Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations
We report Raman light scattering in the phase separated superconducting single crystal Rb₀.₇₇Fe₁.₆₁Se₂ with Tc = 32 K over a wide temperature region 3–500 K. The observed phonon lines from the majority vacancy ordered Rb₂Fe₄Se₅ (245) antiferromagnetic phase with TN = 525 K demonstrate modest anomali...
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| Zitieren: | Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations / Yu. Pashkevich, V. Gnezdilov, P. Lemmens, T. Shevtsova, A. Gusev, K. Lamonova, D. Wulferding, S. Gnatchenko, E. Pomjakushina, K. Conder // Физика низких температур. — 2003. — Т. 42, № 6. — С. 628-643. — Бібліогр.: 72 назв. — англ. |
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Pashkevich, Yu. Gnezdilov, V. Lemmens, P. Shevtsova, T. Gusev, A. Lamonova, K. Wulferding, D. Gnatchenko, S. Pomjakushina, E. Conder, K. 2018-01-16T17:00:30Z 2018-01-16T17:00:30Z 2016 Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations / Yu. Pashkevich, V. Gnezdilov, P. Lemmens, T. Shevtsova, A. Gusev, K. Lamonova, D. Wulferding, S. Gnatchenko, E. Pomjakushina, K. Conder // Физика низких температур. — 2003. — Т. 42, № 6. — С. 628-643. — Бібліогр.: 72 назв. — англ. 0132-6414 https://nasplib.isofts.kiev.ua/handle/123456789/129161 We report Raman light scattering in the phase separated superconducting single crystal Rb₀.₇₇Fe₁.₆₁Se₂ with Tc = 32 K over a wide temperature region 3–500 K. The observed phonon lines from the majority vacancy ordered Rb₂Fe₄Se₅ (245) antiferromagnetic phase with TN = 525 K demonstrate modest anomalies in the frequency, intensity and halfwidth at the superconductive phase transition. We identify phonon lines from the minority compressed RbδFe₂Se₂ (122) conductive phase. The superconducting gap with dx₂₋y₂ symmetry has been detected in our spectra. In the range 0–600 cm–¹ we observe a weak but highly polarized B₁g-type backgroundwhich becomes well-structured upon cooling. A possible magnetic or multiorbital origin of this background is discussed. We argue that the phase separation in M₀.₈₊xFe₁.₆₊ySe₂ is of pure magnetic origin. It occurs below the Néel temperature when the magnetic moment of iron reaches a critical value. We state that there is a spacer between the majority 245 and minority 122 phases. Using ab initio spin-polarized band structure calculations we demonstrate that the compressed vacancy ordered Rb₂Fe₄Se₅ phase can be conductive and therefore may serve as a protective interface spacer between the purely metallic RbδFe₂Se₂ phase and the insulating Rb₂Fe₄Se₅ phase providing percolative Josephson-junction like superconductivity all throughout of Rb₀.₈₊xFe₁.₆₊ySe₂. Our lattice dynamics calculations show significant differences in the phonon spectra of the conductive and insulating Rb₂Fe₄Se₅ phases. This paper is devoted to the memory of academician Kirill Borisovich Tolpygo — prominent Physicist, Teacher and Citizen, who made a great contribution to the lattice dynamics theory and many other branches of solid state physics. Authors thanks to Vladimir Pomjakushin for useful discussions. This work was supported in part by the State Fund of Fundamental Research of Ukraine and by the Ukrainian-Russian Grant No. 9-2010, NTH school Contacts in Nanosystems, and DFG. The calculations have been supported by resources of Ukrainian National GRID under NASU Grant No. 232. Yu. Pashkevich acknowledges partial support from the Swiss National Science Foundation (grant SNSF IZKOZ2 134161). en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур К 100-летию со дня рождения К.Б. Толпыго Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| title |
Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations |
| spellingShingle |
Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations Pashkevich, Yu. Gnezdilov, V. Lemmens, P. Shevtsova, T. Gusev, A. Lamonova, K. Wulferding, D. Gnatchenko, S. Pomjakushina, E. Conder, K. К 100-летию со дня рождения К.Б. Толпыго |
| title_short |
Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations |
| title_full |
Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations |
| title_fullStr |
Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations |
| title_full_unstemmed |
Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations |
| title_sort |
phase separation in iron chalcogenide superconductor rb₀.₈₊xfe₁.₆₊yse₂ as seen by raman light scattering and band structure calculations |
| author |
Pashkevich, Yu. Gnezdilov, V. Lemmens, P. Shevtsova, T. Gusev, A. Lamonova, K. Wulferding, D. Gnatchenko, S. Pomjakushina, E. Conder, K. |
| author_facet |
Pashkevich, Yu. Gnezdilov, V. Lemmens, P. Shevtsova, T. Gusev, A. Lamonova, K. Wulferding, D. Gnatchenko, S. Pomjakushina, E. Conder, K. |
| topic |
К 100-летию со дня рождения К.Б. Толпыго |
| topic_facet |
К 100-летию со дня рождения К.Б. Толпыго |
| publishDate |
2016 |
| language |
English |
| container_title |
Физика низких температур |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| format |
Article |
| description |
We report Raman light scattering in the phase separated superconducting single crystal Rb₀.₇₇Fe₁.₆₁Se₂ with Tc = 32 K over a wide temperature region 3–500 K. The observed phonon lines from the majority vacancy ordered Rb₂Fe₄Se₅ (245) antiferromagnetic phase with TN = 525 K demonstrate modest anomalies in the frequency, intensity and halfwidth at the superconductive phase transition. We identify phonon lines from the minority compressed RbδFe₂Se₂ (122) conductive phase. The superconducting gap with dx₂₋y₂ symmetry has been detected in our spectra. In the range 0–600 cm–¹ we observe a weak but highly polarized B₁g-type backgroundwhich becomes well-structured upon cooling. A possible magnetic or multiorbital origin of this background is discussed. We argue that the phase separation in M₀.₈₊xFe₁.₆₊ySe₂ is of pure magnetic origin. It occurs below the Néel temperature when the magnetic moment of iron reaches a critical value. We state that there is a spacer between the majority 245 and minority 122 phases. Using ab initio spin-polarized band structure calculations we demonstrate that the compressed vacancy ordered Rb₂Fe₄Se₅ phase can be conductive and therefore may serve as a protective interface spacer between the purely metallic RbδFe₂Se₂ phase and the insulating Rb₂Fe₄Se₅ phase providing percolative Josephson-junction like superconductivity all throughout of Rb₀.₈₊xFe₁.₆₊ySe₂. Our lattice dynamics calculations show significant differences in the phonon spectra of the conductive and insulating Rb₂Fe₄Se₅ phases.
|
| issn |
0132-6414 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/129161 |
| citation_txt |
Phase separation in iron chalcogenide superconductor Rb₀.₈₊xFe₁.₆₊ySe₂ as seen by Raman light scattering and band structure calculations / Yu. Pashkevich, V. Gnezdilov, P. Lemmens, T. Shevtsova, A. Gusev, K. Lamonova, D. Wulferding, S. Gnatchenko, E. Pomjakushina, K. Conder // Физика низких температур. — 2003. — Т. 42, № 6. — С. 628-643. — Бібліогр.: 72 назв. — англ. |
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Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6, pp. 628–643
Phase separation in iron chalcogenide superconductor
Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering
and band structure calculations
Yu. Pashkevich1, V. Gnezdilov2, P. Lemmens3, T. Shevtsova1, A. Gusev1, K. Lamonova1,
D. Wulferding4, S. Gnatchenko2, E. Pomjakushina5, and K. Conder5
1A.A. Galkin Donetsk Institute for Physics and Engineering, NAS of Ukraine, 03680 Kyiv, Ukraine
E-mail: yu.pashkevich@gmail.com
2B.I. Verkin Institute for Low Temperature Physics and Engeneering, NAS of Ukraine, 61103 Kharkov, Ukraine
3Institute for Condensed Matter Physics, TU Braunschweig, D-38106, Germany
4Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science, Pohang 790-784, Korea
5Laboratory for Developments and Methods, PSI, CH-5232 Villigen PSI, Switzerland
Received April 7, 2016, published online April 25, 2016
We report Raman light scattering in the phase separated superconducting single crystal Rb0.77Fe1.61Se2 with
Tc = 32 K over a wide temperature region 3–500 K. The observed phonon lines from the majority vacancy or-
dered Rb2Fe4Se5 (245) antiferromagnetic phase with TN = 525 K demonstrate modest anomalies in the frequen-
cy, intensity and halfwidth at the superconductive phase transition. We identify phonon lines from the minority
compressed RbδFe2Se2 (122) conductive phase. The superconducting gap with 2 2x yd
−
symmetry has been de-
tected in our spectra. In the range 0–600 cm–1 we observe a weak but highly polarized B1g-type background
which becomes well-structured upon cooling. A possible magnetic or multiorbital origin of this background is
discussed. We argue that the phase separation in M0.8+xFe1.6+ySe2 is of pure magnetic origin. It occurs below the
Néel temperature when the magnetic moment of iron reaches a critical value. We state that there is a spacer be-
tween the majority 245 and minority 122 phases. Using ab initio spin-polarized band structure calculations we
demonstrate that the compressed vacancy ordered Rb2Fe4Se5 phase can be conductive and therefore may serve
as a protective interface spacer between the purely metallic RbδFe2Se2 phase and the insulating Rb2Fe4Se5 phase
providing percolative Josephson-junction like superconductivity all throughout of Rb0.8+xFe1.6+ySe2. Our lattice
dynamics calculations show significant differences in the phonon spectra of the conductive and insulating
Rb2Fe4Se5 phases.
PACS: 78.30.−j Infrared and Raman spectra;
74.25.Jb Electronic structure (photoemission, etc.);
74.25.Kc Phonons;
74.70.Xa Pnictides and chalcogenides.
Keywords: iron pnictides, high-Tc superconductors, Raman scattering, first-principles calculations, phonons,
band structure.
Introduction
Analogous to oxygen intercalated La2CuO4+d [1], an
intrinsic structural phase separation into the antiferro-
magnetic insulating and nonmagnetic superconducting
phases has been shown to occur in the alkali iron selenides
M0.8+xFe1.6+ySe2 (M = K, Rb, Cs alkali metals) [2–14].
Immediately after this discovery the question about coex-
istence and competition of two phases arose. While one
phase evidences robust superconductivity with Tc ~ 30 K
the other phase develops antiferromagnetic (AFM) order
with an anomalously large Néel temperature of TN ~ 500 K
and with the highest magnetic moment ~ 3.3 μB/Fe. Exper-
imental data collected at an early stage favored a simple
phase separation scenario with two structurally distinctive
© Yu. Pashkevich, V. Gnezdilov, P. Lemmens, T. Shevtsova, A. Gusev, K. Lamonova, D. Wulferding, S. Gnatchenko, E. Pomjakushina,
and K. Conder, 2016
Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering
and non-interacting phases. However, following studies
pointed out that some remnant interplay occurs [15,16].
Particularly, it was noticed that an intermediate phase
should exist between insulating and superconducting re-
gions [17] and that the AFM phase can be related to the
unusual electronic properties of M0.8+xFe1.6+ySe2 com-
pounds through formation of an orbital-selective Mott
phase [18–21].
In this paper we examine spectral fingerprints of the non-
magnetic superconducting phase and the insulating antiferro-
magnetic phase in the superconductor Rb0.77(2)Fe1.61(3)Se2
with Tc = 32 K using Raman light scattering and ab initio
band structure and lattice dynamics calculations. So far,
only one Raman study on K0.68Fe1.57Se2 has considered a
phase separation [22] whereas in other investigations the
Raman spectra have been analyzed using the of the one
phase concept [23–28]. In addition, the Raman studies of the
rubidium based compound mainly focused on superconduct-
ing properties of the sample [28].
To date much information about details of the phase
separation in the M0.8+xFe1.6+ySe2 (M–Fe–Se) compounds
has been accumulated. General consensus has been
achieved among the following observations: 1) The insu-
lating phase is an antiferromagnetic semiconductor [8]
with an enormously large Néel temperature (TN ~ 500 K)
an iron magnetic moment of 3.3 μB [29] which is highest
among all iron superconductor parent compounds; 2) The
antiferromagnetic phase represents an iron vacancy or-
dered layered structure 5 5 1× × that corresponds to the
so-called “245” stoichiometry with two formula units of
the M2Fe4Se5 in the primitive cell and with space group
I4/m [6,7]; 3) The phase separation sets in at a temperature
TP that is a few tens of Kelvin below TN which in turn oc-
curs about 10–20 K below the vacancy ordering tempera-
ture TS [4–6,13,30]; 4) The volume fraction ratio of metal-
lic to antiferromagnetic phase is about 1/9, i.e., the
majority constitutes of the insulating vacancy ordered
phase [3,6,9,13], triggering a question about the
percolative limit in the superconductive state; 5) The me-
tallic nonmagnetic phase has an averaged I4/mmm tetrago-
nal symmetry with set of Bragg rods 2 2 1× × which is
expected for an averaged vacancy disordered structure
[6,7,13,14]. The respective lattice constants are compressed
in the ab plane and expanded along c axis compared to the
AFM phase [4–7,13,14]. 6) The minority metallic phase
forms a complex 3D nanoscale stripe-like network embed-
ded into the insulating phase with plate-shaped features
aligned along the {113} planes and elongated along the
〈301〉 directions [10,16,31,32]. Such a structure suggests
that the superconductive state in M0.8+xFe1.6+ySe2 may be
realized through Josephson junctions. The latter behavior
has been observed in optical conductivity studies [2,33]. 7)
In both phases the c axes coincide which means both phases
are crystallographically coherent [3–5,16].
The exact structure of the metallic superconductive
phase is not known. Its average structure can be well de-
scribed by the I4/mmm BaFe2As2 structure but with alkali
deficient chemical content MδFe2Se2 (δ < 1). This model
approximation was firstly suggested in the paper [13] and
now this crystallographic attribution of the superconduc-
tive phase is generally accepted [34,35]. However, the mi-
nority phase MδFe2Se2 has never been successfully syn-
thesized separate from the majority M2Fe4Se5 phase.
Recently [16] it was explained by specific mechanism of
the superconductive phase formation which relies on the
occurrence of an “imperfect” iron-vacancy order-disorder
transition. The superconductive phase is a remnant of a
high-temperature iron-vacancy-disordered phase with more
iron concentration but less alkali ion concentration com-
pared to the iron-vacancy-ordered phase.
In this scenario of phase separation the minority phase
should consist mainly of the MδFe2Se2 vacancy free phase
together with a small amount of phases which include iron
vacancies. Using ab initio band structure calculations we
demonstrate that the M2Fe4Se5 vacancy ordered compressed
phase, can be in the metallic state. We suggest that this third
phase can serve as an interface to ensure the percolative su-
perconductivity in M0.8+xFe1.6+ySe2. We provide lattice
dynamics calculations for both metallic and insulating
Rb2Fe4Se5 phases to distinguish their phonon spectra. How-
ever, in spite of a large difference observed in the high-
frequency part of our spectra we did not find a direct mani-
festation of this third phase in our Raman spectra.
The distinctive feature of the M0.8+xFe1.6+ySe2 supercon-
ductors is the absence of hole pockets near the Brillouin
zone (BZ) center that were established by angle resolved
photoemission spectroscopy studies [12,36–40]. This dis-
covery has altered the dominant channel of superconduc-
tive pairing from the s+-type, which is based on the interac-
tion between electron and hole pockets, to the 2 2x y
d
−
channel which based on the interaction between electron
pockets on the BZ boundary [41]. The 2 2x y
d
−
channel has
been detected in Raman studies of Rb0.8Fe1.6Se2 [28].
However, another important feature of M0.8+xFe1.6+ySe2
compounds is an insensitivity of the superconducting
transition temperature to the doping of alkali ions δ in the
metallic phase MδFe2Se2 which remains at Tc ~ 30 K for
a wide doping range [42,43]. The alkali content δ deter-
mines the electron doping level and highly affects the
topology of Fermi surface [44]. Therefore one should
expect that the structure of the superconductive gap in
M0.8+xFe1.6+ySe2 should display some modifications which
depend on the history of sample preparation (e.g., it should
be a function of the doping level δ). In our Raman spectra
we observed 2 2x y
d
−
symmetry of the superconductive gap,
typical for iron selenides while some specific features are
different from the Raman spectra of Rb0.8+xFe1.6+ySe2
reported previously [28]. Additionally our first-principal
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6 629
Yu. Pashkevich et al.
band structure calculations for the metallic RbFe2Se2 mod-
eling phase reveal an absence of hole pockets at the center
of the Brillouin zone.
We did not see a noticeable change in the volume of
superconductive fraction upon decreasing temperature
through Tc in contrast to the phase separated La2CuO4+d
[45]. However, our observations reveal some alteration of
the electronic structure in the metallic phase which starts
slightly above Tc.
Experimental details
A Rb0.8+xFe1.6+ySe2 single crystal with chemical con-
tent Rb0.77(2)Fe1.61(3)Se2 was synthesized following the
procedure described in [46]. Chemical homogeneity and
elemental composition of the cleaved crystals were studied
using x-ray fluorescence spectroscopy (XRF, Orbis Micro-
XRF Analyzer, EDAX). Elemental distribution maps for
Rb, Fe and Se were collected in a vacuum applying white
x-ray radiation produced by a Rh tube (35 kV and 500 μA).
The x-ray primary beam was focused to a spot of 30 μm
diameter. A primary beam Ti filter (25 μm thickness) was
applied. An area of ~ 0.5 cm2 was scanned. Prior to the
measurements, elemental calibration was done using as a
standard carefully weighted, homogenized, and pressed
into a pellet mixture of Se, Fe, and corresponding alkali
metal carbonates. The applied calibration procedure results
in ~ 2% accuracy of the determined stoichiometric coeffi-
cient values. The superconducting properties of the plate-
like crystal were characterized with a Quantum Design
MPMS XL SQUID magnetometer. The temperature de-
pendence of the magnetic susceptibility is shown in Fig. 1.
The crystal was cleaved and a flat shiny surface was ob-
tained. The freshly-cleaved sample was immediately trans-
ferred into an evacuated cryostat for Raman scattering
(RS) studies avoiding surface contamination and degrada-
tion. Raman scattering measurements were performed in
quasibackscattering geometry with the excitation line
λ = 532.1 nm of a solid-state laser. Raman spectra were
measured from the crystallographic ab plane. The laser power
of less than 3 mW was focused to a 0.1 mm diameter spot on
the sample surface. Spectra of the scattered radiation were
collected by a Dilor-XY triple spectrometer and a micro-
Raman setup (Horiba Labram) equipped with liquid nitrogen
cooled charge coupled device (CCD) detector (Horiba Jobin-
Yvon, Spectrum One CCD-3000V). Temperature dependenc-
es were obtained in a variable temperature closed cycle cryo-
stat (Oxford/Cryomech Optistat, RT-2.8 K). We have collect-
ed additional Raman spectra over an extended temperature
range to 500 K in a constantly evacuated heating chamber.
Results and discussion
The tetragonal insulating majority phase of
Rb0.8+xFe1.6+ySe2 possesses a 5 5 1× × iron vacancies
ordering pattern that is described by I4/m (N 87) space
group symmetry with two M2Fe4Se5 formula units per
primitive cell (so-called “245” phase). The Fe ions occupy
general type 8i (x,y,z)-positions, while the Se ions are dis-
tributed between 8i (x,y,z) (Se1)- and 2e (1/2,1/2,z) (Se2)-
positions, and the Rb ions reside at 4h (x,y,0)-positions.
Note that Se2 atoms are located at special positions in the
layer of FeSe4 tetrahedrons, being the corner shared ions
between four tetrahedrons in the same plaquet while the
connection between different plaquets is realized through
Se1 atoms. In spite of the high symmetry of Se2 ions their
z coordinate is not fixed. Thus, the free coordinates of the
Se1 and the Se2 sites make the structure of the M2Fe4Se5
compounds much more susceptible to possible orbital re-
ordering processes and iron spin state changes compared to
other chalcogenides, like FeSe and FeTe. The highly dis-
torted FeSe4 tetrahedron shape, in which the Fe–Se2 dis-
tance is substantially larger than the Fe–Se1 one, also sup-
ports this tendency.
The block checker-board type of antiferromagnetic or-
der in Rb0.8+xFe1.6+ySe2 is formed by a plaquet (square) of
four nearest-neighbor (in block) iron atoms with ferromag-
netic order along c axis while other in-plane and out-of-
plane nearest-neighbor plaquets are ordered antiferromag-
netically [6,7,9]. There is no multiplication of the crystal-
lographic primitive cell. The neutron diffraction studies as
well polarized muon spin relaxation measurements did not
reveal the spin reorientation phase transition in this com-
pound under lowering temperature [7,9]. This type of
magnetic order parameter (belonging to the τ2 or Au irre-
ducible representation [6]) breaks inversion symmetry but
retains tetragonal symmetry. The unitary subgroup of
magnetic group I4/m′ is I4 (N 79) with rotational symmetry
C4 so that the magnetically ordered part of the sample re-
mains in the ferroelectric state at all temperatures of our
experiment. The crystallographic and magnetic structures
of Rb2Fe4Se5 are shown in Fig. 2.
Fig. 1. Temperature dependence of the magnetic susceptibility of
our Rb0.77Fe1.61Se2 sample. The sharp transition into the super-
conductive state at Tc = 32 K demonstrates the high quality of
the crystal.
630 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6
Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering
The symmetry analysis reveals that for the M2Fe4Se5
vacancy ordered structure in the paramagnetic phase there
are in general 66 phonon modes including 3 acoustic ones.
The mechanical representation Γ consists of the following
irreducible representations (IRp):
Γ = 9Ag + 8Bg + 8Eg + 8Au + 7Bu + 9Eu.
Out of 25 Raman-active modes only 17 modes (9Ag + 8Bg)
are accessible for the given backscattering geometry of
our Raman experiment with allowed incident-scattered light
polarization for Ag(XX+YY, ZZ) modes and Bg(XX–YY,
XY) modes. Here the X and Y axes are directed along the
a and b lattice constants of the 245 phase shown in Fig. 2.
Raman-active modes are distributed among the atom po-
sitions in following way: 4h (Rb) = 2Ag + 2Bg; 8i (Se1
and Fe) = 3Ag + 3Bg; and 2e (Se2) = Ag.
In accordance with the generally accepted description
the minority phase is alkali deficient, iron vacancy free,
nonmagnetic and metallic. It consists of layers of the edge
shared FeSe4 tetrahedrons stacked along the c axis with the
Rb ions in between. This RbδFe2Se2 phase (so-called
“122” phase) has an averaged I4/mmm symmetry [13] with
Rb content δ depending on the initial x, y chemical compo-
sition of the Rb0.8+xFe1.6+ySe2 [42,43]. The occupancy of
atoms (Fe at 4d, Se at 4e, and Rb at 2a) should result in
two additional Raman active modes A1g(Se) and B1g(Fe)
into Raman spectra measured in our geometry of scatter-
ing. The Raman tensor of the B1g(Fe) mode has (X′X′–Y′Y′)
symmetry. Here the X′ and Y′ axes of the 122 phase are rotat-
ed by 27° from the X–Y axes of the 245 phase. The align-
ment of the C4 axes remains the same for the 245 and the
122 phases [3–5].
The evolution of the Rb0.77Fe1.61Se2 Raman spectra in
parallel (XX) and crossed (YX) polarizations as a function
of temperature from 3 to 490 K is shown in Fig. 3. In total
at least fifteen phonon modes can be clearly identified
from both polarizations. All of these phonon modes are
located in the frequency region below 300 cm−1 similar to
previous studies of K0.8+xFe1.6+ySe2 [24,25,27]. The num-
ber of observed lines evidences the presence of the vacan-
cy ordered state already at T = 490 K just below the Néel
temperature TN = 525 K.
The tetragonal symmetry of the sample is clearly seen
in the polarization dependence of the spectra. This is il-
lustrated in Fig. 4 where we compare the low-temperature
XX and YX spectra. The Ag and Bg phonons can be easy
distinguished in our well polarized spectra. However, it is
not so straightforward to identify the B1g phonon of the
minority I4/mmm phase from the Bg phonons with strong
intensity of the I4/m majority phase. In Raman studies of
the K0.8+xFe1.6+ySe2 the weak B1g line of the 122 phase
has been extracted by using its specific intensity depend-
ence from the angle between polarizations of incident and
scattered light [24], while the A1g phonon has been as-
signed under the assumption about of similar Raman spec-
tra from (Sr,K)Fe2As2 and KδFe2Se2. The observed fre-
quencies of the 122 phase at 85 K A1g (180 cm–1) and B1g
(207 cm–1) [24] turn out to be strikingly equal (with the
accuracy of one wave number) to the frequencies of the
same symmetry phonons in FeSe at 100 K [47]. Relying
on that identity we compare the Raman spectra of the
FeSe [47] and Rb0.77Fe1.61Se2. in Fig. 4. We did not ob-
serve a direct coincidence of the phonon lines in both
spectra as it takes place in K0.8+xFe1.6+ySe2 [24]. Howev-
er, considering the similarity of some lines in the spectra
of both compounds, one can preliminary attribute the
mode at 178.4 cm–1 in the XX spectra as the A1g mode and
the mode at 212 cm–1 in the XY spectra at the B1g mode of
the RbδFe2Se2 metallic minority phase. Below we will
give more evidence to substantiate our attribution.
Fig. 2. (Color online) The crystallographic and magnetic structures of Rb2Fe4Se5. The plaquets (blocks) of four nearest-neighbor iron atoms
are marked by shaded planes on left side. The block type of AFM order with a layer of FeSe4 tetrahedrons is shown on the right side.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6 631
Yu. Pashkevich et al.
The temperature-dependent Raman spectra of
Rb0.77Fe1.61Se2 phonon lines were described explicitly by
Lorentz functions. All of them show a conventional pho-
non line shape without any evidence of the Fano-like
asymmetry. In Figs. 5 and 6 the results of a temperature
analysis of the representative phonons is shown. Interesting-
ly, a like asymmetry indicative for an electron-phonon cou-
pling did appear in the Raman spectra of the superconduc-
tive K0.75Fe1.75Se2 [27], while being absent in the Raman
spectra of the superconductive K0.68Fe1.57Se2 [24]. Hence,
the question arises whether there is a connection to details
of the phase separation caused by different potassium con-
tent, which might change the doping level of the metallic
phase as well as the ratio between minority and majority
phase, whether this is simply an issue of sample quality.
We did not observe a structural phase transition which
takes place in K0.75Fe1.75Se2 at 250 K [27]. Instead, the
frequencies and halfwidths of selected Ag phonon lines
show a small kink around 270 K in their temperature de-
pendences shown in Fig. 5. Among high-frequency Bg
phonons with iron-dominated vibrations the Bg(5) phonon
shows the strongest frequency increase under cooling. This
resembles a frequency anomaly of the B1g phonon in FeSe
detected in our Raman studies [47], supporting our as-
signment of the Bg(5) phonon as B1g phonon of the 122
phase with I4/mmm symmetry.
Further evidence is found in the behavior of the scatter-
ing intensity, which increases anomalously for the Bg(5)
phonon under approaching and just below superconductivi-
ty onset, illustrated in Fig. 7. The main contribution to the
intensity gain comes from the B1g(XX–YY) channel that is
clearly seen from comparing of the Raman spectra in
crossed and RL-polarizations (Fig. 7, right panel). Here
Fig. 3. Temperature dependences of Rb0.77Fe1.61Se2 Raman spectra in the diagonal polarization for Ag-type phonon lines (left side) and crossed
polarization for Bg-type phonon lines (right side). The observed phonons are marked by arrows.
Fig. 4. Comparison of the XX and XY Raman scattered polarized
spectra of Rb0.77Fe1.61Se2 at 3 K with the aa-polarized spectra of
FeSe at 7 K adopted from [47]. All strong Ag lines from the XX
polarization are absent in the XY polarization. Almost all Bg lines
which are seen in the XY polarization have visible counterparts in
the XX polarization that is in accordance with I4/m symmetry of
the Bg Raman tensor (XX–YY, XY). The one exception is the high-
est and most intensive Bg mode which has a very weak counter-
part in the XX spectra. The assignment of the A1g and B1g phonon
lines to the minority I4/mmm metallic RbδFe2Se2 (122) phase are
marked by arrows.
632 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6
Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering
and below the RL (RR) denotes opposite (the same) chiral-
ity of the incident and scattered light with right (R) or left
(L) circular polarizations. Simultaneously some inelastic
wide band bump with distinct B1g(XX–YY) symmetry con-
tinually increases under cooling and persists in the super-
conductive state.
In spite of the same symmetry of the broad band and the
B1g phonon at 213 cm–1 there is no visible Fano-like spec-
tral feature. The intensities of the bump band and the B1g
phonon are both nicely fitted by an oscillator-like shape
and a Lorentz function, respectively. Similarly, the Bg(7)
phonon at 272 cm–1 also does not show Fano-like features.
However, in the last case there is no XX–YY contribution to
the Raman tensor of the Bg(7) phonon. This is supported
by the similarity in the temperature evolution of the Bg(7)
intensity in both RL and crossed polarizations (Fig. 7) as
well by the absence of significant contribution of Bg(7)
phonon mode in the XX — Raman spectra (Fig. 4). In other
words the Bg(7) XY phonon cannot interact with excitations
manifested in the XX–YY bump. The absence of a Fano-
like features for the B1g phonon at 212 cm–1 can be inter-
preted in two ways: (i) the bump excitations originate from
another phase, e.g., from the insulating 245 phase and can-
not interact with a phonon of the 122 metallic phase, regard-
less of the symmetry. (ii) the bump band does not present a
many-particle continuum excitation but rather a set of one-
particle excitations and therefore no Fano-like interaction is
expected. Below we will discuss both possibilities.
The anomalous intensity increase of the B1g phonon is
not related to a possible increase of the 122 phase volume
fraction as we do not see the simultaneous increase of the
A1g phonon intensity. At the same time the B1g mode does
not evidence any frequency shift, thereby signaling a high-
ly specific electron structure alteration as the origin for the
intensity anomaly that contributes solely to the B1g-type
Fig. 5. Temperature dependent parameters of selected Ag (left panel, solid symbols) and Bg (right panel, open symbols) phonon
modes in Rb0.77Fe1.61Se2. Temperature dependences of the frequency (left side of each panel) and linewidth, FWHM (right side of
each panel). Solid lines are guides to the eyes.
Fig. 6. Temperature dependences of the intensity of selected Ag
(left panel, solid symbols) and Bg (right panel, open symbols)
phonon modes in Rb0.77Fe1.61Se2. Solid lines are guides to the
eyes. On the right side we display the symmetry assignment,
enumeration and frequency of observed phonons.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6 633
Yu. Pashkevich et al.
Raman tensor and not to the A1g-type Raman tensor. The
difference in the electronic structure above and below Tc in
Rb 245 has been detected in ARPES studies [12]. Note that
we observe the change in intensity already at T = 40 K just
before the onset superconductivity.
The above mentioned bump belongs to the low intensive
background which we obtain from the Raman spectra by
subtracting the phonon contribution. The background be-
comes clearly structured under cooling and persists in the
superconductive state (see Fig. 8). This background can be
described very well with four wide oscillator shaped bands.
It has essentially B1g(XX–YY) symmetry in which the
nondiagonal XY components are absent. The background
almost disappears in the nondiagonal spectra by respective
choice of the axes (for instance it shows up in the YX spectra
shown in the Fig. 3 but it is absent in the yx spectra shown in
the Fig. 7).
Let us discuss the possible origin of this background.
The block type AFM order with eight magnetic ions in the
primitive cell should lead to four double degenerated
transversal spin waves at k = 0. The energy of the first
wide band of background equals to 50 cm–1. It nicely coin-
Fig. 7. (Color online) Comparison of Raman spectra for crossed and RL polarizations at different temperatures (left panel). Temperature-
dependent intensities of selected Bg phonons for different polarizations (right panel). The inset the left panel shows a broad band of
B1g(XX–YY) symmetry which is centered around 245 cm–1.
Fig. 8. (Color online) Temperature dependences of the low-energy electronic background for two different polarizations in
Rb0.77Fe1.61Se2. The smooth development of the well defined structure is clearly seen under decreasing temperature in the RL polariza-
tion. This structure persists even in the superconductive state.
634 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6
Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering
cides with a gap of the acoustic transversal spin wave that
has been found below 8 meV by inelastic neutron scatter-
ing in the superconductive Rb0.82Fe1.68Se2 with Tc = 32 K
[48] as well with spin wave gap at 6.5 meV found in the
superconductive (Tl,Rb)2Fe4Se5 with Tc ~ 32 K [49]. Due
to the specific symmetry of acoustic spin waves this possi-
ble one-magnon scattering should not appear in the B1g
channel neither by Faraday nor by exchange mechanism
scattering. Further bumps are centered at 247, 355 and 485
cm–1 and have no counterparts in the optical spin wave
spectra at k = 0 (Γ point of Brillouin zone) [48]. However,
a respective symmetry analysis shows that one-magnon
scattering in the B1g channel is possible on an optical
branch of the longitudinal magnons. It is caused by the
exchange mechanism of scattering. Such a scattering can
be assisted by specific phonons with vibration of atoms
that are involved into pathway of the superexchange inter-
actions. Interestingly the frequency of the most prominent
background bump at 247 cm–1 lies exactly in between fre-
quencies of the two highly intensive Ag phonons. One can
demonstrate that this phonon type can assist longitudinal
iron moment oscillations.
Another possible explanation of the structured and po-
larized background can be related to the multiorbital na-
ture of the iron HTSC. The multiband electronic structure
allows some interband transitions across the Fermi sur-
face caused by charge fluctuations [50,51] and a d-wave
Pomeranchuk instability [52]. According to Ref. 50 the
scattering in the B1g-symmetry channel corresponds to
intraorbital transitions at the electron pockets, which are
expected in our 122 metallic phase. The energy scale of
the background bumps also points towards a metallic
phase origin. Below we will see that the electronic band
structure of the metallic compressed Rb2Fe4Se5 phase
allows some low-energy interband transitions. If we as-
sume this mechanism to be valid in the insulating 245
phase then the energy of the respective interband excita-
tions should exceed the energy of the semiconducting gap
that is roughly 0.4 eV. Interestingly, the highly polarized
broad bump, centered at low T at 450 cm–1, has also been
observed in FeSe [47].
Raman spectroscopy is a suitable tool to elucidate the
symmetry of the superconducting gap [53]. The distinctive
difference between superconducting and normal states ap-
pears in RL polarized spectra but is absent in spectra of RR
polarization (see insets in Fig. 9). This difference evidenc-
es the presence of at least two types of superconducting
gaps with B1g and B2g symmetries in our Rb0.77Fe1.61Se2
sample. As shown in [54] the gap with pure 2 2x y
d
−
sym-
metry should be the dominant superconducting gap in the
iron selenides where the Fermi surface of the metallic
phase contains electronic pockets at the BZ boundary and
does not have the hole pockets at the center of BZ. The
presence of a subdominant B2g superconducting gap
should be seen in Raman spectra as the appearance of new
low-energy excitations in the superconducting state
which could be interpreted as Bardasis–Schrieffer modes
[55]. In iron-based superconductors these collective
modes can arise due to a coexistence and competition of
two different pairing channels [56]. Since we did not see
new features in the superconducting state we conclude that
the B2g superconducting gap is absent in Rb0.77Fe1.61Se2.
In fact, a previous study reported the absence of a B2g su-
perconducting gap in Rb0.8Fe1.6Se2 as well [28]. On other
hand, collective in-gap modes have been found in
K0.75Fe1.75Se2 in the RL channel [27] which evidences a
coexistence and strong competition of the s-wave and the
d-wave pairing mechanisms [57]. The discrepancy in de-
tails of the superconducting state between Rb- and K- rela-
tives of the M0.8+xFe1.6+ySe2 superconductors is connected
with an uncontrolled and different electron doping value δ
in the metallic phase MδFe2Se2 for the samples of different
origin. Such an uncertainty is reflected in the phase dia-
gram of M0.8+xFe1.6+ySe2 where the samples of different
alkali metal and iron content nevertheless fall within the
superconducting dome [42,43]. The relation between dif-
ferent pairing channels and the level of doping in
Fig. 9. (Color online) Manifestation of the superconducting state in Raman spectra of Rb0.77Fe1.61Se2 for the RL (left) and RR (right) circu-
lar polarizations. The spectra are corrected by Bose factor. The insets plot the intensity difference of the spectra above and below Tc = 32 K.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6 635
Yu. Pashkevich et al.
MδFe2Se2 has been analyzed in a recent paper [58]. To
this point one can add a difference between Raman spec-
tra of the superconducting phase in Rb0.8+xFe1.6+ySe2
samples taken from different sources. While our spectra of
Rb0.77Fe1.61Se2 did not show any intensity gain in super-
conducting state for the phonons of insulating 245 phase at
80 and 115 cm–1 spectra of Rb0.8Fe1.6Se2 clearly evidence
changes in B1g intensity [28].
We did not see additional modes caused by the loss of in-
version symmetry in the magnetically ordered and ferroelec-
tric 245 phase. In this phase the 15 far-infrared active modes
(7Au + 8Eu) as well 7 previously silent Bu modes should be
Raman active. The group–subgroup relation between the par-
amagnetic phase (belonging to the –I4/m group) and the
unitary subgroup (–I4) of magnetic group of the antiferro-
magnetic phase yields the following transformations: Ag and
Au transition to A (XX+YY, ZZ); Bg and Bu transition to
B (XX–YY, XY); and Eg and Eu transition to E (XZ, YZ). In
particular, 14 additional modes (previously of 7Au + 7Bu
symmetry) should be seen in our scattering geometry.
However, our ab initio band structure and lattice dynamic
calculations demonstrate that the mixing between former
g- and u-type of modes in the magnetically ordered phase is
very small and can be neglected. This is due to an almost
indiscernibly difference between the I4/m and the I4 crystal-
lographic structures which has been obtained under structure
optimization. Hence we conclude that u-type modes of the
Rb2Fe4Se5 insulating magnetically ordered phase can not be
observed in Raman experiment.
Finally, our phonon Raman spectra which can be assigned
to the insulating vacancy and antiferromagnetically ordered
Rb2Fe4Se5 phase show several distinctive features at Tc (see
Figs. 5 and 6): 1) clear frequency jump of the Ag modes at Tc;
2) frequency and linewidth anomaly of the Bg modes at Tc;
3) and an intensity anomaly at Tc.
The observed anomalies are reminiscent of the super-
conductivity-induced features in the μSR spectra of FeSe
under pressure [59] and in optical conductivity spectra of
Rb [8] and K [60] members of the “245” family. It can be
explained by a rearrangement in the electronic subsystem
trough under the onset of superconductivity. A remarkable
alteration of the low-frequency range (below 8 meV) in the
optical response with a kink at Tc has been observed in
Rb2Fe4Se5 [8]. Furthermore, in this frequency region they
observed a double peak structure around 2 meV appears
which is buried by the electronic background at high tem-
peratures but persists into the superconducting state. Par-
ticularly interesting is a drop of the μSR frequency below
Tc in FeSe in the regime where magnetic and superconduc-
tive states coexist [59]. This drop signals the lowering of
the absolute iron magnetic moment. The change in magnet-
ic moment, in turn, reflects the change of electron density
distribution among d-orbitals in the magnetically ordered
phase, which nevertheless occurs below the superconduct-
ing phase transition.
First-principal band structure and lattice dynamic
calculations
Density-functional (DFT) calculations can provide an
adequate description of the band structure and the lattice
dynamics of iron pnictides and chalcogenides if the iron spin
states are explicitly taken into account [61–64]. We applied
the all-electron full-potential linearized augmented —
plane-wave method (ELK code) [65] with revised local
spin density approximation [66] for the exchange-correl-
ation potential in the spin-polarized mode. To obtain band
structure and phonon frequencies for the given lattice pa-
rameters and magnetic structure without the full optimiza-
tion procedure we assume that at the true value of magnet-
ic moments the crystal structure is already optimized.
Thus, for the first step of calculations we use the value of
magnetic moment as a fitting parameter to optimize the
ion’s coordinates while experimental lattice constants re-
main to be fixed. We successfully applied this procedure in
previous lattice dynamic calculations of the FeTe [67,68]
and FeSe [47].
We used the experimental unit-cell parameters taken at
room temperature for Rb0.8+xFe1.6+ySe2 [13] that adopt the
vacancy ordered structure and I4/m space symmetry with
two formula units in the primitive cell with iron atoms lo-
cated at the general 8(i) positions, Rb atoms at 4(h) posi-
tions, and Se atoms at two positions 8(i) and 2(e). For the
self-consistent and phonon calculations we considered a
5×5×5 gamma-centered k-mesh, which corresponds to 21
points in the irreducible part of the Brillouin zone. The
phonon frequencies were calculated in the “fix-spin” mode
at which the iron magnetic moment was fixed at
2.62 μB/Fe and does not change throughout the calcula-
tions. The magnitude of the iron magnetic moment was ob-
tained from our preliminary calculations which were carried
out for the antiferromagnetic state of the Rb2Fe4Se5 with
fixed lattice constants. The magnetic structure has been ac-
counted for explicitly by the so-called “block antiferromag-
netic order” in which a plaquet of the four nearest-neighbor
iron atoms has magnetic moments aligned ferromagnetically
along the c axis and all nearest-neighbor plaquets align
antiferromagnetically (see Fig. 2). The calculated magnitude
of the iron magnetic moment is in good agreement with ex-
perimental data taken at 300 K [13]. The partially optimized
structural data are summarized in the Table 1.
Our band structure calculations for the vacancy or-
dered magnetic state result in the insulating solution
shown in Fig. 10 for the stoichiometric Rb2Fe4Se5 com-
pound similar to the results mentioned in [8,12,69,70] for
the whole stoichiometric “245” family. The density of
states (DOS), presented in Fig. 11, highlights that this
insulating state can be qualified as a semiconducting state
with a very small interband gap of around 0.2–0.3 eV that
is in accordance with experimental measurements of opti-
cal conductivity [8].
636 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6
Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering
We carried out self-consistent band-structure calcula-
tions for the metallic vacancy free RbFe2Se2 phase (see
Fig. 12). We used the structural parameters of the I4/mmm
phase (see Table 2) which has been identified at 200 K
in the superconducting sample of Rb0.8+xFe1.6+ySe2 as
the minority phase [35]. For the calculations we consid-
ered (2a) Rb site to be fully occupied. Calculations were
carried out using the same parameters as previously, ex-
cept for a change of the k-mesh into 9×9×5 due to the
reduced size of the RbFe2Se2 unit cell compare to
Rb2Fe4Se5. The magnetic moment of Fe after self-
consistent calculations without fixation turned out about
0 μB. Our results demonstrate the absence of the hole-like
Fermi surface at the BZ center and the presence of the
electron-like double degenerated pockets at the BZ M-
points that are in accordance with many angle-resolved
photoemission experiments on alkali iron selenides.
We argue that the phase separation phenomenon in
M0.8+xFe1.6+ySe2 is of magnetic origin. The driving force
of the phase separation is an internal in-plane pressure,
which develops in the vacancy ordered phase at temperatures
below its AFM ordering temperature. The phase separation
occurs when the internal pressure reaches a critical value un-
der the static magnetic moment growth. Particularly in
Rb0.8+xFe1.6+ySe2, the phase separation temperature coin-
Table 1. The partially optimized structural data
Parameter Atomic and cell parameters (after optimization)
for the insulating phase of Rb2Fe4Se5 for the conductive phase of Rb2Fe4Se5
M, μB/Fe 2.62 0.01
a, Å 8.7996 8.5682
b, Å 8.7996 8.5682
c, Å 14.5762 14.636
Position
of atom
x/a y/b z/c x/a y/b z/c
Rb (8h) 0.399 0.803 0 0.3567 0.8122 0
Fe (16i) 0.2958 0.5920 0.2524 0.3061 0.6040 0.2489
Se1 (16i) 0.3880 0.7944 0.6495 0.4036 0.8022 0.6714
Se2 (4e) 0.5 0.5 0.1455 0.5 0.5 0.1596
Fe–Fe, Å Fe–Se, Å Fe–Fe, Å Fe–Se, Å
in block between
blocks
in block between blocks
2.7872 2.896 2.4284
2.4238
2.4217
2.5124(Se2)
2.6636 2.6784 2.2718
2.2214
2.2717
2.2930(Se2)
Fig. 10. Band structure of the insulating magnetically ordered
Rb2Fe4Se5 (245) phase in the superconducting Rb0.8+xFe1.6+ySe2.
Fig. 11. (Color online) Density of states in the antiferromagnetic
Rb2Fe4Se5. The black line presents the total DOS. Partial DOS
for Fe, Se(1), Se(2) and Rb species are shown for each muffin-tin
part of the primitive cell. Note that the iron electron states con-
tribute largely to the DOS at both side of the gap.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6 637
Yu. Pashkevich et al.
cides with the temperature at which the iron magnetic mo-
ments attain their maximum (see Fig. 8 in Ref. 13). Hence,
the magnetic moment value is a key parameter which de-
fines the level of internal nonuniform compression and the
appearance of the vacancy free 122 phase. In the case of
nonfixed boundaries of the sample the phase separation
phenomenon is a response of the crystal to minimize the
elastic free energy. Note that the influence of the suggested
internal pressure is different from the influence of an ex-
ternal hydrostatic pressure. The latter should decrease all
lattice constants whereas internal pressure has a
nonuniform effect and leads to a decrease in ab constants
and an increase of the c lattice constant.
The mutual crystallography coherence of the majority
and minority phases implies that a spacer should exist in
between them. We suppose that the compressed vacancy
ordered 245 phase could play a role of this spacer. Surpris-
ingly our ab initio band structure calculations demonstrate
that the compressed 245 phase resides in the metallic state
with iron magnetic moments close to zero. The amount of
this third phase can be smaller than the amount of the mi-
nority 122 phase. However, being metallic this phase may
serve as the protective interface phase between the pure
metallic vacancy free 122 phase and the insulating vacancy
ordered 245 phase ensuring the percolative superconduc-
tivity in the Rb0.77Fe1.61Se2.
To perform the spin-polarized band structure calcula-
tions we assume that the lattice constants of the metallic
vacancy ordered Rb2Fe4Se5 phase with I4/m space sym-
metry are the same as the lattice constants of the minority
122 phase from [13] whis a245(met) = 5 a122 and
c245(met) = c122. The atomic coordinates were determined
under the optimization procedure (see Table 1). We found
that the z coordinate of Se ions undergoes a remarkable
shift and the distance betwen Fe–Se decreases. In addition,
the difference in the Fe–Fe distances between in-block
and out-of-block iron atoms which is inherent to the insu-
lating 245 phase almost disappears in the metallic 245
phase (see Table 1).
The band structure of the compressed metallic 245 phase is
shown in Fig. 13. The most prominent features of this phase
are the presence of both the hole-like Fermi surface pockets
at the BZ center and the electron-like Fermi surface pockets at
the BZ M-point. The variety of electron bands at the Fermi
surface in this phase supplies more possibilities for interband
transitions than in the metallic 122 phase. Consequently,
one can speculate that this spacer phase is the source of the
low-intensive background. Furthermore, the coexistence of
hole- and electron-like pockets at the Fermi surface in the
spacer phase supports the scenario of two competing chan-
nels of the superconducting pairing in alkali iron selenides.
The frequencies and eigenvectors displacement patterns
of the phonon modes were calculated using the frozen
phonon approach [65] in the “fix-spin” mode for both insu-
lating and metallic Rb2Fe4Se5 phases. We used the struc-
Fig. 12. Band structure of the metallic nonmagnetic RbFe2Se2
(122) phase in the superconducting Rb0.8+xFe1.6+ySe2.
Table 2. Structural data of the I4/mmm minority RbδFe2Se2 phase
Parameter Experiment at 200 K [35], δ = 0.516 Theory after optimization, δ = 1; M = 0
a = b, Å 3.84702 3.84702
c, Å 14.7680 14.7680
Position of atom x/a y/b z/c x/a y/b z/c
Rb (2a) 0 0 0 0 0 0
Fe (4d) 0 0.5 0.25 0 0.5 0.25
Se (4e) 0 0 0.35655 0 0 0.331054
Fig. 13. Band structure of the metallic vacancy ordered com-
pressed Rb2Fe4Se5 phase.
638 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6
Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering
tural data given in Table 1. The representative displace-
ment patterns and phonon frequencies for the Raman-
active Ag and Bg modes in the 245 insulating phase are
shown in Figs. 14 and 15.
The phonon energies as obtained from our calculations
are grouped in accordance to atomic weight of the respec-
tive vibrating ions, i.e., the high-frequency part of the pho-
non spectra is dominated by Fe ionic displacements (the
lightest element in the composition) while the low-energy
part of the phonon spectra is formed by the displacements of
the most heavy Rb atoms within the almost empty ab-layer.
This is important information for our Raman studies because
such a motion implies a weak polarizability and, therefore,
low-intensive Raman lines.
In Table 2 we compare experimental and theoretical data
including the calculated phonon frequencies of the insulating
and metallic 245 phases.
In Table 3 we denote most intensive lines in “bold” and
we use “cursive” for a considerable deviation (more then
10 cm–1) between theoretical data for the insulating 245
phase and experiment. Also we indicate by “weak” the lines
which are not clearly seen at room temperature but have
counterparts in our spectra at lowest temperature. In general,
one can conclude a good agreement between experiment and
theory for the insulating 245 phase since the difference for
most frequencies does not exceed 10%. However, the theory
tends to underestimate the high-frequency part spectra cal-
culated of the insulating 245 phase and overestimates those
of the conducting 245 phase.
Let’s analyze the most instructive deviations between
experiment and theory for the insulating phase. The largest
discrepancy is found for two Ag phonon modes at 106.6
and 184 cm–1 that are mainly formed by z-displacements
of Se(2) atoms. As was noted above, the z-coordinate of
Se(2) atoms defines the FeSe4 tetrahedral distorsions dis-
tortions and thereby affects the iron spin state. The dis-
placements of Se(1) in the Bg phonon at 177.2 cm–1 lead to
the decrease-increase of the volume of the FeSe4 tetrahe-
Fig. 14. (Color online) The calculated frequency and main atomic displacement patterns for nine Raman-active Ag modes in the insulat-
ing magnetically and vacancy ordered phase of Rb2Fe4Se5. Only those displacements exceeding 0.2 are shown with their level of dis-
placement given in parentheses. Note the special position of the Se(2) atoms denote as 9 and 10 which are corner shared atoms of four
FeSe4 tetrahedrons. In accordance with the assignment shown in Table 3 three out of the four most intensive Ag modes involve dis-
placements of the Se(2) along the z axis.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6 639
Yu. Pashkevich et al.
dron. This calculated phonon frequency also differs from
the experimental Bg(4) frequency. The most striking devia-
tions occur in the high-frequency part of the spectra, i.e., in
the range of following iron atoms vibrations: the Ag line at
255 cm–1 and the Bg lines at 206 and 264.4 cm–1 at 290 K.
Also, as it follows from the respective displacement patterns,
these vibrations induce strong distortions of the FeSe4 tetrahe-
dron. Note that in all cases the experimental frequencies
are higher than the calculated ones. Using ab initio lattice
dynamic calculations for FeTe it was previously shown that
the high-frequency part of the phonon spectrum always in-
creases if the iron magnetic moment (the iron spin state) de-
creases whereas the low-lying part of the spectrum is mainly
insensitive to the iron spin state [67,68]. Considering this,
the calculated phonon frequencies of the metallic 245 phase
are highly overestimated compared to experimental ones.
While part of this deviation is related to the compressed lat-
tice of the 245 metallic phase, it is mainly induced by the low
spin state of iron.
Thus we conclude that in the phonon calculations of the
iron-based superconductors the influence of the iron-spin
state is not accounted for sufficiently in the “fix-spin”
mode approximation to realistically reproduce the iron
involved vibrations. One should probably take a dynamical
(nonadiabatic) change of the iron spin state into account
which appears already during the iron atom displacements.
Such a phenomenon has been observed in recent femtosecond
pulse experiment of induced lattice distortions in BaFe2As2
where a transient change of the iron magnetic moments upon
lattice oscillations has been detected [71,72].
In spite of a remarkable difference between phonon fre-
quencies of the metallic and insulating 245 phases we do not
expect to detect the former phase in the phonon Raman spec-
tra as the fraction of the metallic 245 phase should be much
smaller than the amount of the minority metallic 122 phase.
Conclusion
We studied the phase separation phenomenon in the su-
perconducting single crystal Rb0.77Fe1.61Se2 with Tc = 32 K
by Raman spectroscopy and ab initio band structure and
lattice dynamics calculations. We identify phonon lines
from the insulating magnetically and vacancy ordered
Rb2Fe4Se5 phase and from the vacancy free nonmagnetic
and superconducting RbδFe2Se2 phase. At temperatures
below Tc we observed an isotropic superconducting gap
with 2 2x y
d
−
symmetry. This observation agrees well
with the Fermi topology which does not contain any hole
pockets at the Brillouin zone center. Our band structure
calculations of the 122 metallic phase confirmed this
conclusion.
The only interplay between the two phases is manifested
in the spectra of the insulating phase as modest anomalies in
the frequency, intensity and halfwidth at the superconducting
phase transition. However, some specific alteration of the
electronic structure in the 122 metallic phase below Tc is seen
in our spectra as an enormous increase of the B1g (212 cm–1)
phonon intensity which belongs to this phase.
We suggest a magnetic origin for the phase separation
which acts if the static magnetic moment of iron exceeds
some threshold level under the magnetic ordering process.
This mechanism implies the appearance of intermediate inter-
face phases between the majority 245 insulating phase and the
minority 122 metallic phase. We show that the compressed
vacancy ordered 245 phase can be one of those intermediate
phases and we highlight that this phase is metallic. This
property can explain how a Josephson coupling mechanism
can be realized in the superconducting M0.8+xFe1.6+ySe2 com-
pounds in the presence of an insulating majority phase.
Table 3. Comparison between experimental data and theory for Ag and Bg modes
Ag modes Bg modes
Experiment
at 290 K
Тheory: insulating
245 phase,
M = 2.62 μB/Fe
Тheory: metallic
245 phase,
M = 0
Experiment
at 290 K
Тheory: insulating
245 phase,
M = 2.62 μB/Fe
Тheory: metallic
245 phase,
M = 0
cm–1 cм–1
Ag (1) – 58.7 61.0 65.8 51.8 62.0
weak 68.8 72.4 weak 58.2 76.9
weak 76.7 85.7 Bg(1) – 79.7 76.9 96.8
Ag(2) – 94.2 94.8 141.8 Bg(2) – 107.8 104.1 128.9
Ag(3) – 120.7 106.6 154.7 Bg(3) – 132.6 127.5 186.9
Ag(4) – 155.7 159.3 208.6 Bg(4) – 188.7 177.3 236.2
Ag(5) – 173.7 A1g mode of 122 phase Bg(5) – 212.4 B1g mode of 122 phase
Ag(6) – 197.6 184.0 271.7 Bg(6) – 215.7 190.2 264.8
Ag(7) – 231.5 199.8 279.1 Bg(7) – 264.4 216.9 319.3
Ag(8) – 255.7 224.8 308.1
640 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6
Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering
Our lattice dynamics calculations show a significant
difference in the phonon spectra of the conductive and in-
sulating Rb2Fe4Se5 phases which is mainly related to dif-
ferent iron spin states in these two phases. We did not ob-
serve additional lines in our spectra which could be
assigned to the conductive Rb2Fe4Se5 phase. However, we
argue that the occurrence of a weak, well-structured, and
highly polarized background observed in our spectra can
be connected with this phase.
Acknowledgments
This paper is devoted to the memory of academician Kirill
Borisovich Tolpygo — prominent Physicist, Teacher and Citi-
zen, who made a great contribution to the lattice dynamics
theory and many other branches of solid state physics.
Authors thanks to Vladimir Pomjakushin for useful dis-
cussions. This work was supported in part by the State
Fund of Fundamental Research of Ukraine and by the
Ukrainian-Russian Grant No. 9-2010, NTH school Con-
tacts in Nanosystems, and DFG. The calculations have
been supported by resources of Ukrainian National GRID
under NASU Grant No. 232. Yu. Pashkevich acknowledg-
es partial support from the Swiss National Science Founda-
tion (grant SNSF IZKOZ2 134161).
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Introduction
Experimental details
Results and discussion
First-principal band structure and lattice dynamic calculations
Conclusion
Acknowledgments
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