Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy
Andreev reflection spectra have been measured in a new superconductor EuAsFeO₀.₈₅F₀.₁₅ having an unexpectedly low superconducting transition temperature Tc≈11.3 K among related FeAs compounds on a base Sm and Gd surrounding Eu in the series of lanthanides. The nearly fivefold lower Tc, as against th...
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| Zitieren: | Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy / V.M. Dmitriev, E.P. Khlybov, D.S. Kondrashov, A.V. Terekhov, L.F. Rybaltchenko, E.V. Khristenko, L.A. Ishchenko, I.E. Kostyleva, A.J. Zaleski // Физика низких температур. — 2010. — Т. 37, № 4. — С. 360–368. — Бібліогр.: 37 назв. — англ. |
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irk-123456789-1185342017-05-31T03:08:35Z Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy Dmitriev, V.M. Khlybov, E.P. Kondrashov, D.S. Terekhov, A.V. Rybaltchenko, L.F. Khristenko, E.V. Ishchenko, L.A. Kostyleva, I.E. Zaleski, A.J. Сверхпроводимость, в том числе высокотемпературная Andreev reflection spectra have been measured in a new superconductor EuAsFeO₀.₈₅F₀.₁₅ having an unexpectedly low superconducting transition temperature Tc≈11.3 K among related FeAs compounds on a base Sm and Gd surrounding Eu in the series of lanthanides. The nearly fivefold lower Tc, as against the expected value, is attributed to the divalent properties of Eu ions when in the compound investigated along with the weakly magnetic Eu³⁺ ions may be present and the strongly magnetic Eu²⁺ ones that is a strong destructive factor for superconductivity. Most of the spectra measured showed features that corresponds to two energy gaps whose values varied from contact to contact within 2Δ s/kTc = 2.2–4.7 and 2Δ1/kTc = 5.1–11.7 for small and large gap, respectively. The corresponding variations for single-gap spectra are 2Δ/kTc = 2.6–6.4. The relatively large size of crystallites (no less than ~25 µm) and the large number of contacts measured (several tens) suggest with a high degree of probability that the spectra obtained account quite fully for the gap distribution practically in all crystallographic directions. The data obtained and the absence of zero gaps in the measured spectra evidence in favor of the anisotropic s- or s±-symmetry of the order parameter in EuAsFeO₀.₈₅F₀.₁₅ that was revealed in other similar compounds with higher Tc. Thus, the character of the gap function Δ(k) in this compound is inconsistent with the d-wave superconductivity observed in some low-Tc pnictides. 2011 Article Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy / V.M. Dmitriev, E.P. Khlybov, D.S. Kondrashov, A.V. Terekhov, L.F. Rybaltchenko, E.V. Khristenko, L.A. Ishchenko, I.E. Kostyleva, A.J. Zaleski // Физика низких температур. — 2010. — Т. 37, № 4. — С. 360–368. — Бібліогр.: 37 назв. — англ. 0132-6414 PACS: 74.70.Dd, 74.70.–b http://dspace.nbuv.gov.ua/handle/123456789/118534 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Сверхпроводимость, в том числе высокотемпературная Сверхпроводимость, в том числе высокотемпературная |
| spellingShingle |
Сверхпроводимость, в том числе высокотемпературная Сверхпроводимость, в том числе высокотемпературная Dmitriev, V.M. Khlybov, E.P. Kondrashov, D.S. Terekhov, A.V. Rybaltchenko, L.F. Khristenko, E.V. Ishchenko, L.A. Kostyleva, I.E. Zaleski, A.J. Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy Физика низких температур |
| description |
Andreev reflection spectra have been measured in a new superconductor EuAsFeO₀.₈₅F₀.₁₅ having an unexpectedly low superconducting transition temperature Tc≈11.3 K among related FeAs compounds on a base Sm and Gd surrounding Eu in the series of lanthanides. The nearly fivefold lower Tc, as against the expected value, is attributed to the divalent properties of Eu ions when in the compound investigated along with the weakly magnetic Eu³⁺ ions may be present and the strongly magnetic Eu²⁺ ones that is a strong destructive factor for superconductivity. Most of the spectra measured showed features that corresponds to two energy gaps whose values varied from contact to contact within 2Δ s/kTc = 2.2–4.7 and 2Δ1/kTc = 5.1–11.7 for small and large gap, respectively. The corresponding variations for single-gap spectra are 2Δ/kTc = 2.6–6.4. The relatively large size of crystallites (no less than ~25 µm) and the large number of contacts measured (several tens) suggest with a high degree of probability that the spectra obtained account quite fully for the gap distribution practically in all crystallographic directions. The data obtained and the absence of zero gaps in the measured spectra evidence in favor of the anisotropic s- or s±-symmetry of the order parameter in EuAsFeO₀.₈₅F₀.₁₅ that was revealed in other similar compounds with higher Tc. Thus, the character of the gap function Δ(k) in this compound is inconsistent with the d-wave superconductivity observed in some low-Tc pnictides. |
| format |
Article |
| author |
Dmitriev, V.M. Khlybov, E.P. Kondrashov, D.S. Terekhov, A.V. Rybaltchenko, L.F. Khristenko, E.V. Ishchenko, L.A. Kostyleva, I.E. Zaleski, A.J. |
| author_facet |
Dmitriev, V.M. Khlybov, E.P. Kondrashov, D.S. Terekhov, A.V. Rybaltchenko, L.F. Khristenko, E.V. Ishchenko, L.A. Kostyleva, I.E. Zaleski, A.J. |
| author_sort |
Dmitriev, V.M. |
| title |
Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy |
| title_short |
Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy |
| title_full |
Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy |
| title_fullStr |
Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy |
| title_full_unstemmed |
Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy |
| title_sort |
andreev reflection spectroscopy of the new fe-based superconductor euasfeo₀.₈₅f₀.₁₅: evidence for the strong order parameter anisotropy |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2011 |
| topic_facet |
Сверхпроводимость, в том числе высокотемпературная |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118534 |
| citation_txt |
Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy / V.M. Dmitriev, E.P. Khlybov, D.S. Kondrashov, A.V. Terekhov, L.F. Rybaltchenko, E.V. Khristenko, L.A. Ishchenko, I.E. Kostyleva, A.J. Zaleski // Физика низких температур. — 2010. — Т. 37, № 4. — С. 360–368. — Бібліогр.: 37 назв. — англ. |
| series |
Физика низких температур |
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© V.M. Dmitriev, E.P. Khlybov, D.S. Kondrashov, A.V. Terekhov, L.F. Rybaltchenko, E.V. Khristenko, L.A. Ishchenko, I.E. Kostyleva, and
A.J. Zaleski, 2011
Fizika Nizkikh Temperatur, 2011, v. 37, No. 4, p. 360–368
Andreev reflection spectroscopy of the new Fe-based
superconductor EuAsFeO0.85F0.15: evidence
for the strong order parameter anisotropy
V.M. Dmitriev1,2, E.P. Khlybov1,3, D.S. Kondrashov2, A.V. Terekhov2, L.F. Rybaltchenko2,
E.V. Khristenko2, L.A. Ishchenko2, I.E. Kostyleva1,3, and A.J. Zaleski4
1International Laboratory for High Magnetic Fields and Low Temperatures
Gajowicka 95, 53–421 Wroclaw, Poland
2B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine
47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: rybal@ilt.kharkov.ua
3L.F. Vereshchagin Institute for High-Pressure Physics, RAS, Troitsk 142190, Russia
4W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences
P.O.Box 1410, 50–950, Wroclaw, Poland
Received June 11, 2010, revised August 27, 2010
Andreev reflection spectra have been measured in a new superconductor EuAsFeO0.85F0.15 having an unex-
pectedly low superconducting transition temperature Tc ≈ 11.3 K among related FeAs compounds on a base Sm
and Gd surrounding Eu in the series of lanthanides. The nearly fivefold lower Tc, as against the expected value,
is attributed to the divalent properties of Eu ions when in the compound investigated along with the weakly
magnetic Eu3+ ions may be present and the strongly magnetic Eu2+ ones that is a strong destructive factor for su-
perconductivity. Most of the spectra measured showed features that corresponds to two energy gaps whose val-
ues varied from contact to contact within 2Δs/kTc = 2.2–4.7 and 2Δ1/kTc = 5.1–11.7 for small and large gap, re-
spectively. The corresponding variations for single-gap spectra are 2Δ/kTc = 2.6–6.4. The relatively large size of
crystallites (no less than ~25 µm) and the large number of contacts measured (several tens) suggest with a high
degree of probability that the spectra obtained account quite fully for the gap distribution practically in all crys-
tallographic directions. The data obtained and the absence of zero gaps in the measured spectra evidence in favor
of the anisotropic s- or s±-symmetry of the order parameter in EuAsFeO0.85F0.15 that was revealed in other simi-
lar compounds with higher Tc. Thus, the character of the gap function Δ(k) in this compound is inconsistent with
the d-wave superconductivity observed in some low-Tc pnictides.
PACS: 74.70.Dd Ternary, quaternary, and multinary compounds (including Chevrel phases, borocarbides, etc.);
74.70.–b Superconducting materials other than cuprates.
Keywords: superconductivity, Andreev reflection, energy gap, point contact.
Introduction
The discovery of a basically novel high–Tc LaO1–xFeAsFx
superconductor with the onset of the superconducting tran-
sition at Tc ≈ 26 K [1] has stimulated a search for other
similar compounds (briefly denoted as 1111–type sys-
tems). In some cases, substitution of La with other Ln-se-
ries elements (Ln — lanthanide) raised Tc significantly, for
example, to Tc ≈ 55 K for Fe-based 1111 compound with
Sm [2] and to Tc ≈ 54 K for compound with Gd [3]. As the
temperature lowers, the parent LnOFeAs compounds con-
sisting of alternating LnO and FeAs layers undergo struc-
tural and successive/simultaneous antiferromagnetic
(AFM) transitions in the interval 160–180 K. The transi-
tions can be suppressed when O is partially substituted
by F. On such substitution excessive electrons appear in
the LnO layer, which then pass over to the FeAs layer and
activate the superconducting state there.
Andreev reflection spectroscopy of the new Fe-based superconductor
Fizika Nizkikh Temperatur, 2011, v. 37, No. 4 361
Later on, superconductivity was also observed in other
FeAs similar systems which contained no oxygen. Three–
component AFe2As2 (briefly 122) [4] compounds are such
systems in which the FeAs layers have practically identical
crystalline structure. In these systems superconductivity
appears when divalent element A (Ba, Ca, Sr) is partially
substituted with a univalent one (usually K) that induces a
hole doping of the FeAs layers. The highest Tc ≈ 38 K is
achieved at A = Ba1–xKx. The physical properties of both
types of superconductors are quite similar, but the prepara-
tion technology of 122 compounds is much simpler. Be-
sides, superconductivity was detected in some materials
that do not need doping (e.g., LiFeAs with Tc ≈ 18 K [5])
and in the non-stoichiometric monolayers of Fe chalcoge-
nides FeX1–x (X = Se, Te) with Tc ≈ 8 K [6].
The discovery of high-Tc superconductivity in Fe-con-
taining compounds has initiated intensive investigations
aimed at clarifying the mechanism of the Cooper pairing
and the symmetry of the superconducting order parameter.
The nature of electron attraction in these compounds is not
yet clear completely and the preference is given mainly to
the magnetic mechanism. Most of the experimental results
obtained on relatively high-Tc Fe pnictides show consider-
able variations of the gap near the Fermi level, though it
never turns zero. This suggests the existence of the aniso-
tropic s-wave type gap function in these compounds. Nev-
ertheless, nodes or lines of nodes of the gap observed in
some compounds, when partially substituting Co for Fe or
P for As, and in a number of low-Tc pnictides that evi-
dences the d-wave symmetry of Cooper pairing.
Andreev reflection spectroscopy of the N-S point con-
tacts is one of the simplest and sufficiently reliable me-
thods of estimating the value and symmetry of the order
parameter (gap) in various kinds of superconductors. It has
the advantage of finding the gap structure in different crys-
tallographic directions avoiding intricate fitting procedures.
By such the technique, single BCS-like gap 2Δ0/kTc ≈ 3.7
in SmO0.9FeAsF0.1 was obtained for the first time by Chen
et al. [7]. However, most of the subsequent investigations
on 1111 systems revealed two gaps, each varying widely
for the same compound [8–11]. For example, in
NdO0.9FeAsF0.1 the small and the large gaps varied within
2Δs/kTc = 1.8–2.7 and 2Δl/kTc = 4.1–5.9, respectively.
Such a scatter of gaps found by different authors may
be due to the anisotropy of the gap function in the k-space
in different sheets of the Fermi surface (FS). When the
number of probes is small (which is for some reasons typi-
cal of PC spectroscopy), only a limited number, if not sin-
gle, of crystallographic directions are scanned. Therefore,
the gap values measured in different investigations do not
coincide. One more factor — the quality of the sample
especially its surface — is no less important. With an im-
proper control over the onset of the superconducting transi-
tion in each contact, its central part may contain a region
with a disturbed stoichiometry or significant surface con-
tamination. In this case the resulting PC spectra will not
display the characteristics of the bulk sample.
In this study the Andreev reflection spectra have been
investigated in point contacts based on the polycrystalline
EuAsFeO0.85F0.15 compound having an unexpectedly low
Tc ≈ 11.3 К, as against other 1111-type systems. The large-
size crystallites (no less than ~25 μm) and a great number
of measured contacts (several tens) give reasonable confi-
dence that the spectra obtained account quite fully for the
gap distribution practically along all crystallographic direc-
tions. Both one-gap and two-gap spectra (in most cases)
were observed in our Au-EuAsFeO0.85F0.15 contacts. In
the one-gap spectra the relative gap varied within 2Δ/kTc =
= 2.6–6.4. In the two-gap spectra the relative small Δs and
the relative large Δl gaps varied within 2Δs/kTc = 2.2–4.7
and 2Δl/kTc = 5.1–11.7, respectively. For any of these con-
tacts the ratio Δl/Δs was within 2–4. The results obtained
and the absence of zero gaps in the spectra measured evi-
dence in favor of the anisotropic s-wave (or s±-wave)
symmetry of the order parameter in EuAsFeO0.85F0.15,
which was previously revealed in other similar com-
pounds.
Among the abundance of information about Fe-based
oxypnictides, we have failed to find at least one report of
synthesizing a Eu–containing 1111 compound. Because of
the Eu position in the periodic table of the elements be-
tween in Sm and Gd, which are constituents of the 1111
systems with Tc > 50 K [2,3], such attempts might be made
but possibly with no success. At the same time, there are
many publications about using Eu for fabricating the 122
systems with relatively high Tc > 30 K. This may be be-
cause Eu, like most lanthanides, is a polyvalent metal hav-
ing 2+ or 3+ valence in different chemical compounds.
However, unlike other lanthanides, the lower valence of Eu
is preferable for forming metallic bonds, such as in 122
systems, where under certain conditions the divalent metal-
lic layers dope holes to the FeAs layers generating super-
conductivity in them. In 1111-type compounds doping
electrons come to the FeAs layers from the adjacent lan-
thanide oxide ones, where Ln should be in trivalent state.
Such a Ln-state is typical of the compounds with strongly
electronegative metalloids, for example, F or As. This sort
of compounds is normally present in the mixture of the
starting components for synthesis of 1111 systems.
Experiment
Such the ingredients as EuF3, EuAs, Fe2O3 and Fe were
used for preparing the EuAsFeO0.85F0.15 compound. The
chemical solid-phase reaction proceeded in an Ar-filled
quartz ampoule at T = 1150 °C for 24 h. For homogeniza-
tion, the samples were ground and kept at this temperature
for 30 h. As was expected, with this technological process-
ing Eu should retain its trivalent state. The typical curve
describing the resistive transition to the superconducting
V.M. Dmitriev et al.
362 Fizika Nizkikh Temperatur, 2011, v. 37, No. 4
state in one of the samples is illustrated in Fig. 1. (A simi-
lar transition was also registered in the temperature depen-
dence of magnetic susceptibility.) Of surprise is the unex-
pectedly low Tc ≈ 11.3 K (the onset of the transition) in
comparison with other Fe-based 1111 compounds, includ-
ing the neighboring rare-earth elements Sm and Gd, with
Tc > 50 [2,3]. (Eu is located between Sm and Gd in the
lanthanide series).
Previously [12] we tried to correlate the low Tc of
EuAsFeO0.85F0.15 with the atomic radius of Eu, which is
rather large in comparison with other lanthanides. This as-
sumption seems rather doubtful because literature data on
the atomic radius of Eu are rather controversial. On the
other hand, the literature data on the ionic radius account-
ing most accurately for ionic bonds are practically identical
for the trivalent state of these elements. It is therefore rea-
sonable to assume that decrease in Tc is due to the magnet-
ic, rather than structural factor. The point is that Eu2+ ions
have the largest spin magnetic moment S ~ 7µB among the
lanthanide elements. This feature is determined by the fact
that a half of the f-electrons (14 altogether) have identical
spin orientations precisely corresponding to the Hunde
rule. In the trivalent state one electron leaves the f-shell,
which decreases the spin moment and induces an orbital
moment which partially compensates for the spin one. We
are unaware of the total magnetic moment in Eu3+ ions
but, according to the indirect evidence, it can be high
enough. Besides, because of the mixed-valence effect typi-
cal of many rare-earth compounds, EuAsFeO0.85F0.15 can
also contain Eu2+ ions, which enhances the destructive in-
fluence of magnetism on singlet superconductivity. It is
quite possible that the presence of Eu2+ ions provides a
certain level of hole doping which counterbalances the
main mechanism — electron doping and thus decreases the
effective number of carriers in the FeAs layer, hence Tc as
well.
The Andreev reflection spectra, dI/dV(V) characteristics,
were measured on point contacts (PC) having metallic
conductivity (without an additional insulating interlayer)
between a mechanically-sharpened chemically-polished
Au needle (N-electrode) and freshly-fractured surface of
EuAsFeO0.85F0.15 (S-electrode). The S-electrode consisted
of small (2–3 mm across) pieces broken off from a sintered
bulk. The fracture was a conglomeration of brilliant crys-
tallites about 100 µm across. Some of them were split into
smaller (~25 µm) blocks with small-angle misorientation.
Besides, the fracture had dull areas possibly of amorphous
slag that took about one-third of the fracture surface. As a
result, the share of the superconducting phase could hardly
exceed two thirds of the sample volume. Taking into con-
sideration the 100% diamagnetic screening, we can state
that the dull areas do not degrade the electric contact be-
tween individual crystallites. The comparatively small
width of the superconducting transition rules out signifi-
cant variation of the superconducting parameters over the
sample volume.
The electrodes were brought together in liquid He. A spe-
cial device was used to move the electrodes relative to each
other in two perpendicular directions. We were thus able to
change the point of contact on the S-electrode without
heating the sample. The PC spectra were registered using
the standard modulation technique of lock-in detection at
the frequency 437 Hz. With this technique we could make
contacts in a wide interval of resistance. To preclude ther-
mal effects and to ensure good mechanical stability, the
preference was given to point contacts with moderate resis-
tance scatter (2–10 Ω). Most of such contacts exhibited a
spectroscopic regime, which is proved by high-level excess
(Andreev) current close in some cases to the theoretical
value, which did not change up to the voltage no less than,
at least, several Δ/e. Some spectra had additional features
at eV >> Δ, which were most likely due to the reduced crit-
ical current in the inter-crystallite layers, typically ob-
served in materials prepared by solid-state synthesis, and
could not influence the basic (informative) portion of the
spectra.
The size of the contact can be found by using the Shar-
vin formula which corresponds to the ballistic regime.
However, this is impossible in our case because the Fermi
parameters are not known for the compound investigated.
We estimated no more than the upper limit of contact sizes
using the Maxwell formula d = ρ0/RN most suitable for
diffusive regime. It is obvious that the residual resistivity
ρ0 of the bulk sample (Fig. 1) is excessively large. Most
likely this is due to the influence of the aforementioned
intercrystalline layers having poor electric conductivity.
Therefore, this ρ0 does not account for the electric proper-
ties of the crystallites themselves.
The calculation of d using ρ0 of Fig. 1 would yield
anomalously large micron-scale sizes of the contact, which
is not compatible with the spectroscopic character of the
registered spectra. The real sizes of the contact can be ob-
tained from ρ0 measured on single crystals whose proper-
8 9 10 11 12
0
0.2
0.4
0.6
H = 0
EuO FeAsF0.85 0.15
R
es
is
ti
v
it
y,
m
·c
m
�
T, K
Fig. 1. Resistive transition to the superconducting state in the
EuAsFeO0.85F0.15 compound investigated in this study.
Andreev reflection spectroscopy of the new Fe-based superconductor
Fizika Nizkikh Temperatur, 2011, v. 37, No. 4 363
ties may be similar to those of the crystallites of the sin-
tered samples. Unfortunately, this kind of information is
unavailable for EuAsFeO0.85F0.15. As for the electric cha-
racteristics of single crystals of similar pnictides, their re-
sidual resistivity is known to be over one and a half order
of magnitude lower than that presented in Fig. 1. For ex-
ample, for single crystalline LaFePO ρ0 is about 5 μΩcm
[13]. In this case the calculation of the contact sizes would
give quite reasonable values within approximately 5–25 nm.
This is a rough estimation of the upper limit of our contact
sizes because the Maxwell formula yields essentially larger
values than the Sharvin one.
The order parameter Δ was estimated on the basis of the
Blonder–Tinkham–Klapwijk (BTK) theory [14] which pro-
vides an adequate description of the electric characteristics
of N-S contacts produced on a base of conventional super-
conductors. The experimental PC spectra were fitted to the
extended BTK formulae [15] including an additional pa-
rameter Г characterizing the Cooper pair lifetime [16],
which defines the smearing of the spectra in the region of
gap energies. In reality this parameter also accounts for the
effects of the crystal structure imperfection in the contact
area which can cause an inhomogeneous distribution of the
order parameter at submicron-scale dimensions. There is
also another parameter Z characterizing a possible potential
barrier at the N-S interface that can be generated by the
dielectric interlayer or by the discrepancy between the
Fermi parameters on both sides of contact. Such a method
modified for two-band superconductor is widely used for
analyzing the iron-pnictide PC spectra with an acceptable
accuracy.
Recently a new generalized theory of the Andreev and
tunneling conductance of the normal metal−multiband su-
perconductor contacts has been published [17,18]. PC
spectra computed there intuitively seem to be quite realis-
tic. But the ultimate conclusion can be made only after
comparison between experimental and theoretical curves
on the basis of the reliable computer program.
Results and discussion
All of the measured electric characteristics (spectra)
dI/dV(V) of Au-EuAsFeO0.85F0.15 point contacts had spec-
tral features that pointed to a high Andreev reflection in-
tensity close in many cases to the theoretically predicted
value. Some spectra had the standard form typical of tradi-
tional one-band superconductors with a single gap. How-
ever, in most cases the registered spectra could be describ-
ed only in the two-gap approximation. Figure 2 illustrates
two spectra of the first type whose BTK fitted [15] gap
parameters differ considerably: Δmin ≈ 1.3 meV (Fig. 2,a) and
Δmax ≈ 3.1 meV (Fig. 2,b). The corresponding characte-
ristic ratios 2Δ/kTc are 2.6 and 6.4, respectively. (The es-
timates were obtained for Tc ≈ 11.3 К, corresponding to
the onset of the superconducting transition.) These data
point to high anisotropy of the order parameter in
0.85 0.15EuAsFeO F .
Of interest is the low intensity of the double maxima
in the gap energy region (Fig. 2,a) or even their absence
(Fig. 2,b). It is known that such maxima are always observed
in the spectra of N-S contacts based on traditional s-wave
superconductors when the Fermi velocities are different in
the two electrodes and/or there is a thin dielectric interlayer
in the contact area (Z > 0), as is stated in the BTK theory.
Assuming that the Fermi velocity of the electrons is low in
Fe oxypnictides, we could expect the intensive double
maxima or even a tunnel regime (Z >> 0) in our contacts.
This has not occurred. Note that low intensity of this struc-
ture was also observed in other Fe pnictides [19–21].
This discrepancy between theory and experiment was
also observed in contacts based on the superconducting
copper-oxide and heavy-fermion compounds. The pheno-
menon was analyzed by Deutscher and Nozieres [22] who
assume that the electron mass renormalization responsible
for the effective Fermi velocity is much weaker in the PC
region than in the bulk material. A detailed analysis of the
processes of quasiparticle transition and relaxation in the
contact area using Green function technique supported the
assumption at the microscopic level. The original BTK
theory contains some simplifying assumptions which disre-
gard the real distribution of the pairing potential at the N-S
interface and the electron structure of the superconductor.
In this context it is hardly possible to calculate correctly
the effective Fermi velocity in multiband superconductors
using the parameter Z from the BTK analysis of PC spectra.
This is evident in the calculation of the Fermi velocity
vFS of EuAsFeO0.85F0.15 based on the formula following
from the BTK theory [23]
–10 –5 0 5 10 –10 –5 0 5 10
a
T = 1.9 K
H = 0
Au-EuO FeAsF0.85 0.15 Au-EuO FeAsF0.85 0.15
0.1 S
d
I
d
V
/
,
re
la
ti
v
e
u
n
it
s
d
I
d
V
/
,
re
la
ti
v
e
u
n
it
s
Voltage, mV
experiment experiment
theory theory
b
T = 1.8 K
H = 0
0.09 S
1/ = 0.14 SR
N
1/ = 0.25 SR
N
Voltage, mV
Fig. 2. Two typical one-gap dI/dV(V) spectra of
0.85 0.15Au-EuAsFeO F point contacts (solid lines — experiment,
dash lines — BTK fitting) differing in gap size and degree of
smearing of the spectral lines ( fitting parameter Г). 1/RN ≈ 0.25 S,
Δ ≈ 1.3 meV, Г ≈ 0.01 meV, Z ≈ 0.1 (a); 1/RN ≈ 0.14 S, Δ ≈
≈ 3.1 meV, Г ≈ 1.8 meV, Z ≈ 0.15 (b).
V.M. Dmitriev et al.
364 Fizika Nizkikh Temperatur, 2011, v. 37, No. 4
2 2 2
0 (1– ) /4Z Z r r⎡ ⎤= +⎣ ⎦
where r = (vFS/vFN). The estimation was made using the
barrier parameter Z = 0.1–0.15 for the contacts in Fig. 2.
Since the Andreev current is high, the possible dielectric
layer at the N-S boundary can be neglected and hence Z0
can be assigned zero. Taking vFN = 1.4·108 cm/s for Au,
we obtain only a ∼20–30% decrease in vFS, which is un-
likely for iron pnictides. According to the data published
for some compounds of this family, vFS varies within
∼(0.3–2.4)·107 cm/s in different sheets of the Fermi surface
[24,25]. The significant (2- to 9-fold, according to different
sources) increase in the free electron mass, calculated from
experimental data on the photoemission spectroscopy, de
Haas-van Alphen effect and heat capacity [26,27] for dif-
ferent crystallographic directions, is another point in favor
of low vFS.
The low electron mass renormalization in the contact
region (as follows from the BTK analysis of PC spectra for
such classes of nonconventional superconductors as copper
oxides, iron pnictides and heavy-fermion systems) may
also be dependent on the type of Cooper pairing. Let us
assume that Cooper pairs are formed by some other (e.g.,
magnetic) mechanism different from the phonon one, as is
postulated by the classical Bardeen-Cooper-Schrieffer
(BCS) theory. In this case magnons along with phonons
would participate in the scattering processes involving the
superconducting excitations (bogolons). As a result, the
relative part of the electron-phonon scattering events could
be reduced. We believe that the contribution of the elec-
tron-magnon interaction to the electron mass renormaliza-
tion in iron pnictides cannot be large. Thus, the small
height of the potential barrier in the contacts based on mul-
tiband superconductors can be attributed both to the specif-
ic transition of various types of charge carriers trough the
N-S boundary [22] and to a non-phonon mechanism of
Cooper pairing.
The structure of most of the measured spectra had addi-
tional (as compare to conventional superconductors) fea-
tures in the region of gap voltages. Therefore, the BTK-fitt-
ing of these spectra in the one-gap approximation induces a
large error. It is reasonable to relate the additional features
to the second energy gap. Two typical spectra with nearly
equal contributions of each gap to the excess current are
shown in Fig. 3 (arrows mark the second gap-related fea-
tures, Fig. 3,b).
The possibility of revealing two gaps in a two-band su-
perconductor was demonstrated convincingly in 2001 by
Szabo et al. [28] for the first time through measuring the
Andreev reflection spectra in MgB2-based N-S contacts. In
the line with this study we separated the experimentally
observed Andreev reflection amplitude into two compo-
nents assuming that these parts take contributions from
different sheets of the Fermi surface. Each component was
then BTK-fitted.
First, the low-energy part of the spectrum was fitted,
which enabled us to use the obtained barrier parameter Z
for fitting the high-energy part of the spectrum, indepen-
dent estimation of Z being impossible for the spectra regis-
tered. The procedure used is quite reasonable because the
barrier height can hardly vary in a very narrow energy in-
terval (several meV). The smearing parameter Г was not
always identical for both the parts of the spectrum and this
is quite normal because the intensity of quasiparticle scat-
tering at impurities and structural defects can differ essen-
tially from band to band. And this is not surprising since in
the nonconventional superconductors inelastic scattering
can initiate the pair-breaking effects, which have been reli-
ably established for oxide high-Tc compounds. Recently,
the well justified assumptions about the existence of a sim-
ilar effect in the iron pnictides have appeared.
The BTK formulas were used to calculate the conduc-
tances σs(eV) and σl(eV) dependent on the small and large
gaps, respectively. At the final stage the calculated total
conductance (1 )s lσ = ασ + −α σ was fitted to the experi-
mental one to find the relative contribution of each gap,
that is, weight factors α and (1 )−α . For most contacts
these contributions were quite close α = 0.45–0.55. The es-
timated small Δs and large Δl gaps for different contacts varied
within 1.1–2.3 meV (2Δs/kTc = 2.2–4.7) and 2.5–5.9 meV
(2Δl/kTc = 5.1–11.7), respectively. (The ratio Δl/Δs for any
contact was within 2–4). It is obvious that the upper bo-
unds of 2Δ/kTc intervals are inconsistent with the phonon
model of Cooper pairing and an alternative mechanism
should be considered for the compound investigated.
It is unlikely that the variations of gap parameters are
caused by an inhomogeneous distribution of the critical pa-
rameters over the sample volume. The processed spectra show
that the superconducting transition started at practically the
same temperature in all the contacts (Tc ≈ 11.3 K), imply-
Fig. 3. Two examples of dI/dV(V) spectra that can be fitted ac-
curately in the two–gap BTK model [25]: 1/RN ≈ 0.13 S, Δs ≈
≈ 1.1 meV, Гs ≈ 0.4 meV, Δl ≈ 4.3 meV, Гl ≈ 1.1 meV, Z ≈ 0.1 (a);
1/RN ≈ 0.15 S, Δs ≈ 1.3 meV, Δl ≈ 5.9 meV, Г ≈ 0.5 meV, Z ≈ 0.1 (b).
–10 –5 0 5 10 –10 –5 0 5 10
a
T = 2.7 K
H = 0
Au-EuO FeAsF0.85 0.15 Au-EuO FeAsF0.85 0.15
0.02 S
d
I
d
V
/
,
re
la
ti
v
e
u
n
it
s
d
I
d
V
/
,
re
la
ti
v
e
u
n
it
s
Voltage, mV
experiment experiment
theory theory
b
T = 1.9 K
H = 0
0.03 S
1/ = 0.15 SR
N
1/ = 0.13 SR
N
Voltage, mV
Andreev reflection spectroscopy of the new Fe-based superconductor
Fizika Nizkikh Temperatur, 2011, v. 37, No. 4 365
ing the invariability of order parameter over a contact area.
This also indicates that only one, sufficiently perfect, crys-
tallite is present in the contact area. If the contact spot oc-
curred at the joint of two (or three) neighboring crystal-
lites, the related spectra usually had parasitic features of
non-spectroscopic character because of the weakened elec-
tric coupling between the crystallites. Such spectra are not
considered here. We thus believe that the information
about two gaps arrives from the same microscopic volume
of the sample.
The considerable variations of the gap parameters ob-
served in the study can be attributed to the anisotropy of
the gap function Δ(k) in EuAsFeO0.85F0.15. This idea was
supported in a number of experiments on different Fe-bas-
ed 1111- and 122-type compounds. For example, Δs =
= 1.5 meV and Δl = 9 meV were obtained in μSR mea-
surements on single crystalline (BaK)Fe2As2 (Tc = 32 K)
[29]. The measurement of the first critical field in a similar
compound gave Δs = 2.0 meV and Δl = 8.9 meV [30]. The
results obtained in the NMR [31] and angle-resolved pho-
toemission [32] experiments as well as in measurements of
thermal conductivity [25] also point to the order parameter
anisotropy in iron pnictides. Theoretically (e.g., [33–35])
these experimental facts are explained within the aniso-
tropic s±-wave model of Cooper pairing in the Fe-based
superconductors.
In PC measurements the relative contribution to the
spectrum from individual sheets of the Fermi surface,
where superconductivity is realized, can depend on the ori-
entation of the contact axis relative to the crystallographic
directions of the probed crystallite. Despite the polycrystal-
line structure of the samples, each crystallite (subcrystal-
lite), no less than ~25μ in size, is actually a small single
crystal. The crystallite size exceeding the expected contact
diameter allows the regime of directional spectroscopy of
the order parameter along some, even if uncertain, crystal-
lographic axis. Such spectroscopy is feasible, at least in a
rough approximation, due to the limited width of the
bunched beam of quasiparticles incoming to the N-S boun-
dary. The limitation is caused by the large difference be-
tween kF values in both electrodes as well as a contact
geometry, which actually represents the narrow elongated
channel. Unfortunately, we cannot estimate the angular
selectivity of the PC technique used in this study because
kF of the compound investigated and the exact geometry of
the constriction formed when the electrodes come into a
contact are unknown.
As follows from many photoemission experiments and
theoretical calculations of the band structure of iron pnic-
tides, there are two hole FS cylinders at the center of the
Brillouin zone (Г point) and two cylindrical electron FS
sheets at the corners of the zone (M points). It is found that
the angular distribution of the order parameter is anisotrop-
ic at least in the electron sheets.
We believe that the observation of one- or two-gap
spectra is dependent on the orientation of the contact axis
relative to the principal crystallographic axes. When the
contact axis is close to the ΓX-direction, the PC spectrum
can exhibit the one-gap features (Fig. 2,a). The reason is
that in this case only hole FS sheets near the Γ-point can
form the structure of the spectrum. (Note that there are no
other FS sheets near the X-points located between the M-
points in the Brillouin zone). Correspondently, a two-gap
spectrum is expected along the ΓM-direction (Fig. 3,b).
Such a spectrum can furnish information about the averaged
gap values on both the hole FS sheets around the Γ-point
and the electron FS sheets near the M-point. In a certain
approximation, the spectra in Figs. 2,a and 3,b can be per-
ceived as quite “pure”. In these spectra the parameter Γ
characterizes not only the spatial distribution of the order
parameter typical of anisotropic superconductors but the
processes of Cooper pair breaking as well, which deter-
mines the broadening of the gap itself. (See Fig. 2,a, Г ≈
≈ 0.01 meV and Fig. 3,b, Г ≈ 0.5 meV).
For the contacts measured in the intermediate directions
(between ΓX- and ΓM-lines) the peripheral areas of the
electron FS sheets situated near the M-point can modify
significantly the one-gap spectral lines present in all spec-
tra. (The latter are determined only by hole FS sheets lo-
cated around the Γ-point.) The spectra appeared to be
smeared appreciably and were characterized by a sharply
increased fitting parameter Γ. In this case the parameter
accounts essentially for the non-uniform angular distribu-
tion of the order parameter and to a much lesser degree for
its broadening caused by the breaking of the Cooper pairs.
Evidently, the spectra shown in Fig. 2,b (Г ≈ 1.8 meV) and
Fig. 3,a (Г ≈ 1.1 meV) belong to this type.
The considerable scatter of the gap parameters meas-
ured in different PC experiments on identical FeAs com-
pounds published so far (see survey [11]) may be con-
nected not only with the quality of the samples, but with
anisotropy of the gap function near the Fermi-level as well.
The effects of intra- and inter-band scattering of quasipar-
ticles at impurities can also influence the order parameter
value, because in nonconventional superconductors the
elastic scattering of quasiparticles has pair-breaking effect.
Note that none of the measured spectra had close-to-zero
gap parameters, which indicates the absence of zeros or
lines of zeros in the Δ(k)-dependence in EuAsFeO0.85F0.15.
Thus, the assumed [36,37] existence of zero gaps in some
low temperature pnictides lacks a support in this case.
For one-gap spectra the gap was estimated quite accu-
rately at different temperatures below Tc through fitting to
the modified BTK theory [15]. The typical set of one-gap
PC spectra measured at different temperatures and used to
estimate the temperature dependence of the gap is shown
in Fig. 4. The minima in the dI/dV(V)-dependences near
V = 4 mV can be due to the relaxation processes in the
contact area. The slowed-down charge equalization be-
V.M. Dmitriev et al.
366 Fizika Nizkikh Temperatur, 2011, v. 37, No. 4
tween both branches of the quasiparticle spectrum, that is
responsible for these minima, is typical for comparatively
low-resistance contacts with high-level Andreev current.
The contact in Fig. 4 belongs to this group.
The corresponding Δ(T)-dependence (Fig. 5, solid squares)
can be compared, within the experimental error, to the
BCS theory (dashed line) excluding the region close to Tc.
The tail-like deviation of experimental data from the
theory in this region is typical of many nonconventional
superconductors. One of the reasons for this deviation is
the increased width of the superconducting transition in
some structural elements, which causes a disagreement
between the fitted BCS
cT close to the midpoint of the tran-
sition and the temperature of its onset. Crystal structure
defects near grain boundaries occurring typically in most
multicomponent materials are the main cause for the above
smearing. The presence of a thin normal layer at the S-
electrode surface can be another influencing factor, though
it has no appreciable effect on the critical temperature
BCS
cT and Andreev current. This factor is more probable
because in the contact discussed, like in many others, the
value of the Andreev current is close to the theoretically
expected one.
The temperature dependences of the large Δl(T) and the
small Δs(T) gaps were found by the same procedure for one
of the contacts whose spectra are described adequately in
the two-gap approximation (Fig. 6). The temperature de-
pendence of the large gap is roughly similar to the BCS
theory, while the behavior of the small gap deviates consi-
derably from the theory. The deviation (Fig. 6, lower curve)
can be attributed to the low stability of the low-energy part
of the spectrum near V = 0, which shows up as appreciable
variations of the dI/dV-amplitude in this range. Such varia-
tions are typical of contacts based on magnetic supercon-
ductors and are caused by spontaneous or current-induced
shifts of the domain wall in the contact area. For this rea-
son it was impossible to fit this small gap-related part of
the spectrum to the BTK theory with a good accuracy.
Conclusions
The spectra of Andreev reflection have been measured
in point contacts based on the polycrystalline Fe-based
oxipnictide EuAsFeO0.85F0.15 having the lowest tempera-
ture of the superconducting transition Tc ≈ 11.3 K among
other related materials. We believe that the low Tc is con-
–10.0 –7.5 –5.0 –2.5 0 2.5 5.0 7.5 10.0
experiment
theory
0.3 S
11.5 K
11.2 K
10.4 K
8.7 K
7.4 K
5.7 K
6.3 K
5.1 K
4.4 K
C
o
n
d
u
ct
an
ce
,
re
la
ti
v
e
u
n
it
s
Voltage, mV
H = 0
Au-EuO FeAsF0.85 0.15
1/ = 0.14 SR
N
Fig. 4. A temperature set of dI/dV(V) spectra taken on a one–gap
contact; Δ(4.4 K) ≈ 1.5 meV, Г ≈ 0.01 meV, Z ≈ 0.1.
Fig. 5. The temperature dependence of the gap parameter Δ(T)
obtained from the BTK fitting of the one–gap spectra of Fig. 4.
4 5 6 7 8 9 10 11 12
0
0.4
0.8
1.2
1.6
Experiment
BCS theory
T
c
onset
T, K
O
rd
er
p
ar
am
et
er
m
eV
,
H = 0
Au-EuO FeAsF0.85 0.15
3 4 5 6 7 8 9 10 11 12
0
1
2
3
4
Large gap
BCS theory
Small gap
BCS theoryO
rd
er
p
ar
am
et
er
,
m
eV
T
c
onset
T, K
H = 0
Au-EuO FeAsF0.85 0.15
Fig. 6. The temperature behavior of the small and large gap pa-
rameters specified in the BTK analysis of one of the contacts
exhibiting two-gap features; Δs(4.1 K) ≈ 2.3 meV, Δl(4.1 K) ≈
≈ 4.2 meV.
Andreev reflection spectroscopy of the new Fe-based superconductor
Fizika Nizkikh Temperatur, 2011, v. 37, No. 4 367
nected to the polyvalency of Eu ions when a part of weakly
magnetic trivalent ions traps additional electrons and
changes to the bivalent state. In this state Eu ions have
very high spin moment S ∼7 μB that result in a strong pair-
breaking effect in spin-singlet superconductors.
The fitting of the spectra to the modified BTK theory
[15] shows that some spectra can be characterized by a
single gap parameter, whereas the two-gap approximation
is necessary for most of them. In both cases the gap para-
meters varied considerably from contact to contact being
within 1.3–3.1 meV (2Δ/kTc = 2.6–6.4) in the one-gap
spectra or within 1.1–2.3 meV (2Δs/kTc = 2.2–4.7) and
2.5–5.9 meV (2Δl/kTc = 5.1–11.7) for the small Δs and
large Δl gap, respectively, in the two-gap ones. The anoma-
lously high value of the characteristic parameter 2Δ/kTc
obtained for some contacts point to a non-phonon mechan-
ism of pairing in the compound investigated.
We attribute the observed variations of the gap parame-
ters in EuAsFeO0.85F0.15 to the anisotropy of the gap func-
tion Δ(k) near the Fermi level. This assumption agrees
with some theoretical studies substantiating the existence
of the anisotropic s±-symmetry of the order parameter in
iron pnictides. The varying from contact to contact intensi-
ty of the inter- and intra-band scattering of quasiparticles at
impurities and structural defects can be contributory too.
The result obtained in this study imply the absence
of zeros or lines of zeros in the Δ(k)-dependence. This ex-
cludes the d-wave symmetry of the order parameter in
EuAsFeO0.85F0.15, which was assumed to exist in some
low-Tc pnictides.
The work was partially supported by grants #09-02-01370
and #08-08-00709 from Russian Fundamental Research
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