Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors
The magnetic susceptibility χ of the FeSe and FeTe compounds, which form the simplest FeSe(Te) system of the novel iron-based superconductors, is studied in the normal state under hydrostatic pressure. A substantial positive pressure effect on χ is detected at low temperatures for both compounds. At...
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| Опубліковано в: : | Физика и техника высоких давлений |
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| Дата: | 2012 |
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Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України
2012
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| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors / G.E. Grechnev, A.S. Panfilov, V.A. Desnenko, A.V. Fedorchenko, I.P. Zhuravleva, S.L. Gnatchenko, V. Tsurkan, D.A. Chareev, E.S. Kozlyakova, O.S. Volkova, A.N. Vasiliev // Физика и техника высоких давлений. — 2012. — Т. 22, № 4. — С. 28-38. — Бібліогр.: 22 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1860254045854236672 |
|---|---|
| author | Grechnev, G.E. Panfilov, A.S. Desnenko, V.A. Fedorchenko, A.V. Zhuravleva, I.P. Gnatchenko, S.L. Tsurkan, V. Chareev, D.A. Kozlyakova, E.S. Volkova, O.S. Vasiliev, A.N. |
| author_facet | Grechnev, G.E. Panfilov, A.S. Desnenko, V.A. Fedorchenko, A.V. Zhuravleva, I.P. Gnatchenko, S.L. Tsurkan, V. Chareev, D.A. Kozlyakova, E.S. Volkova, O.S. Vasiliev, A.N. |
| citation_txt | Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors / G.E. Grechnev, A.S. Panfilov, V.A. Desnenko, A.V. Fedorchenko, I.P. Zhuravleva, S.L. Gnatchenko, V. Tsurkan, D.A. Chareev, E.S. Kozlyakova, O.S. Volkova, A.N. Vasiliev // Физика и техника высоких давлений. — 2012. — Т. 22, № 4. — С. 28-38. — Бібліогр.: 22 назв. — англ. |
| collection | DSpace DC |
| container_title | Физика и техника высоких давлений |
| description | The magnetic susceptibility χ of the FeSe and FeTe compounds, which form the simplest FeSe(Te) system of the novel iron-based superconductors, is studied in the normal state under hydrostatic pressure. A substantial positive pressure effect on χ is detected at low temperatures for both compounds. At room temperature, this effect is found to be also strong, but it is negative for FeSe and positive for FeTe. Ab initio calculations of the pressure dependent electronic structure and magnetic susceptibility indicate that FeSe and FeTe are close to magnetic instability with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and the height Z of chalcogen species from the Fe plane. The observed large positive pressure effects on χ in FeTe and FeSe at low temperatures are related to considerable sensitivity of the paramagnetism to the internal parameter Z. It is shown that available experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with the calculated behavior of the density of electronic states at the Fermi level.
Магнитная восприимчивость χ соединений FeSe и FeTe, которые образуют простейшую систему FeSe(Te) новых сверхпроводников на основе железа, изучена в нормальном состоянии под действием гидростатического давления. Значительный положительный эффект влияния давления на χ обнаружен при низкой температуре для обоих соединений. При комнатной температуре этот эффект оказывается также сильным, но отрицательным − для FeSe и положительным − для FeTe. Расчеты из первых принципов зависимости электронной структуры и магнитной восприимчивости от давления показали, что FeSe и FeTe близки к магнитной неустойчивости с преобладанием обменно-усиленного спинового парамагнетизма. Вычисленная парамагнитная восприимчивость проявляет сильную зависимость от объема элементарной ячейки и высоты Z слоя халькогена над плоскостью железа. Наблюдаемые большие положительные эффекты давления на χ в FeTe и FeSe при низких температурах связаны со значительной чувствительностью парамагнетизма к внутреннему параметру Z. Показано, что имеющиеся экспериментальные данные о сильной и немонотонной зависимости температурного сверхпроводящего перехода от давления в FeSe качественно коррелируют с рассчитанным поведением плотности электронных состояний на уровне Ферми.
Магнітну сприйнятливість χ сполук FeSe і FeTe, які утворюють найпростішу систему FeSe(Te) нових надпровідників на основі заліза, вивчено в нормальному стані під дією гідростатичного тиску. Значний позитивний ефект тиску на χ визначено при низьких температурах для обох сполук. При кімнатній температурі цей ефект виявляється також сильним, але негативним − для FeSe і позитивним − для FeTe. Розрахунки з перших принципів залежності електронної структури й магнітної сприйнятливості від тиску показали, що FeSe і FeTe близькі до магнітної нестійкості з переважанням обмінно-посиленого спінового парамагнетизму. Обчислена парамагнітна сприйнятливість проявляє сильну залежність від об’єму елементарної комірки й висоти Z шару халькогену над площиною заліза. Великі позитивні ефекти тиску на χ в FeTe і FeSe, які спостерігалися при низьких температурах, пов’язані зі значною чутливістю парамагнетизму до внутрішнього параметру Z. Показано, що наявні експериментальні дані про сильну й немонотонну залежність температурного надпровідного переходу від тиску в FeSe якісно корелюють із розрахованою поведінкою густини електронних станів на рівні Фермі.
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Физика и техника высоких давлений 2012, том 22, № 4
© G.E. Grechnev, A.S. Panfilov, V.A. Desnenko, A.V. Fedorchenko, I.P. Zhuravleva,
S.L. Gnatchenko, V. Tsurkan, D.A. Chareev, E.S. Kozlyakova, O.S. Volkova, A.N. Vasiliev, 2012
PACS: 74.70.Xa, 74.62.Fj, 75.10.Lp, 75.30.Cr, 74.20.Pq
G.E. Grechnev1, A.S. Panfilov1, V.A. Desnenko1, A.V. Fedorchenko1,
I.P. Zhuravleva1, S.L. Gnatchenko1, V. Tsurkan2, D.A. Chareev3,
E.S. Kozlyakova4,5, O.S. Volkova5, A.N. Vasiliev5
PRESSURE EFFECTS ON ELECTRONIC STRUCTURE AND MAGNETIC
PROPERTIES OF THE FeSe(Te) SUPERCONDUCTORS
1B. Verkin Institute for Low Temperature Physics and Engineering, National Academy of
Sciences of Ukraine
47 Lenin Ave., 61103 Kharkov, Ukraine
2Institute of Applied Physics, Academy of Sciences of Moldova
MD-2028 Chisinau, Republic of Moldova
3Institute of Experimental Mineralogy, Russian Academy of Sciences
Chernogolovka, Moscow District 142432, Russia
4Moscow State University, Department of Material Science
Moscow 119991, Russia
5Moscow State University, Physics Department
Moscow 119991, Russia
Received September 19, 2012
The magnetic susceptibility χ of the FeSe and FeTe compounds, which form the simplest
FeSe(Te) system of the novel iron-based superconductors, is studied in the normal state
under hydrostatic pressure. A substantial positive pressure effect on χ is detected at low
temperatures for both compounds. At room temperature, this effect is found to be also
strong, but it is negative for FeSe and positive for FeTe. Ab initio calculations of the
pressure dependent electronic structure and magnetic susceptibility indicate that FeSe
and FeTe are close to magnetic instability with dominating enhanced spin paramag-
netism. The calculated paramagnetic susceptibility exhibits a strong dependence on the
unit cell volume and the height Z of chalcogen species from the Fe plane. The observed
large positive pressure effects on χ in FeTe and FeSe at low temperatures are related to
considerable sensitivity of the paramagnetism to the internal parameter Z. It is shown
that available experimental data on the strong and nonmonotonic pressure dependence of
the superconducting transition temperature in FeSe correlate qualitatively with the cal-
culated behavior of the density of electronic states at the Fermi level.
Keywords: iron-based superconductors, FeSe, FeTe, magnetic susceptibility, pressure
effect, electronic structure
1. Introduction
The novel superconducting FeSe1−xTex compounds have attracted extensive
attention due to the simplest crystal structure among the new families of iron-
Физика и техника высоких давлений 2012, том 22, № 4
29
based layered compounds exhibiting high temperature superconductivity (see
Refs. [1,2] and references therein). This structural simplicity favors experi-
mental and theoretical studies of chemical substitution and high pressure ef-
fects, which are aimed at better understanding of a mechanism of the super-
conductivity, and also at modifying properties of the novel superconducting
materials.
For the FeSe1−xTex compounds, a nonmonotonic behavior of the supercon-
ducting transition temperature with x was found, rising from TC ~ 8 K at x = 0 to a
maximum value of ~ 15 K at x ~– 0.5. Additionally, a large enhancement of TC up
to 35−37 K was observed in FeSe under high pressures P ≈ 90 kbar, indicating
that FeSe is indeed a high temperature superconductor [3,4]. Similar pressure ef-
fects on TC have been also reported for FeSe0.5Te0.5 [1].
The parent compound FeTe is not a superconducting one, but it exhibits pecu-
liar magnetic properties. A drastic drop in the temperature dependence of its mag-
netic susceptibility χ(T) with decreasing temperature was observed at T ~– 70 K,
which is related to the first-order structural phase transition accompanied by the
onset of antiferromagnetic (AFM) order [1,2]. Though the attempts to obtain a
superconducting phase in FeTe under high pressure appeared to be unsuccessful,
the superconductivity at 13 K was detected by applying tensile stress conditions in
thin films of FeTe, which involved in-plane extension and out-of-plane contrac-
tion of the lattice [5].
The tetragonal phase P4/nmm of FeSe undergoes a weak distortion upon cool-
ing to the lower symmetry orthorhombic Cmma phase [3,6]. This transition occurs
within a broad temperature range, about 70−100 K, depending on stoichiometry of
the FeSe1−x samples. Also, the tetragonal FeSe undergoes structural transitions
under high pressures (P ≳ 100 kbar) to the hexagonal non-superconducting
P63mmmc NiAs-type phase, and then to its orthorhombic modification (Pbnm,
MnP-type) [3,7,8].
Though a substantial increase of TC was observed in FeSe under pressure
[3,9−11], these studies did not detect any trace of magnetic ordering. How-
ever, recent NMR studies provided some indication of magnetic phase tran-
sition under pressure [12]. Recently, a static magnetic ordering has been ob-
served above P ~ 100 kbar by means of zero-field muon spin rotation (ZF μSR)
and neutron diffraction [13]. These studies indicated that as soon as magnetic
ordering emerges, the magnetic and the superconducting states coexist, and
both the transition temperatures grow simultaneously with increasing pres-
sure.
Also, it was found that upon applying a pressure, the increase of TC in FeSe1−x
appeared to be nonmonotonic and exhibits a local maximum at P ~– 8 kbar, which
is followed by a local minimum at P ~– 12 kbar [9,10,13]. Thus, there is still a
controversy regarding an interplay between electronic structure, magnetism and
superconductivity in FeSe(Te) compounds. In order to elucidate the role of ex-
Физика и техника высоких давлений 2012, том 22, № 4
30
pected spin fluctuations in superconductivity, it is very important to investigate
the nature of magnetism in FeSe and FeTe and its evolution with temperature and
pressure.
Here we report results of detailed experimental studies of the effect of hydro-
static pressure on magnetic susceptibility of FeSe and FeTe compounds in the
normal state. The experimental investigations are supplemented by ab initio cal-
culations of the electronic structure and magnetic susceptibility of FeSe and FeTe
within the density functional theory (DFT).
2. Experimental details and results
The single crystals of FeSe1−x superconductor were grown in evacuated quartz
ampoules using the KCl/AlCl3 flux technique at stationary gradient of tempera-
ture. The energy dispersive X-ray spectroscopy revealed the FeSe0.95 composition,
labelled in the following as FeSe for simplicity. The FeTe single crystal was
grown by a slow-cooling self-flux method [14] and polycrystalline FeTe0.95 was
prepared by a conventional solid-state synthesis. The tetragonal P4/nmm structure
was checked by X-ray diffraction technique.
Magnetic properties of the samples were tested at T = 4.2–300 K by using
SQUID magnetometer. For FeSe, the superconducting transition is detected in the
range of 6−8 K. The magnetization dependencies of the samples in magnetic field
up to 5 T appeared to be close to linear, indicating that concentrations of ferro-
magnetic impurities are negligibly small.
The temperature dependence of magnetic susceptibility χ(T) of the FeSe single
crystal, measured in magnetic field around H ~– 1 T, is shown in Fig. 1. As is seen,
a substantial growth of susceptibility with temperature was revealed in the normal
state of FeSe, as well as large magnetic anisotropy. The figure also shows the χ(T)
dependence for the FeSe sample, which includes about 50 small arbitrarily ori-
ented single crystals. This sample, further called as «polycrystalline» FeSe, was
used for study of the pressure effect on the magnetic susceptibility. In Fig. 2 the
measured temperature dependencies χ(T) are shown for the FeTe single crystal
(H||c) and for the polycrystalline FeTe0.95. The data exhibit a distinct peculiarity at
T ~– 70 K, which is presumably related to simultaneous structural and magnetic
transition [2].
The study of magnetic susceptibility under helium gas pressure P up to
2 kbar was performed at fixed temperatures of 78 and 300 K by using a pen-
dulum-type magnetometer placed directly in the nonmagnetic pressure cell
[16]. High level of hydrostaticity of pressure is essential to obtain suffi-
ciently precise pressure dependencies of χ, because FeSe and FeTe have in-
homogeneous compressibilities associated with the layered structure. The
measurements were carried out in the field H = 1.7 T and their relative errors
did not exceed 0.5% for FeSe sample and 0.2% for FeTe one. The experi-
mental pressure dependencies χ(P) at different temperatures are shown in
Физика и техника высоких давлений 2012, том 22, № 4
31
Figs. 3 and 4, which demonstrate a linear character. The obtained pressure
effects on magnetic susceptibility dlnχ/dP for the FeSe and FeTe compounds
are compiled in Table 1.
Fig. 1. Temperature dependencies of magnetic susceptibility in the normal state for the
single-crystalline sample FeSe and the «polycrystalline» sample FeSe (dashed line). Data
for the single crystal corresponding to magnetic field directions H⊥c axis and H||c are
denoted by ○ and △ symbols, respectively
Fig. 2. Temperature dependence of the magnetic susceptibility for the FeTe single crystal
(○) and the FeTe0.95 polycrystalline sample (▲)
Fig. 3. Pressure dependencies of the magnetic susceptibility, normalized to its value at P = 0,
for the «polycrystalline» FeSe compound at temperatures 78 (1) and 300 K (2). The solid
lines are guides for the eye
Fig. 4. Pressure dependencies of the magnetic susceptibility, normalized to its value at P = 0,
for the single-crystalline FeTe (solid line) and the polycrystalline FeTe0.95 (dashed line)
compounds at temperatures, K: 1 – 55, 2 – 78, 3 – 300
Физика и техника высоких давлений 2012, том 22, № 4
32
Table 1
Pressure effect on magnetic susceptibility d lnχ/dP for the FeSe and FeTe compounds
dlnχ/dP, Mbar−1
Method T, K
FeSe FeTe
78 10 ± 3 23 ± 1.5Experiment 300 −6.5 ± 1 13 ± 1
Theory ~– 10 ~– 12 ~ 20
Theory* ~– 10 ~– 8 –
*Present calculations with structural parameters from Ref. [15].
3. Computational details and results
Ab initio calculations of the electronic structure and paramagnetic susceptibil-
ity were aimed at the pressure effect on magnetic properties of FeSe and FeTe
compounds in the normal state. At ambient conditions these compounds possesses
the tetragonal PbO-type crystal structure (space group P4/nmm), which is com-
posed by alternating triple-layer slabs. Each iron layer is sandwiched between two
nearest-neighbor layers of chalcogen atoms, which form edge-shared tetrahedron
around the iron sites. The positions of layers are fixed by the structural parameter
Z, which represents the relative height of chalcogen atoms above the iron plane.
The structural parameters of FeSe and FeTe were determined by means of X-ray
and neutron diffraction and given in Refs. [1−3,6,8,17,18].
The purpose of the present calculations was evaluation of the paramagnetic re-
sponse in an external magnetic field and elucidation of the nature and features of
magnetism in the FeSe and FeTe compounds. The electronic structure calculations
were performed by employing a full-potential linear muffin-tin orbital method
(FP-LMTO, code RSPt [19]). The exchange-correlation potential was treated
within the local spin density approximation. The calculated basic features of elec-
tronic structures of FeSe and FeTe are in a qualitative agreement with the results
of previous calculations (see Refs. [15,20]).
To evaluate the paramagnetic susceptibilities of FeSe and FeTe, FP-LMTO
calculations of the field-induced spin and orbital (Van Vleck) magnetic moments
were carried out within the approach described in Refs.[21,22]. The spin-orbit
coupling was incorporated, and the effect of an external magnetic field H was
taken into account self-consistently by means of the Zeeman term:
( )B
ˆˆ2ZH = μ +H s l . (1)
Here μB is the Bohr magneton, ŝ and l̂ are the spin and orbital angular mo-
mentum operators, respectively. The induced in the external field of 10 T spin and
orbital magnetic moments provide the related contributions to the magnetic sus-
ceptibility, χspin and χorb.
It is found that magnetic response to the external field is very sensitive to the
unit cell volume, as well as to the internal parameter Z. The calculated dependen-
Физика и техника высоких давлений 2012, том 22, № 4
33
cies of susceptibility of FeSe as functions of the volume and parameter Z are
given in Figs. 5 and 6, respectively. The similar but more pronounced dependen-
cies of susceptibility as those in Figs. 5 and 6 were also obtained for FeTe.
Fig. 5. Calculated paramagnetic susceptibility of FeSe as a function of unit cell volume. Z
is taken to be 0.26. The arrows indicate the theoretical (1) and experimental (2) equilib-
rium volume values
Fig. 6. Calculated paramagnetic susceptibility of FeSe as a function of Z for the experi-
mental unit cell volume
4. Discussion
Above TC, a substantial growth of susceptibility with temperature was revealed
in the normal state of FeSe up to 300 K (Fig. 1). This fact apparently indicates the
itinerant nature of electronic states of Fe near EF, however the scale of the effect
is larger than might be expected. It is presumably related to a fine structure of
DOS N(E) at EF, but one should expect that FeSe system is driven far from the
ground state at room temperatures. As is seen in Fig. 2, in FeTe χ(T) exhibits an
anomaly at T ~– 70, which is in agreement with the literature data [1,2] and appar-
ently associated with the magnetic and structural transitions.
In order to analyse the experimental data on χ(P) in FeSe and FeTe, we used
the calculated paramagnetic contributions to susceptibility, χspin and χorb. The or-
bital χorb term amounts to about 15% of the total paramagnetic susceptibility for
FeSe and FeTe. The ab initio calculations of the paramagnetic susceptibility of the
tetragonal FeSe and FeTe compounds revealed that these systems are in close
proximity to the quantum critical point and magnetic instability (see Figs. 5 and 6),
and this nearness can result in strong spin fluctuations.
In fact, for FeSe the calculated paramagnetic contribution χspin + χorb has to be
substantially compensated by a diamagnetic one in order to conform with the ex-
perimental data in Fig. 1. From comparison of the calculated paramagnetic suscep-
tibilities in Figs. 5 and 6 with the experimental dependence of χexp(T) in Fig. 1, one
Физика и техника высоких давлений 2012, том 22, № 4
34
can estimate a diamagnetic contribution to magnetic susceptibility of FeSe to be
about –1.5·10–4 emu/mol. This diamagnetism is comparable in absolute value with
the paramagnetic contribution, and presumably comes from conduction electrons.
The observed large pressure effects on magnetic susceptibility of FeSe and
FeTe are intriguing and require examination. Firstly, as can be seen in Fig. 3 and
in Table 1, there is a striking sign difference for the pressure effects on χ in FeSe
at low and room temperatures. Also, the absolute value of this effect is substan-
tially larger than that observed in strongly enhanced itinerant paramagnets [22]. In
FeTe the observed pressure effect on χ appeared to be even larger and positive at
low and room temperatures (see Fig. 4 and Table 1).
In order to clarify mechanisms of the behavior of χ(P), we carried out field-
induced ab initio calculations of paramagnetic susceptibility for FeSe in the range
of 0−10 kbar, based upon the pressure dependent structural parameters obtained
and listed in Ref. [15]. The corresponding calculated Z(P) are in agreement with
the experimental data [3,6,8,18], as is seen in Fig. 7. As a result, the calculated
values of dlnχ/dP for FeSe appeared to be in a qualitative agreement with the ex-
perimental low temperature data, see Table 1.
In order to elucidate the main mechanism of the experimentally observed
strong increase in the magnetic susceptibility of FeSe and FeTe under pressure,
we have also analyzed the pressure effect in terms of the corresponding change of
the volume and Z parameters by using the relation:
d ln ln d ln ln d
d ln d d
X
X
V Z
P V P Z P
χ ∂ χ ∂ χ
= +
∂ ∂
. (2)
The required values of the partial volume and Z derivatives of χ were estimated
from the results of ab initio calculations (as those presented in Figs. 5 and 6 for FeSe),
and were found to be ∂lnχ/∂lnV ~– 8 and ∂lnχ/∂Z ~– 65 for FeSe, and ∂lnχ/∂lnV ~– 40
and ∂lnχ/∂Z ~– 350 for FeTe. The value dlnV/dP = −3 Mbar−1 is taken for the com-
pressibility of FeSe and FeTe, which agrees closely with the experimental values
reported in Refs. [4,6,7]. Also, the optimized value dZ/dP = 0.55 Mbar−1 [15] was
Fig. 7. Calculated pressure behavior of the
internal chalcogen structural parameter Z
for FeSe (taken from Ref. [15], solid line).
Experimental data on parameter Z in FeSe
for the tetragonal phase at T = 190 (△, [6]),
295 (□, [17]) and 300 K (⋄, [18]), and for
the orthorhombic phase at T = 16 (○, [3])
and 50 K (●, [6]). The dashed line is a
guide for the eye
Физика и техника высоких давлений 2012, том 22, № 4
35
adopted for evaluation of Eq. (2). As is seen in Fig. 7, this value of dZ/dP at lower
pressures is in agreement with the experimental data of Ref. [6]. The experiments of
Ref. [3] were basically focused on the higher pressures (up to 120 kbar), and have
not provided detailed data for the lower pressure region.
As far as all parameters entering Eq. (2) are estimated, the first term in (2) re-
sults in a large negative value of about −24 Mbar−1, whereas the second term ap-
pears to be large and positive: ∂lnχ/∂Z × dZ/dP ≃ 36 Mbar−1. The both terms in
Eq. (2) taken together yield the theoretical estimation dlnχ/dP ≃ 12 Mbar−1 for
FeSe, which is qualitatively consistent with the low temperature experimental data
and results of our direct ab initio calculations of χ(P) (see Table 1).
In the absence of reliable data on dZ/dP for FeTe, we can fit the experimental
pressure effect dlnχ/dP ~ 20 Mbar−1 for FeTe in PM state within Eq. (2) by setting
dZ/dP = 0.40 Mbar−1, which provides a large positive value of the second term in
Eq. (2), ∂lnχ/∂ZX × dZ/dP ~– 140 Mbar−1. Actually, the above choice of the dZ/dP
derivative is consistent with the available data on the pressure dependence of Z
parameter for the related FeSe compound, as is seen in Fig. 7.
The above estimations allowed to shed light on the nature of the observed hydro-
static pressure effect on χ in FeTe and FeSe (at low temperatures) by separating ef-
fects of change in structural parameters V and Z. Based on results of the ab initio
calculations, shown in Figs. 5 and 6, it is found that the pressure effect on χ for
FeSe (and also for FeTe) can be represented as a sum of two large in size and com-
peting contributions, related to the pressure dependence of the structural parameters
V and Z. Therefore, the large positive values of the pressure effect dlnχ/dP are de-
termined by a change of Z under pressure, which plays a substantial role in Eq. (2),
taking into account the calculated strong dependences of susceptibility on Z.
The nature of this large positive pressure effect on χ in FeSe is similar to that
for FeTe compound. However, in the case of FeTe, such effect is twice more pro-
nounced, and also takes place at room temperatures, whereas for FeSe dlnχ/dP is
found to be negative at 300 K (see Table 1). The grounds of this difference are
unclear. At the present stage one can presume, that the negative sign of dlnχ/dP
derivative is probably related to the nature of the observed anomalous growth of
χ(T) up to room temperatures (Fig. 1), which is not the case for FeTe.
Basically, the observed positive pressure effect on χ in FeSe at low tempera-
tures correlates with the calculated behavior of the density of states at the Fermi
level N(EF) at low pressures (Fig. 8). At higher pressures, one can see non-
monotonic variation of N(EF) in Fig. 8 which clearly exhibits the consecutive
maximum at 5 kbar and the minimum at 22 kbar. It should be noted, that the pres-
ently calculated behavior of N(EF) under pressure is qualitatively consistent with
the reported experimental dependencies of TC(P) in FeSe (the corresponding
maximum and minimum of TC(P) were observed at ~– 8 kbar and ~– 13 kbar, re-
spectively [9,10,13]). This correlation of pressure dependencies of N(EF) and TC
might be considered as an indication of BCS-like pairing mechanism in FeSe.
Физика и техника высоких давлений 2012, том 22, № 4
36
5. Conclusions
The intrinsic magnetic susceptibility of FeSe compound is found to rise sub-
stantially with temperature, which points to the itinerant nature of the electronic
states of Fe. The origin of the observed about twofold increase of χ in FeSe up to
300 K is puzzling. The precision measurements of magnetic susceptibility were
carried out under hydrostatic gaseous pressure and the strong positive pressure
effect on χ is revealed for FeTe and for FeSe at low temperatures, whereas at
room temperature for FeSe the pressure effect is found to be also strong, but
negative.
Ab initio calculations of the paramagnetic susceptibility of FeSe and FeTe re-
vealed that these systems are in close proximity to the quantum critical point, and
this nearness can result in strong spin fluctuations. Paramagnetic susceptibility of
FeSe and FeTe indicates a strong sensitivity to the unit cell volume V and the height
Z of chalcogen species above the Fe plane. It is found that the observed large posi-
tive pressure effect on χ in FeSe at low temperatures and in FeTe is related to con-
siderable sensitivity of the paramagnetic susceptibility to the internal parameter Z,
determining the dominant positive term in Eq. (2). At higher temperatures, the
anomalous growth of χ(T) in FeSe is apparently reduced by applied pressure, how-
ever the grounds of the negative sign of dlnχ/dP derivative at 300 K are unclear.
The present calculations demonstrate that for the FeSe compound, the behavior
of superconducting transition temperature with pressure correlates with the den-
sity of electronic states at the Fermi level. This fact indicates a possibility of reali-
zation of the BCS-like pairing mechanism in this system. In general, our results
suggest that the itinerant magnetism approach within the DFT-LSDA methods is
relevant to describe the paramagnetic normal state of FeSe and FeTe.
This work was supported by the Russian-Ukrainian RFBR-NASU project 01-
02-12 and 12-02-90405, by NASU Young Scientists Grant 3-2012, and by a grant
of the President of the Russian Federation for State Support of Young Russian
Scientists (MK-1557.2011.5).
Fig. 8. Calculated pressure dependencies of
the density of states at the Fermi level for
FeSe (in states/eV/f.u.). The pressure de-
pendent structural parameters, including
lattice constants and atomic positions, were
taken from the optimization of Ref. [15]
(○) with the small upward shift ΔZ =
= +0.004 to start from the experimental
value of Z (see Z(P) behavior in Fig. 7).
The solid line is a guide for the eye
Физика и техника высоких давлений 2012, том 22, № 4
37
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Физика и техника высоких давлений 2012, том 22, № 4
38
Г.Є. Гречнєв, А.С. Панфілов, В.О. Десненко, О.В. Федорченко, І.П. Журавльова,
С.Л. Гнатченко, В. Цуркан, Д.О. Чарєєв, О.С. Волкова, О.М. Васильєв
ВПЛИВ ТИСКУ НА ЕЛЕКТРОННУ СТРУКТУРУ Й МАГНІТНІ
ВЛАСТИВОСТІ НАДПРОВІДНИКІВ FeSe(Te)
Магнітну сприйнятливість χ сполук FeSe і FeTe, які утворюють найпростішу систе-
му FeSe(Te) нових надпровідників на основі заліза, вивчено в нормальному стані
під дією гідростатичного тиску. Значний позитивний ефект тиску на χ визначено
при низьких температурах для обох сполук. При кімнатній температурі цей ефект
виявляється також сильним, але негативним − для FeSe і позитивним − для FeTe.
Розрахунки з перших принципів залежності електронної структури й магнітної
сприйнятливості від тиску показали, що FeSe і FeTe близькі до магнітної
нестійкості з переважанням обмінно-посиленого спінового парамагнетизму. Об-
числена парамагнітна сприйнятливість проявляє сильну залежність від об’єму еле-
ментарної комірки й висоти Z шару халькогену над площиною заліза. Великі пози-
тивні ефекти тиску на χ в FeTe і FeSe, які спостерігалися при низьких температурах,
пов’язані зі значною чутливістю парамагнетизму до внутрішнього параметру Z.
Показано, що наявні експериментальні дані про сильну й немонотонну залежність
температурного надпровідного переходу від тиску в FeSe якісно корелюють із роз-
рахованою поведінкою густини електронних станів на рівні Фермі.
Ключові слова: надпровідники на основі заліза, FeSe, FeTe, магнітна сприйнят-
ливість, ефект тиску, електронна структура
Г.Е. Гречнев, А.С. Панфилов, В.А. Десненко, А.В. Федорченко, И.Л. Гнатченко,
В. Цуркан, Д.А. Чареев, Е.С. Козлякова, О.С. Волкова, А.Н. Васильев
ВЛИЯНИЕ ДАВЛЕНИЯ НА ЭЛЕКТРОННУЮ СТРУКТУРУ
И МАГНИТНЫЕ СВОЙСТВА СВЕРХПРОВОДНИКОВ FeSe(Te)
Магнитная восприимчивость χ соединений FeSe и FeTe, которые образуют простей-
шую систему FeSe(Te) новых сверхпроводников на основе железа, изучена в нормаль-
ном состоянии под действием гидростатического давления. Значительный положи-
тельный эффект влияния давления на χ обнаружен при низкой температуре для обоих
соединений. При комнатной температуре этот эффект оказывается также сильным, но
отрицательным − для FeSe и положительным − для FeTe. Расчеты из первых принци-
пов зависимости электронной структуры и магнитной восприимчивости от давления
показали, что FeSe и FeTe близки к магнитной неустойчивости с преобладанием об-
менно-усиленного спинового парамагнетизма. Вычисленная парамагнитная восприим-
чивость проявляет сильную зависимость от объема элементарной ячейки и высоты Z
слоя халькогена над плоскостью железа. Наблюдаемые большие положительные эф-
фекты давления на χ в FeTe и FeSe при низких температурах связаны со значительной
чувствительностью парамагнетизма к внутреннему параметру Z. Показано, что имею-
щиеся экспериментальные данные о сильной и немонотонной зависимости темпера-
турного сверхпроводящего перехода от давления в FeSe качественно коррелируют с
рассчитанным поведением плотности электронных состояний на уровне Ферми.
Ключевые слова: сверхпроводники на основе железа, FeSe, FeTe, магнитная вос-
приимчивость, эффект давления, электронная структура
|
| id | nasplib_isofts_kiev_ua-123456789-69571 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0868-5924 |
| language | English |
| last_indexed | 2025-12-07T18:46:36Z |
| publishDate | 2012 |
| publisher | Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України |
| record_format | dspace |
| spelling | Grechnev, G.E. Panfilov, A.S. Desnenko, V.A. Fedorchenko, A.V. Zhuravleva, I.P. Gnatchenko, S.L. Tsurkan, V. Chareev, D.A. Kozlyakova, E.S. Volkova, O.S. Vasiliev, A.N. 2014-10-16T19:35:11Z 2014-10-16T19:35:11Z 2012 Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors / G.E. Grechnev, A.S. Panfilov, V.A. Desnenko, A.V. Fedorchenko, I.P. Zhuravleva, S.L. Gnatchenko, V. Tsurkan, D.A. Chareev, E.S. Kozlyakova, O.S. Volkova, A.N. Vasiliev // Физика и техника высоких давлений. — 2012. — Т. 22, № 4. — С. 28-38. — Бібліогр.: 22 назв. — англ. 0868-5924 PACS: 74.70.Xa, 74.62.Fj, 75.10.Lp, 75.30.Cr, 74.20.Pq https://nasplib.isofts.kiev.ua/handle/123456789/69571 The magnetic susceptibility χ of the FeSe and FeTe compounds, which form the simplest FeSe(Te) system of the novel iron-based superconductors, is studied in the normal state under hydrostatic pressure. A substantial positive pressure effect on χ is detected at low temperatures for both compounds. At room temperature, this effect is found to be also strong, but it is negative for FeSe and positive for FeTe. Ab initio calculations of the pressure dependent electronic structure and magnetic susceptibility indicate that FeSe and FeTe are close to magnetic instability with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and the height Z of chalcogen species from the Fe plane. The observed large positive pressure effects on χ in FeTe and FeSe at low temperatures are related to considerable sensitivity of the paramagnetism to the internal parameter Z. It is shown that available experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with the calculated behavior of the density of electronic states at the Fermi level. Магнитная восприимчивость χ соединений FeSe и FeTe, которые образуют простейшую систему FeSe(Te) новых сверхпроводников на основе железа, изучена в нормальном состоянии под действием гидростатического давления. Значительный положительный эффект влияния давления на χ обнаружен при низкой температуре для обоих соединений. При комнатной температуре этот эффект оказывается также сильным, но отрицательным − для FeSe и положительным − для FeTe. Расчеты из первых принципов зависимости электронной структуры и магнитной восприимчивости от давления показали, что FeSe и FeTe близки к магнитной неустойчивости с преобладанием обменно-усиленного спинового парамагнетизма. Вычисленная парамагнитная восприимчивость проявляет сильную зависимость от объема элементарной ячейки и высоты Z слоя халькогена над плоскостью железа. Наблюдаемые большие положительные эффекты давления на χ в FeTe и FeSe при низких температурах связаны со значительной чувствительностью парамагнетизма к внутреннему параметру Z. Показано, что имеющиеся экспериментальные данные о сильной и немонотонной зависимости температурного сверхпроводящего перехода от давления в FeSe качественно коррелируют с рассчитанным поведением плотности электронных состояний на уровне Ферми. Магнітну сприйнятливість χ сполук FeSe і FeTe, які утворюють найпростішу систему FeSe(Te) нових надпровідників на основі заліза, вивчено в нормальному стані під дією гідростатичного тиску. Значний позитивний ефект тиску на χ визначено при низьких температурах для обох сполук. При кімнатній температурі цей ефект виявляється також сильним, але негативним − для FeSe і позитивним − для FeTe. Розрахунки з перших принципів залежності електронної структури й магнітної сприйнятливості від тиску показали, що FeSe і FeTe близькі до магнітної нестійкості з переважанням обмінно-посиленого спінового парамагнетизму. Обчислена парамагнітна сприйнятливість проявляє сильну залежність від об’єму елементарної комірки й висоти Z шару халькогену над площиною заліза. Великі позитивні ефекти тиску на χ в FeTe і FeSe, які спостерігалися при низьких температурах, пов’язані зі значною чутливістю парамагнетизму до внутрішнього параметру Z. Показано, що наявні експериментальні дані про сильну й немонотонну залежність температурного надпровідного переходу від тиску в FeSe якісно корелюють із розрахованою поведінкою густини електронних станів на рівні Фермі. This work was supported by the Russian-Ukrainian RFBR-NASU project 01-02-12 and 12-02-90405, by NASU Young Scientists Grant 3-2012, and by a grant of the President of the Russian Federation for State Support of Young Russian Scientists (MK-1557.2011.5). en Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України Физика и техника высоких давлений Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors Влияние давления на электронную структуру и магнитные свойства сверхпроводников FeSe(Te) Вплив тиску на електронну структуру й магнітні властивості надпровідників FeSe(Te) Article published earlier |
| spellingShingle | Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors Grechnev, G.E. Panfilov, A.S. Desnenko, V.A. Fedorchenko, A.V. Zhuravleva, I.P. Gnatchenko, S.L. Tsurkan, V. Chareev, D.A. Kozlyakova, E.S. Volkova, O.S. Vasiliev, A.N. |
| title | Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors |
| title_alt | Влияние давления на электронную структуру и магнитные свойства сверхпроводников FeSe(Te) Вплив тиску на електронну структуру й магнітні властивості надпровідників FeSe(Te) |
| title_full | Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors |
| title_fullStr | Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors |
| title_full_unstemmed | Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors |
| title_short | Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors |
| title_sort | pressure effects on electronic structure and magnetic properties of the fese(te) superconductors |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/69571 |
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