defects of
dimensions of at least the coherence length are required for
effective pinning, the defect size for quasiparticle scatter-
ing pinning may be much smaller [92]. In superconductors
with isotropic gap the δl effect influences only the kinetic
energy of the supercurrent of a vortex. In the framework of
collective pinning, Griessen et al. [93] interpreted their
results as evidence for this pinning mechanism in YBCO
thin films. Since then, the presence of an order parameter
with
2 2x yd − or some mixed symmetry (s+d) has been es-
tablished in YBCO [94]. In this case the Anderson theorem
[95] is not applicable and elastic quasiparticle scattering
breaks the Cooper pairs. Consequently, vortices interact
with defects by the δl effect also via condensation energy.
The results by Jooss et al. [96] prove this interaction
mechanism of vortices at APB’s in YBCO.
3.4. Fluctuation conductivity.
There is a general consensus that a complete under-
standing of the high Tc cuprate superconductors must in-
clude their normal state properties, especially in the range
between Tc and the temperature where a pseudogap opens
in the underdoped region of the generic phase diagram. As
already mentioned, in this range the resistivity is nearly
two orders of magnitude larger than in simple metals and
shows frequency and temperature dependences that are
incompatible with the conventional theory of metals; the
regime above Tc is called the regime of ‘strange metals’.
The behavior is frequently described in terms of the so
called “marginal-Fermi-liquid” phenomenology [97]. The
structure of the pseudogap in momentum space can be di-
rectly analyzed by ARPES experiments and it formally
mimics the d-wave superconducting gap. The pseudogap
only appears in the antinodal direction of the Brillouin
zone where the d-wave gap is largest. This experimental
observation suggests that the pseudogap opens at a temper-
ature where pairs already start to form. Phase fluctuations,
however, prevent phase coherence and thus superconduct-
ing order. There are several experimental hints for the ex-
istence of these preformed pairs, a prominent strong one is
the observation of diamagnetism [98]. Furthermore, a de-
tailed analysis of fluctuation conductivity σ’(T) can reveal a
deeper insight in the pseudogap properties. Solovjov et al.
[99] studied the fluctuation conductivity in YBCO thin
films with controlled oxygen deficiency so the doping lev-
el could be tailored. The basic concept of this work is ad-
dressing the question of scattering due to the interaction
with different types of excitations. One model is based on
the resonant valence scenario of Anderson [100], here; the
excitations are spinons and holons, which contribute to the
resistivity. The holon contribution is assumed to be the
governing one, whereas the spinons, which are efficiently
coupled to the magnetic field H, should determine the tem-
perature dependence of the Hall effect. The other models
are based on the concept of anisotropy of the carrier scat-
tering, arising from the complex band structure of the
Fig. 18. Illustration of the formation of an antiphase boundary at a substrate step: domains nucleating on adjacent terraces are shifted
vertically by a fraction of the unit cell (a); high-resolution cross-section TEM image showing an extended antiphase boundary (APB)
marked by arrowheads (b) [46].
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1091
H.-U. Habermeier
cuprates or the influence of spin fluctuations. There are
two fluctuation contributions to σ’(T). The direct contribu-
tion, which is given in a theoretical framework by
Aslamazov and Larkin (AL) [101] arises as a result of the
spontaneous formation of fluctuation-induced Cooper pairs
above Tc. An additional contribution, introduced by Maki
and Thompson (MT) [102] as an extension of the AL theo-
ry, is interpreted as being the result of an interaction of
already existing fluctuating pairs with normal charge carri-
ers and is governed by pair-breaking processes in the par-
ticular sample. The MT contribution depends on the life-
time ϕτ of the fluctuating pairs and is dominant in the
region of two-dimensional fluctuations, whereas the AL
mechanism dominates in three dimensional region of fluc-
tuation conductivity near Tc. In layered structures, includ-
ing HTSCs the AL contribution is usually described within
the frame of the Lawrence–Doniach model [103], which
predicts a smooth dimensional crossover from 2D to 3D
fluctuation behavior for T approaching Tc. Solovjov’s ex-
periments show a clear MT–AL 2D to 3D dimensional
crossover on the σ’(T) curves by reducing the temperature
towards Tc as shown in Fig. 19.
Solovjov and Dmitriev reviewed the study of excess
and fluctuation conductivity σ’(T) in YBa2Cu3O7−y and
Y1−xPrxBa2Cu3O7−y epitaxial thin films [104]. Their anal-
ysis is based on the idea that excess conductivity σ’(T)
forms in HTSCs at temperatures substantially above criti-
cal temperature Tc as a result of the formation of pair states
in the form of non-interacting strongly bound bosons, ex-
hibiting with decreasing temperature a transition from a
regime with localized pairs compatible with the Bose–
Einstein condensation theory into a regime with fluctuation
Cooper pairs which can be described within the frame of
the BCS approach.
During the further development of complex oxide depo-
sition technologies heterostructures and superlattices with
antagonistic order parameters (e.g. superconductive and
ferromagnet ones) could be synthesized and fluctuation
conductivity measurements give rise to more insight on the
interrelation of phase fluctuations, preformed pairs and
dimensionality.
4. Complex oxide heterostructures and superlattices
based on superconducting cuprates
4.1. General remarks
The technological advances in the atomic-scale synthe-
sis of complex oxide heterostructures have provided a
flourishing ground to explore electronic interactions either
at their interfaces or across the layers in analogy to the
mechanisms giving rise to the GMR effect in magnetic
multilayers. In several recent review papers Hwang et al.
[105], Keimer et al. [18] and Mannhart et al. [106] high-
lighted the fundamental physics background emerging
from the breakthroughs based on advanced oxide thin film
technology. In contrast to bulk materials, different sym-
metry constraints can be artificially imposed at the inter-
faces and thus offer the opportunity to design structures
exhibiting new phenomena with potentially intriguing nov-
el properties. Arising from the experimental results of gen-
erating conducting layers between oxide band insulators
[107] and the analysis of cuprate/manganite hetero-
structures and superlattices [108], the concept of recon-
struction of the charge, spin and orbital states at interfaces
at the nanometer scale turns out to be a new research topic
with strong impact on fundamental physics and possible
applications as well. Interface superconductivity, magneto-
electric coupling, and the quantum Hall effect in oxide
heterostructures are representative examples of the scien-
tific and technological achievements in this rapidly grow-
ing field. Common to all these activities is the participation
of at least one transition metal ion with the potential of
occurring in different valence states.
Transition metal oxides (TMOs) are at the core of these re-
search activities and are the ideal playground for the study of
electron correlations. The transition metal s electrons are
transferred to the oxygen ions, and the remaining d electrons
are strongly correlated and determine the physical properties
of these complex oxides including electrical transport, mag-
netism, optical response and thermal conductivity. These elec-
tron correlations constrain the number of electrons at a given
lattice site, and induce a local entanglement of the charge, spin
Fig. 19. Comparison of the experimental data of ln(σ’) (full circles)
vs ln(ε) with the fluctuation theories: curve 1 — MT contribution
(scaling factor C2D = 0.544, d = 1.17 nm), curve 2 — LD contri-
bution (C3D = 0.55, d = 1.17 nm), curve 3 — AL contribution
(3D) (C3D = 1), curve 4 — MT contribution (C2D = = 0.494, d =
0.42 nm).
1092 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
and orbital degrees of freedom and give rise to a variety of
macroscopic phenomena like Mott insulators, various charge,
spin and orbital orderings, metal–insulator transitions,
multiferroics and superconductivity. In recent years, there has
been a burst of activities to manipulate these phenomena, as
well as to create new ones, using oxide heterostructures. As
described in detail by Hwang et al. [105] the fundamental
basis for understanding the physical properties of TMOs is the
concept of the symmetry of the order parameter. As Landau
has already stated, the essence of phase transitions is the
change in symmetry. Manipulating the symmetry at an
nanoscale level opens the door for artificially designed phase
transitions. The interplay among the electronic degrees of
freedom produces various forms of symmetry-breaking pat-
terns of space inversion, time-reversal and gauge symmetry
(associated with the change of the phase of the wave function)
and is expected to lead to novel emergent phenomena trig-
gered through the collective behavior of electrons.
An especially intriguing case is the combination of ferro-
magnetic (FM) and superconducting (SC) oxide layers as
heterostructures and superlattices. Phenomenologically, fer-
romagnetic and superconducting order has been regarded to
be antagonistic by nature for a long time. Ferromagnetic order
consists in the parallel alignment of spins whereas for the es-
sential ingredient of superconductivity, the Cooper pairs, the
coupling of electrons with antiparallel spin is mandatory. In
this section some of the experiments are reviewed which con-
tributed to the boom of research activities in complex oxide
interface investigations. Amongst them are the early activities
to grow and analyze SC/FM heterostructures and super-
lattices, the determination of the spin diffusion length in the
superconductor and the growth of SC/FM bi- and trilayers for
potential use in planar SFS junctions.
4.2. Ferromagnet-superconducting bilayers and
superlattices
4.2.1. Early experiments
In conventional s-wave superconductors local magnetic
moments break the spin singlet Cooper pairs and thus
strongly suppress superconductivity. There is, however, a
limited class of superconducting compounds where super-
conductivity occurs in the presence of magnetic ions occu-
pying a specific lattice site. Some examples are the rare
earth molybdenum selenides (RMo6Se8) and the rare earth
rhodium borides (RRh4B4). All these materials have in
common that the superconducting ordering temperature is
substantially higher than the Curie temperature. A compre-
hensive review of the physics of the coexistence of ferro-
magnetism and superconductivity in these materials is given
by Maple [109]. The discovery of ferromagnetism and su-
perconductivity in a completely different class of materials,
the cuprate based compound RuSr2GdCu2O8 (Ru1212)
[110] generated much interest mainly due to the fact that the
Curie temperature is much higher (~ 160 K) than the super-
conducting Tc, (~ 50 K), i.e. superconductivity emerges in a
magnetic environment. Since in copper oxides superconduc-
tivity appears upon doping of an antiferromagnetic parent
compound a detailed analysis of the properties of
RuSr2GdCu2O8 can shed some light on the origin of super-
conductivity in the cuprates. Structurally, Ru1212 consists
of a charge reservoir block containing magnetically ordered
Ru5+ ions intercalated by CuO2–Gd–CuO2 layers which are
believed to cause superconductivity. Due to the layered
structure of Ru1212 a natural extension for the research on
magnetic superconductors was to mimic its properties by
combining oxide ferromagnmets (e.g. LaCaMnO) and oxide
superconductors (YBCO) in heterostructures and superlat-
tices. Stimulated by earlier studies of cuprate/manganite
heterostructures [111] Habermeier et al. performed system-
atic studies by varying the individual layer thicknesses in
superlattices and studied their transport and magnetic prop-
erties [112]. The films grown by PLD showed x-ray diffrac-
tion patterns of a superposition of features from YBCO and
LCMO including superlattice peaks indicating the perfection
of their crystallographic structure as shown in Fig. 20.
Figure 21(a) depicts the temperature dependence of
resistance as well as the magnetization for an n/m
YBCO/LCMO superlattice evidencing the existence of a
strong interaction between the SC and FM order parameter,
indicated by the reduction of both ordering temperatures.
At the beginning of these research activities there has been
much concern about sample quality and how they can af-
fect the results. Reproducibility of the results — even
quantitatively — by different groups and detailed micro-
structural analysis including analytical high resolution
TEM could rule out these concerns [113]. Figure 21(b),
e.g., displays a high resolution TEM micrograph of an
YBCO/LCMO SL in the vicinity of the STO substrate
showing the structural perfection of the YBCO and LCMO
layers and the structurally perfect interface.
Additionally, it could be shown by dedicated analytical
TEM investigations that the interfacial stacking patterns are
different for the YBCO–LCMO interface (YBCO–BaO–
CuO2–(La,Ca)O–MnO–LCMO and the LCMO–YBCO one
(LCMO–(La,Ca)O–MnO–BaO–CuO–BaO–CuO2–YBCO)
[114]. The consequences of the details of the microstruc-
ture on the macroscopic properties of the films are not ex-
plores yet, however, they might be of importance for the
charge carrier transport across the interfaces The early
work on these hybrid structures and superlattices com-
posed of a high temperature superconductor (e.g. YBCO)
and a half metallic ferromagnetic oxide (e.g. LCMO) has
initiated further activities to explore the interference be-
tween these antagonistic order parameters. Research on
these structures has revealed charge transfer across the
interfaces as well as interfacial spin and orbital polariza-
tion with consequences for the macroscopic properties in-
cluding charge transport and thermoelectricity.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1093
H.-U. Habermeier
4.2.2. Determination of the spin diffusion length
Measurements of the far infrared dielectric properties of
these superlattices by spectroscopic ellipsometry provided
clear evidence that due to the interaction of the ferromag-
netic LCMO and the superconducting YBCO in the
superlattices the free carrier response is strongly sup-
pressed with an unexpected large length scale in the order
of 10–20 nm. Control experiments replacing the FM
LCMO layer by the paramagnetic LaNiO3 metal do not
show this suppression, so the suppression is due to the
magnetic interaction [115]. Further evidence for the long
range interaction came from an analysis of the Tc reduction
in LCMO/YBCO bilayer samples [116]. In this experiment
on top of a 20 nm LCM film deposited on STO or LSAO
substrates YBCO films have been grown with thicknesses
ranging from 20 to 100 nm and the temperature depend-
ence of the magnetic response as well as their resistance
Fig. 20. (Color online) X-ray diffraction pattern of a YBCO/LCMO (10/10)12 superlattice
Fig. 21. Temperature dependence of resistance and the magnetic moment of a (50nm/20nm)5 YBCO/LCMO superlattice (left) and a
corresponding HRTEM (right).
1094 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
have been measured. Using the LCMO as an electrode the
role of spinpolarized quasiparticle injection into YBCO
and the corresponding reduction of Tc could be investigat-
ed. Using a classical model by Parker [117] the pair break-
ing effect of spinpolarized quasiparticle injection into
YBCO was compared with the experimental results and the
comparison revealed that the pair breaking effect due to the
spinpolarized quasiparticle injection is found to be ~10 nm
compatible with the results from Holden [115].
Figure 22 shows the excellent agreement of the experi-
mental results for YBCO films of different thickness de-
posited on 20 nm LCMO in comparison with the theoreti-
cal model. These experiments clearly show that the
interaction between the SC and the FM layer in the
heterostructures and superlattices are not confined to a
length scale of a few nm, only.
4.2.3. (110)-oriented YBCO/LCMO heterostructures
and trilayers
As already stated, superlattices and heterostructures of
transition metal oxides with strong electron correlations are
gaining increasing interest as a platform to harness a varie-
ty of quantum many-body states (including metal-insulator
transitions, ferromagnetism, superconductivity, multifer-
roicity, as well as charge, spin, and orbital ordering) for
potential applications. Using structures composed of two or
more TMOs, interference between these states across inter-
faces has the potential to generate new phenomena and
functionalities. Due to growth related technological bound-
ary conditions, most of the superlattices and hetero-
structures investigated so far were grown along the [001]
axis of the YBCO crystal structure. Here, the direction
perpendicular to the highly conducting CuO2 planes is par-
allel to the YBCO–LCMO interface. The role of proximity
coupling of superconducting and ferromagnetic order pa-
rameters has not yet been clarified in these structures, pre-
sumably as a consequence of the small superconducting
coherence length ξc ~ 0.3 nm of YBCO in the [001] direc-
tion. Investigations of films with the much larger coher-
ence length ξab ~ 1.6 nm oriented perpendicular to the film
plane can shed more light at the proximity coupling. Mus-
tafa et al. [118] deposited heterostructures using pulsed
laser deposition with deposition parameters optimized for
the growth of (110)-oriented YBCO films. Due to the
problems of the competing (110) and (103)/(103) orienta-
tions, Mustafa made pole figures choosing the YBCO
(117) orientation with no overlap with any of the STO or
LCMO reflections (Fig. 23(a)). The absence of such reflec-
tions in the pole figures clearly demonstrates that the films
are exclusively (110) oriented, within the detection limit of
x-ray diffractometry. To check the orientation of the
LCMO layer with respect to the substrate and the YBCO
layer, the (116) plane of LCMO was chosen for an addi-
tional set of pole figures (Fig. 23(b)). Here, 12 peaks are
observed in the scan, eight of which refer to reflections
from the STO (221) plane, and four of which arise from
the (116) reflection (or equivalent reflections) of LCMO.
No other peaks of LCMO were observed, indicating that
the LCMO layer is grown epitaxially with its (110) plane
parallel to the (110) plane of STO. The pole figure analysis
of the heterostructures thus demonstrates that the (110)
planes of the STO substrate, the YBCO layer, and the
LCMO layer are parallel to each other. Figure 24 presents
the temperature dependence of the resistivity measured
perpendicular (ρab) and parallel (ρc) to the YBCO c-axis
using a standard four-probe arrangement (inset in Fig. 24).
The large anisotropy (ρc/ρab ~ 25 at room temperature,
~ 35 at 180 K) is consistent with the single-phase nature of
the film inferred from the structural analysis and is caused
by the resistivity anisotropy of YBCO. The temperature
dependence of the resistivity of the film can be modeled as
a network of parallel resistors and is hence dominated by
the layer with the smallest resistivity. In the direction of
the CuO2 planes the value is dominated by the YBCO lay-
er whereas in the perpendicular direction both resistivities
(YBCO and LCMO, respectively) are of comparable mag-
nitude and contribute nearly equally to the total value.
The contribution of the LCMO layer can be recognized
in the resistivity peak around the Curie-temperature that is
typical for ferromagnetic LCMO films. This contribution is
presumably responsible for the lower resistivity anisotropy
of the film compared to bulk YBCO, where ρc/ρab ~ 100 at
optimal doping. Both resistivity curves reveal a supercon-
ducting transition with an onset around 90 K, and zero re-
sistivity (within the measurement error) around 75 K. The
rounding of the superconducting transition may reflect
intrinsic interfacial effects (such as the transfer of spin-
polarized quasiparticles across the interface) and/or residu-
al inhomogeneity of the oxygen content within the YBCO
Fig. 22. (Color online) Normalized superconducting transition
temperature of bilayers with varying thickness of the YBCO layer
on STO3 and LSGO4 substrates obtained from the diamagnetic
onset in the zero-field-cooling magnetization measurement (solid
line) [116].
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1095
H.-U. Habermeier
layer. The technological progress accomplished by Musta-
fa et al. [118] offers the opportunity to contribute to the
investigation of the interrelation between crystallographic
and electronic structures and thus to the interface proper-
ties. The comparison of the reduction of Tc in (001) and
(110) — oriented bilayer (82 K for (001) and 60 K for
(110)) indicates that the extension of the proximity effect
between ferromagnetic and superconducting layers arises
from the different coherence length. The different atomic
stacking of the constituents and thus their electronic con-
figuration of (001) and (110) oriented bilayers can be used
as a control parameter to achieve deeper insight in the fun-
damental physics of these heterostructures.
The extension of this so far more technological work is
the fabrication of hybrid oxide superconductor/ferromag-
net/superconductor (SFS) structures with potential applica-
tions in superconducting electronics and quantum compu-
ting. The understanding of the mechanism of the electrical
transport across the oxide ferromagnetic barrier is the
property of interest. Soltan et al. [119] showed that an ul-
tra-thin LCMO ferromagnetic layer with a thickness of few
unit cells can be grown in between of two thick YBCO
superconducting layers. In particular, the ferromagnetic
transition temperature is found to be TCurie = 260 K which
is close to the bulk value This opens up the way to investi-
gate the electronic transport across the thin ferromagnetic
layer with possible perspectives: (i) to fabricate oxide SFS
Josephson junctions and (ii) to understand in detail the
proximity effects in oxide SFS-structures with strong cor-
relation. Therefore the successful fabrication of oxide SFS-
structures may help to understand the long-range proximity
Fig. 23. (Color online) X-ray pole figures of an LCMO (50 nm)–YBCO (100 nm) heterostructure (a) around the (117) reflection of
YBCO, and (b) around the (116) reflection of LCMO [118].
Fig. 24. (Color online) Temperature dependence of resistivity of
an LCMO (50 nm)–YBCO (100 nm) heterostructure parallel and
perpendicular to the YBCO c-axis. The inset shows the arrange-
ment of Au contact pads evaporated on the film surface [118].
Fig. 25. (Color online) Current-voltage curves of current transport
across the ferromagnetic barrier. Junctions with lateral dimensions
of 300 μm × 200 μm were prepared by ion milling out of the
heterostructures of YBCO–LCMO–YBCO (50 nm–1 nm–50 nm)
grown on STO-110 (red, 1) and 001 (blue, 2). A sketch is
shown in the inset of panel (b). (a) — the I–V curve of the 110-
sample shows a vertical slope at V = 0 indicating a supercon-
ducting transport with critical current of 1 ⋅ 10−5 Å. (b) — the
I−V curve of the 001-sample shows nonlinear behavior for I–V
curve without any supercurrent flowing at V = 0 [119].
1096 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
effects between d-wave high-Tc superconductors and half-
metal ferromagnetic LCMO layers. The main difficulty in
accomplishing this type of junctions is a technological one
and arises from the length scale required to obtain a
supercurrent. Charge transfer, spin-polarized effects as
well as structural effects at the interface can strongly sup-
press superconducting transport across the oxide ferro-
magnetic interface. The conjecture is that the thickness of
the ferromagnetic LCMO barrier must be comparable to
the superconducting coherence length.
Soltan et al. succeeded in fabricating ultra-thin ferro-
magnetic LCMO layers in between two epitaxially grown
YBCO films with different crystallographic orientations,
namely, (110) and (001). They could observe a clear and
unexpected ferromagnetic response of 1 nm and 2 nm thick
LCMO films with ordering temperatures above T ≈ 200 K.
In combination with YBCO films grown in (110)-
orientation using sophisticated buffer layers, it is possible
to realize a high-Tc SFS-junction with a ferromagnetic lay-
er thinner than the coherence length ξab of YBCO. The
measurements give evidence of supercurrent transport
across the ferromagnetic barrier (Fig. 25). Barrier rough-
ness and temperature and field dependence of the transport
current strongly suggest that the observed transport current
is flowing at least partly across the ferromagnet.
4.3. Ferromagnet–superconductor interfaces
4.3.1. General remarks
Artificial superlattices (SL’s) represent a well-established
research topic in condensed matter physics and modern de-
vice technology. Semiconductor heterostructures and SL’s
have proven to form the basis for unexpected advances in
science and device physics over the past decades. The de-
velopment of solid state lasers and high electron mobility
transistors (HEMT) may serve as prototypes for the advanc-
es in device physics. An example for achievements in basic
science is the formation of a 2-dimensional electron gas in
III–V-compound semiconductor heterostructures or in sili-
con metal-oxide-semiconductor field-effect transistors ena-
bling the subsequent discovery of the quantum Hall effect by
von Klitzing et al. [120]. Similarly, metallic SL’s consisting
of paramagnetic and ferromagnetic layers give rise to a giant
magnetoresistance [121] and serve now as sensing elements
in reading heads of hard disk drives. The attempt to replicate
such SL’s using transition metal oxides (TMO’s) and ex-
plore their rich potential in “orbital physics” will pave the
way for an even more exciting research area. The delicate
interplay of spin-, charge-, orbital and lattice interactions of
electrons arising from charge transfer across the interface
and the sensitivity of the orbital ordering patterns of elec-
trons in the 3d shells to external perturbations such as strain,
electrical and magnetic fields, photon flux etc. can open a
wide field of applications. Combining such TMO’s into
heterostructures (HS’s) or SL’s gives rise to expectations
that at their interfaces novel correlation driven quantum
states are appearing with functionalities qualitatively beyond
those attainable in metals or semiconductors. Since the phys-
ical properties of TMOs are often dominated by the state
variables of the d-electrons, a natural starting point for the
study of heterostructures, combining different TMO’s is the
investigation of their configurations at the interfaces is.
There are intense efforts addressing this general issue in-
cluding changes of the d-electron configuration by electro-
static boundary conditions that lead to conducting layers at
the insulator interface of SrTiO3 and LaAlO3. In a review
paper Hwang et al. [105] addressed these issues in more
general terms and discussed the science of oxide interfaces
from the viewpoint of emergent phenomena due to strong
electron correlations. Combining advances in experimental
techniques and new concepts supported by first-principles
electronic structure calculations, the oxide HS’s and SL’s
will be an ideal arena for physics, chemistry and technology
in the years to come.
Whereas in the previous section the focus was more on
technological achievements and interlayer interactions, in
this section the more fundamental physics driven research
activities such as the interplay of charge transfer across and
orbital reconstruction at the interface are briefly reviewed.
As shown in Sec. 4.2. the technological requirements to
prepare YBCO/LCMO heterointerfaces in a reproducible
way are accomplished to a high degree of perfection. Tai-
loring the film orientation can be used to exploit the aniso-
tropy of the correlation lengths of the compounds involved
— an area exploited so far from the technological point of
view only. In order to be able to focus on the physics of
interfaces and their intentional manipulation in addition to
precision in the sample preparation, the availability of the
appropriate analytical tools for their characterization is
mandatory. Structural analysis by conventional area aver-
aging x-ray diffraction techniques and local high resolution
transmission electron microscopy have to be supplemented
by more sophisticated local element specific diagnostic
tools to shed some light on the microscopic mechanisms
taking place at the interfaces.
Analytical high resolution TEM with element specific
resolution down to the atomic level will help to structurally
characterize films and interfaces. Complementary Electron
Energy Loss Spectroscopy (EELS) in a TEM is a powerful
tool to reveal structural and chemical element specific in-
formation of solids with atomic resolution. The obvious
disadvantage of all TEM techniques is the rather sophisti-
cated destructive preparation of the films to be investigated
into electron transparent samples with caveats to generate
artifacts due to the thinning process. In contrast to TEM,
the x-ray techniques are nondestructive, eventually element
specific probes with the ability to study the behavior of
electrons at the interfaces — typically buried several na-
nometers below the surface. In the book by Willmott these
mainly synchrotron based techniques are described [122].
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10 1097
H.-U. Habermeier
Resonant x-ray absorption spectroscopy exploits the prop-
erties of x-ray beams from a synchrotron. Their absorption
length is typically more in the μm range rather than in the
nm range thus penetrating deeply into the samples, ena-
bling the analysis of the properties of buried layers. The
photon energy directed to the sample can be tuned and
their polarization parallel and perpendicular to the interface
can be used to extract information about the shape of the
valence-electron clouds (orbitals) around these atoms. The
total electron yield and the fluorescence yield are the quan-
tities to be measured. Resonant elastic x-ray scattering
(REXS) provides a sensitive probe for spatial modulations
of spins, charges, and orbitals. This unique sensitivity is
achieved by merging diffraction and x-ray absorption spec-
troscopy (XAS) into a single experiment, where the scat-
tering provides information about spatial modulations and
the XAS provides sensitivity to the electronic structure.
More precisely, resonant x-ray scattering close to an ab-
sorption edge involves virtual transitions from core levels
into unoccupied states close to the Fermi level and these
virtual transitions depend strongly on the spin, charge and
orbital configuration of the resonant scattering centers.
Furthermore, the resonance process strongly enhances the
scattering cross-section and allows the opportunity to study
selected atomic species in a crystal. Resonant inelastic x-
ray scattering (RIXS) experiments, in which one scatters x-
ray photons inelastically, probes the changes in energy,
momentum, and polarization of the scattered photon from
intrinsic excitations of the material under study. RIXS is
able to probe many elementary excitations such as
plasmons, charge-transfer excitations, crystal-field and
orbital excitations, magnons, and phonons. Besides, similar
to REXS, the resonance in RIXS greatly enhances the
cross-section and adds selectivity to atomic species. In this
aspect, RIXS offers a wide landscape in the study of
strongly correlated electron systems. Combining conven-
tional techniques such as macroscopic measurements of the
temperature dependence of resistivity and magnetic re-
sponse of the samples with more advanced resonant x-ray
techniques is a mandatory prerequisite to get a deeper in-
sight into the processes taking place at the interfaces.
4.3.2. YBCO-LCMO interfaces
The observations described in Sec. 4.2. gave a strong
hint that the macroscopic properties of the YBCO–LCMO
heterostructures and superlattices are determined to a great
deal by the interfaces at an atomic scale. To shed some
light on the electronic mechanisms responsible for these
observations samples with chemically pure, atomically
sharp interfaces are required for both to accurately study
the electronic interactions to the interface. The question to
be addressed is how magnetism is affecting the electronic
properties of the interfaces. Chakhalian et al. [123] and
subsequently other research groups [124,125] applied the
experimental diagnostic machinery described above to
reveal the details of the electronic structure at the
YBCO/LCMO interfaces. Experiments using spin-
polarized neutron reflectometry [126] showed a strong
magnetic contribution coming from the interface. Original-
ly, Chekhovian et al. used symmetric YBCO–LCMO
(10×10 nm) superlattices for the investigations and found a
symmetry forbidden neutron-diffraction peak popping up
at temperatures corresponding to the Curie temperature of
the film. Resonant x-ray magnetic dichroism experiments
revealed that there is an antiferromagnetic coupling of the
Cu spins and Mn spins at the interface that is not compati-
ble with the Goodenough–Kanamori–Anderson rules for
the exchange interactions in oxides. Figure 26 shows the
results of the XMCD measurements. Whereas in YBCO
the conducting electrons are located in the 2 2x yd − orbitals,
the ground state of LCMO is regarded to be an orbitally
disordered state with fluctuating 2 2x yd − and 2 23z rd − or-
bitals it is expected that the superexchange interaction for
the Mn–O–Cu molecule would result in a ferromagnetic
coupling in contrast to the experimental result. Further
evidence for this scenario of an oribital reconstruction
comes from the comparison of resonant linear dichroism
experiments in the total electron yield and fluorescence
modes in comparison with bulk YBCO data. The results
represented in Fig. 27 clearly show that the electronic
structure of the CuO2 layer is modified by covalent bonds
across the interface. These results suggest that the orbital
rearrangement and strong hybridization are at least partial-
ly responsible for the unusual magnetic behavior previous-
Fig. 26. (Color online) XMCD signals obtained from the core-
level absorption spectra for Cu and Mn showing the antiferro-
magnetic coupling of the Mn and Cu spins [123].
1098 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices
ly observed at cuprate-manganate interfaces and contribute
to the suppression of superconductivity close to the inter-
face. It is obvious that this mechanism of orbital recon-
struction does not exclusively apply to the YBCO/LCMO
interface but can be conjectured to apply for other transi-
tion metal oxides with nearly degenerate d-orbitals such as
titanates, vanadates, ruthenates and ferrites.
5. Summary and future perspectives
Research in high temperature superconductivity has gen-
erated a tremendous amount of results during the past three
decades. From the materials science point of view bulk pro-
cessing and thin film deposition technology are the main
fields of interest, both with an impact enabling applications.
However, even after three decades there are plenty of fun-
damental questions to be solved before the focus can be
shifted towards applications. The knowledge associated with
the flux-line behavior in copper oxide superconductors and
especially flux-line pinning is not yet at a stage where the
mechanisms for pinning and thus the current-carrying capac-
ity of superconducting wires are fully understood.
Thin film technology experienced a tremendous boost
during this period. Pulsed Laser Deposition developed to
be the working horse for complex oxide thin film and
heterostructure fabrication. The availability of UV-excimer
lasers with steadily increasing beam homogeneity and con-
stant pulse-to-pulse energy was one technical prerequisite
for this development. Furthermore, the exploration of the
details of the PLD process has been extremely helpful to
improve the film quality in the past decades (e.g. for
YBCO). One of the benefits of PLD is the relative ease to
fabricate not only single layer films of complex oxides but
also heterostructures and superlattices with perovskite-type
complex oxide materials having different functionalities.
The precision of PLD thin film growth could be enhanced
using RHEED control and thickness accuracy down to one
unit cell can be achieved. Oxide- MBE has been demon-
strated to be the method of choice as an excellent research
tool to grow single layer films with superior perfection and
to achieve sub-unit cell layer-by-layer deposition thus
overcoming the limitations of PLD. The future in thin film
technology is seen in a further development of PLD to un-
derstand the plume formation better and find ways to ma-
nipulate and analyze the plume formation process.
Albeit the diagnostic tools to analyze the properties of
complex oxide thin films have improved in the past decade
to an unprecedented degree, especially in the field of ad-
vanced Transmission Electron Microscopy and Synchro-
tron-based resonant x-ray techniques, the ultimate goal of
element specific analysis with atomic resolution in con-
junction with sensitivity to the electronic structure is not
yet achieved.
The development of the experimental “machinery”
ranging from dedicated sample preparation and analysis
has opened — at least in the thin film case — the wing of a
door into a new territory, the oxide interface research. Ox-
ide interfaces are on the one hand an opportunity for novel
types of electronics [106] and on the other hand the tech-
nological advances in the atomic scale synthesis of films
enable the access to so far barely explored novel states at
the interfaces. A rich playground for materials based curi-
osity inspired physics is emerging with bright perspectives
for science and potential applications.
Acknowledgements:
The author is highly indebted to the current and previ-
ous members of the Technology Group of the Max-Planck-
Institute for Solid State Research, Stuttgart, for their enthu-
siasm in the thin film work for complex oxides. He benefit-
ted much from in-depth discussions with M. Cardona,
B. Keimer and O.K. Andersen and the members of their
departments. The financial support of the Max-Planck-
Society, the German Science Foundation (DFG) as well as
the European Union within the frame of several EU-NMP
projects is greatly appreciated.
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1102 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 10
1. Introduction
1.1. The road to copper-oxide based high temperature superconductors
1.2. Basic properties of YBa2Cu3O7–δ
1.2.1. Crystal structure
1.2.2. Electronic phase diagram
1.2.3. Superconducting properties
1.2.4. Current challenges in copper oxide science and technology
2. Physics and technology aspects of high temperature superconductor thin film deposition
2.1. General aspects
2.2. Substrate requirements and opportunities
2.3. Physical vapor deposition techniques for cuprates and other complex oxides
2.3.1. General remarks
2.3.2. Reactive evaporation
2.3.3. Oxide molecular beam epitaxy
2.3.4. Sputtering
2.3.5. Pulsed laser deposition
2.3.6. Comparison of the different deposition techniques
3. Physical properties of single layer high temperature superconductor thin films – some current trends
3.1. General remarks
3.2. Tailoring cuprate thin film orientation
3.3. Controlled generation of antiphase boundaries
3.4. Fluctuation conductivity.
4. Complex oxide heterostructures and superlattices based on superconducting cuprates
4.1. General remarks
4.2. Ferromagnet-superconducting bilayers and superlattices
4.2.1. Early experiments
4.2.2. Determination of the spin diffusion length
4.2.3. (110)-oriented YBCO/LCMO heterostructures and trilayers
4.3. Ferromagnet–superconductor interfaces
4.3.1. General remarks
4.3.2. YBCO-LCMO interfaces
5. Summary and future perspectives
Acknowledgements:
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