Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors
Pressure effects on electronic structure and magnetic properties of iron-based superconductors FeSe1−xTex (x ≃ 0, 0.5 and 1.0) and LaFeAsO are investigated. The superconducting transition was observed at TC ≃ 8, 13.6−14.2, and 26 K in FeSe₀.₉₆₃, FeSe₀.₅Te₀.₅, and LaFeAsO₀.₈₅F₀.₁, respectively. The i...
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Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України
2010
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| Zitieren: | Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors / G.E. Grechnev, V.A. Desnenko, A.V. Fedorchenko, A.S. Panfilov, S.L. Gnatchenko, I.P. Zhuravleva, O.S. Volkova, A.N. Vasiliev, V.V. Tsurkan // Физика и техника высоких давлений. — 2010. — Т. 20, № 4. — С. 60-69. — Бібліогр.: 19 назв. — рос. |
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irk-123456789-693292014-10-12T03:01:43Z Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors Grechnev, G.E. Desnenko, V.A. Fedorchenko, A.V. Panfilov, A.S. Gnatchenko, S.L. Zhuravleva, I.P. Volkova, O.S. Vasiliev, A.N. Tsurkan, V.V. Pressure effects on electronic structure and magnetic properties of iron-based superconductors FeSe1−xTex (x ≃ 0, 0.5 and 1.0) and LaFeAsO are investigated. The superconducting transition was observed at TC ≃ 8, 13.6−14.2, and 26 K in FeSe₀.₉₆₃, FeSe₀.₅Te₀.₅, and LaFeAsO₀.₈₅F₀.₁, respectively. The intrinsic magnetic susceptibility χ in the series of FeSe₀.₉₆₃, FeSe₀.₅Te₀.₅ and FeTe is found to increase gradually approximately 10 times as much with Te content. A kink in the low-field dependence of χ(T) was detected at TM ≃ 135 K for LaFeAsO₀.₈₅F₀.₁. The electronic structures and paramagnetic susceptibilities of the studied compounds are calculated ab initio in external magnetic field as a function of atomic volume and structural parameter Z, which specifies the relative height of chalcogen atoms above iron plane. The calculated field-induced magnetic moments and their volume derivatives indicate that these systems are in a close proximity to a quantum critical point. For FeSe and FeTe the calculated paramagnetic susceptibilities reveal a large magnetovolume effect and a drastic sensitivity to parameter Z. Досліджено вплив тиску на електронну структуру та магнітні властивості надпровідників на основі заліза FeSe1−xTex (x ≃ 0, 0.5 та 1.0) й LaFeAsO. Перехід в надпровідний стан в сполуках FeSe₀.₉₆₃, FeSe₀.₅Te₀.₅ й LaFeAsO₀.₈₅F₀.₁ спостерігався відповідно при TC ≃ 8, 13.6−14.2 та 26 K. Визначено, що притаманна сполукам магнітна сприйнятливість χ в ряду FeSe₀.₉₆₃, FeSe₀.₅Te₀.₅ й FeTe поступово збільшується приблизно на порядок величини зі зростанням концентрації телуру. Для сполуки LaFeAsO₀.₈₅F₀.₁ на виміряній в слабкому полі залежності χ(T) виявлена особливість у вигляді зламу поблизу температури TM ≃ 135 K. Для досліджуваних сполук здійснено ab initio розрахунки електронної структури й парамагнітної сприйнятливості в залежності від атомного об’єму і структурного параметру Z, що визначає висоту атомів халькогену відносно площини атомів заліза. Розрахунки індукованих полем магнітних моментів та їх об’ємних похідних вказують на близькість цих сполук до квантової критичної точки. Для сполук FeSe й FeTe розраховані парамагнітні сприйнятливості виявляють значний магнітооб’ємний ефект та екстремальну чутливість до параметру Z. Изучены эффекты давления на электронную структуру и магнитные свойства сверхпроводников на основе железа FeSe1−xTex (x ≃ 0, 0.5 и 1.0) и LaFeAsO. Переход в сверхпроводящее состояние в соединениях FeSe₀.₉₆₃, FeSe₀.₅Te₀.₅ и LaFeAsO₀.₈₅F₀.₁ наблюдался соответственно при TC ≃ 8, 13.6−14.2 и 26 K. Установлено, что присущая соединениям магнитная восприимчивость χ в ряду FeSe₀.₉₆₃, FeSe₀.₅Te₀.₅ и FeTe монотонно возрастает примерно на порядок величины с повышением концентрации теллура. Для соединения LaFeAsO₀.₈₅F₀.₁ в измеренной в слабом поле зависимости χ(T) была обнаружена особенность в виде излома при TM ≃ 135 K. Для исследуемых соединений выполнены ab initio расчеты электронной структуры и парамагнитной восприимчивости в зависимости от атомного объема и структурного параметра Z, который определяет относительную высоту атомов халькогена над плоскостью железа. Расчеты индуцированных полем магнитных моментов и их объемных производных указывают на близость этих соединений к квантовой критической точке. Для соединений FeSe и FeTe рассчитанные парамагнитные восприимчивости свидетельствуют о значительном магнитообъемном эффекте и сильной чувствительности к структурному параметру Z. 2010 Article Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors / G.E. Grechnev, V.A. Desnenko, A.V. Fedorchenko, A.S. Panfilov, S.L. Gnatchenko, I.P. Zhuravleva, O.S. Volkova, A.N. Vasiliev, V.V. Tsurkan // Физика и техника высоких давлений. — 2010. — Т. 20, № 4. — С. 60-69. — Бібліогр.: 19 назв. — рос. 0868-5924 PACS: 74.70.Xa, 74.20.Pq, 74.25.Ha, 75.30.Cr http://dspace.nbuv.gov.ua/handle/123456789/69329 en Физика и техника высоких давлений Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України |
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Pressure effects on electronic structure and magnetic properties of iron-based superconductors FeSe1−xTex (x ≃ 0, 0.5 and 1.0) and LaFeAsO are investigated. The superconducting transition was observed at TC ≃ 8, 13.6−14.2, and 26 K in FeSe₀.₉₆₃, FeSe₀.₅Te₀.₅, and LaFeAsO₀.₈₅F₀.₁, respectively. The intrinsic magnetic susceptibility χ in the series of FeSe₀.₉₆₃, FeSe₀.₅Te₀.₅ and FeTe is found to increase gradually approximately 10 times as much with Te content. A kink in the low-field dependence of χ(T) was detected at TM ≃ 135 K for LaFeAsO₀.₈₅F₀.₁. The electronic structures and paramagnetic susceptibilities of the studied compounds are calculated ab initio in external magnetic field as a function of atomic volume and structural parameter Z, which specifies the relative height of chalcogen atoms above iron plane. The calculated field-induced magnetic moments and their volume derivatives indicate that these systems are in a close proximity to a quantum critical point. For FeSe and FeTe the calculated paramagnetic susceptibilities reveal a large magnetovolume effect and a drastic sensitivity to parameter Z. |
| format |
Article |
| author |
Grechnev, G.E. Desnenko, V.A. Fedorchenko, A.V. Panfilov, A.S. Gnatchenko, S.L. Zhuravleva, I.P. Volkova, O.S. Vasiliev, A.N. Tsurkan, V.V. |
| spellingShingle |
Grechnev, G.E. Desnenko, V.A. Fedorchenko, A.V. Panfilov, A.S. Gnatchenko, S.L. Zhuravleva, I.P. Volkova, O.S. Vasiliev, A.N. Tsurkan, V.V. Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors Физика и техника высоких давлений |
| author_facet |
Grechnev, G.E. Desnenko, V.A. Fedorchenko, A.V. Panfilov, A.S. Gnatchenko, S.L. Zhuravleva, I.P. Volkova, O.S. Vasiliev, A.N. Tsurkan, V.V. |
| author_sort |
Grechnev, G.E. |
| title |
Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors |
| title_short |
Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors |
| title_full |
Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors |
| title_fullStr |
Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors |
| title_full_unstemmed |
Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors |
| title_sort |
pressure effect on electronic structure and magnetic properties of iron-based high tс superconductors |
| publisher |
Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України |
| publishDate |
2010 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/69329 |
| citation_txt |
Pressure effect on electronic structure and magnetic properties of iron-based high Tс superconductors / G.E. Grechnev, V.A. Desnenko, A.V. Fedorchenko, A.S. Panfilov, S.L. Gnatchenko, I.P. Zhuravleva, O.S. Volkova, A.N. Vasiliev, V.V. Tsurkan // Физика и техника высоких давлений. — 2010. — Т. 20, № 4. — С. 60-69. — Бібліогр.: 19 назв. — рос. |
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Физика и техника высоких давлений |
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Физика и техника высоких давлений 2010, том 20, № 4
© G.E. Grechnev, V.A. Desnenko, A.V. Fedorchenko, A.S. Panfilov, S.L. Gnatchenko,
I.P. Zhuravleva, O.S. Volkova, A.N. Vasiliev, V.V. Tsurkan, 2010
PACS: 74.70.Xa, 74.20.Pq, 74.25.Ha, 75.30.Cr
G.E. Grechnev1, V.A. Desnenko1, A.V. Fedorchenko1, A.S. Panfilov1,
S.L. Gnatchenko1, I.P. Zhuravleva1, O.S. Volkova2, A.N. Vasiliev2,
V.V. Tsurkan3
PRESSURE EFFECT ON ELECTRONIC STRUCTURE AND MAGNETIC
PROPERTIES OF IRON-BASED HIGH TC SUPERCONDUCTORS
1B. Verkin Institute for Low Temperature Physics and Engineering,
National Academy of Sciences of Ukraine
47 Lenin Ave., 61103 Kharkov, Ukraine
E-mail: grechnev@ilt.kharkov.ua
2Moscow State University, Physics Department
119991 Moscow, Russia
3Institute of Applied Physics, Academy of Sciences of Moldova
MD-2028 Chisinau, Republic of Moldova
Pressure effects on electronic structure and magnetic properties of iron-based supercon-
ductors FeSe1−xTex (x ≃ 0, 0.5 and 1.0) and LaFeAsO are investigated. The supercon-
ducting transition was observed at TC ≃ 8, 13.6−14.2, and 26 K in FeSe0.963,
FeSe0.5Te0.5, and LaFeAsO0.85F0.1, respectively. The intrinsic magnetic susceptibility χ in
the series of FeSe0.963, FeSe0.5Te0.5 and FeTe is found to increase gradually approximately
10 times as much with Te content. A kink in the low-field dependence of χ(T) was detected at
TM ≃ 135 K for LaFeAsO0.85F0.1. The electronic structures and paramagnetic suscepti-
bilities of the studied compounds are calculated ab initio in external magnetic field as a
function of atomic volume and structural parameter Z, which specifies the relative height
of chalcogen atoms above iron plane. The calculated field-induced magnetic moments
and their volume derivatives indicate that these systems are in a close proximity to a
quantum critical point. For FeSe and FeTe the calculated paramagnetic susceptibilities
reveal a large magnetovolume effect and a drastic sensitivity to parameter Z.
Keywords: FeAs superconductors, FeSe(Te), pressure effect, magnetization, electronic
structure
Following the discovery of the iron-pnictide high TC superconductors (SCs)
[1], a search for the new SCs extended to a variety of iron-based layered com-
pounds like ROFeAs («1111», R = rare earth metal), AFe2Se2 («122», A = divalent
alkaline earth metal), BFeSe («111», B = alkali metal), and simple FeSe(Te) mate-
rials («11»), which exhibited SC with transition temperatures TC in the range of
Физика и техника высоких давлений 2010, том 20, № 4
61
8–55 K [2–5]. Among iron-based SCs, FeSe1−xTex chalcogenides are distin-
guished by their structural simplicity. Though SC with medium transition tem-
peratures about TC ≃ 8 K was observed for Se deficient FeSe compounds, the
partial replacing of Se with Te has provided TC ≃ 15 K at about 50% Te substitu-
tion [6]. Furthermore, the substantial enhancement of TC was observed in FeSe
under high pressures, yielding the SC transition temperatures as high as TC ≃ 27 K
[7], 34 K [8], 35 K [9], and 37 K [10]. Large pressure effects on TC were also ob-
served for other iron-based SCs [3,11], suggesting an important role of the pres-
sure which provides a controlled modification of superconducting properties
without introducing the disorder effects by chemical substitution or doping.
There is growing anticipation that superconductivity is driven by spin fluctua-
tions due to proximity to magnetic instability in these compounds [2,12]. The itiner-
ant spin-density-wave (SDW) transitions were established in parent compounds of
the Fe-based SCs, which are resulted in relatively small ordered magnetic moments,
and in essentially non-Curie-Weiss behavior of magnetic susceptibility χ with tem-
perature above TSDW [4,5]. On the other hand, the undoped FeTe compound is not
superconducting but magnetically ordered [6], and it was suggested that in FeTe the
local magnetic moments interact via short-range superexchange [4,5].
There is still a controversy regarding an interplay between electronic structure,
magnetism and superconductivity in the iron-based SC compounds. Therefore, fur-
ther studies of magnetic and superconducting properties and their evolution with
pressure can help to elucidate a mechanism of the superconductivity in the Fe-based
SCs. In particular, the studies of pressure effect on magnetic susceptibility of the
iron-based SC systems can shed more light on the nature of their magnetism. We
expect that corresponding pressure (or volume) derivatives of χ are especially sen-
sitive to the mechanism of magnetic interactions. The main objective of this study is
to investigate atomic volume and pressure effects on magnetic properties of the
LaFeAsO and FeSe1−xTex systems, which demonstrate a substantial pressure effect
on TC. The corresponding ab initio calculations of the electronic structure and mag-
netic susceptibility are supplemented by analysis of available experimental data.
The experimental data on magnetic susceptibility behavior of iron-based SCs in
the normal state are still incomplete and contradictory [4–6]. In particular, the
magnetic behaviors of FeSe1−xTex systems are substantially complicated by the
presence of magnetic impurities and secondary phases. Therefore, it is very im-
portant to obtain the intrinsic susceptibility of the Fe-based SCs, in order to verify
theoretical models, which can describe behavior of χ at ambient conditions and
under pressure, and to elucidate effects of spin fluctuations.
In this work the polycrystalline LaFeAsO0.85F0.1 and FeSe1−xTex (x ≃ 0 and 1)
samples were obtained by conventional solid-state synthesis. The single crystals
FeSe1−xTex with x ≃ 0.5 and 1 were grown by slow cooling with the self-flux
method, and two series of samples have been prepared. The phase content of the
samples was checked by the X-ray diffraction method. Hereafter, we will refer to
Физика и техника высоких давлений 2010, том 20, № 4
62
the polycrystalline and single-crystalline samples of FeSe1−xTex as P and S, re-
spectively, followed by the series number. The dc magnetization studies were car-
ried out in the magnetic field up to 50 kOe and the temperature range 4.2−300 K
using a superconducting quantum interference device (SQUID) magnetometer.
For single crystals the magnetic field was applied along the tetragonal c-axis.
As seen in Fig. 1, the studied sample LaFeAsO0.85F0.1 shows the superconducting
transition at temperature TC ≃ 26 K. For the normal state, the χ(T) dependence,
measured in H ≥ 1 T, appears to be relatively flat and roughly described by the
mean value of χav ~ 5·10−4 emu/mol. The peculiar feature of the low-field mag-
netic susceptibility of LaFeAsO0.85F0.1 is a clear cusp at T ≃ 135 K with a sharp
rise of χ with cooling from TM ≃ 135 K to TC ≃ 26 K.
0 50 100 150 200 250 300
–4
0
4
8
12
16
χ,
1
0–4
e
m
u/
m
ol
T, K
Our magnetization M(H) measurements on FeSe1−xTex have revealed a rela-
tively large content of ferromagnetic (FM) impurities in the studied samples. In
high magnetic fields the M(H) dependences show a linear behavior with a slope
determined by the intrinsic magnetic susceptibility of the samples. By their ex-
trapolation to the zero field we have obtained the saturation moment values of FM
impurities in our samples, which fall in the range from 25 to 300 emu/mol, being
weakly dependent on temperature (see Table).
Table
Superconducting transition temperature TC (in K), FM impurity saturation magnetic
moment MS (emu/mol) and host (intrinsic) magnetic susceptibility χ (10−3 emu/mol)
at room and zero temperatures for FeTe1−xSex compounds
χCompound TC MS 290 K 0 K
FeSe0.963 (P) ~ 7 214 0.5 ± 0.1 0.75 ± 0.1
FeSe0.5Te0.5 (S1) 13.5 280 1.3 ± 0.2 1.45 ± 0.2
FeSe0.5Te0.5 (S2) 14.2 9 0.85 ± 0.1 0.4 ± 0.1
FeTe0.95 (P) − 24 2.7 ± 0.2 2.65 ± 0.2
FeTe (S1) − 103 2.9 ± 0.2 3.6 ± 0.2
FeTe (S2) − ~ 0 5.7 ± 0.2 5.45 ± 0.2
Fig. 1. Temperature dependences of dc
magnetic susceptibility of LaFeAsO0.85F0.1
in different magnetic fields H, T: □ – 1.0,
△ – 2.0, ○ – 0.02 [ZFC], ● – 0.02 [FC];
TM ≅ 135 K
Физика и техника высоких давлений 2010, том 20, № 4
63
Despite the pronounced FM impurities effects, the obtained magnetization data
made it possible to estimate with sufficient accuracy the host (intrinsic) magnetic
susceptibilities χhost for FeSe1−xTex samples from the slope of linear part of corre-
sponding M(H) dependence in high fields. The resulted values of χhost at some
fixed temperatures are shown by full circles in Fig. 2. In the figure we also present
the detailed χhost(T) data, which were obtained according to the equation
( )hostχ( ) χ ( ) ( ) ST T M T M H≡ = − , (1)
from temperature dependence of the magnetization M(T) of FeSe1−xTex measured
in magnetic field of 30 kOe. Here the saturation moment value MS of FM impurity
is assumed to be constant and equal to its temperature-averaged value for a given
sample. The experimentally obtained basic superconducting and magnetic char-
acteristics of the FeSe1−xTex samples are summarized in Table.
0 50 100 150 200 250 300
0
1
2
3
4
5
4
3
2
χ,
1
0– 3
e
m
u/
m
ol
T, K
1
To gain a further insight into magnetic properties of the iron-based SC systems
in the normal state, the ab initio calculations of the electronic structure and par-
amagnetic susceptibility are carried out for LaFeAsO, FeSe and FeTe parent com-
pounds within the density functional theory (DFT). The calculations were per-
formed for varying atomic volume at the corresponding experimental lattice pa-
rameter ratios c/a.
Under ambient conditions LaFeAsO and FeSe1−xTex compounds possess the
tetragonal crystal structure (space group P4/nmm). The crystal lattice is composed
of alternating slabs, which are stacked along the c-axis. Each iron layer is sand-
wiched between two chalcogen/pnictogen layers, which form edge-shared tetra-
hedrons around the iron sites. The positions of As (or Se/Te) sheets are fixed by
the internal parameter Z, which represents the height of these atoms above the iron
square plane. This parameter also determines the chalcogen-Fe bond angles.
Crystal structure parameters of LaFeAsO and FeSe1−xTex compounds were estab-
lished in a number of works by means of X-ray and neutron diffraction studies
[1,5–7].
The previous ab initio calculations of the electronic structure of the «1111»-
and «11»-type iron-based SCs were predominantly related to studies of the AFM
Fig. 2. Temperature dependence of the host
magnetic susceptibility for some FeSe1−xTex
compounds: 1 – FeTe (S1), 2 – FeTe0.95 (P),
3 – FeTe0.5Se0.5 (S1), 4 – FeSe0.963 (P).
Full circles correspond to values derived
from the high-field magnetization data
Физика и техника высоких давлений 2010, том 20, № 4
64
and SDW ordering (e.g. Refs. [12–14]).
In this work the electronic structure cal-
culations are carried out for LaFeAsO,
FeSe and FeTe compounds with the
aim to study a paramagnetic response in
an external magnetic field, and to elu-
cidate a nature of pressure effect on
paramagnetism and magnetic instabil-
ity. The ab initio calculations are car-
ried out by employing a full-potential
all-electron relativistic linear muffin-tin
orbital method (FP-LMTO, code RSPt
[15,16]). No shape approximations
were imposed on the charge density or
potential, what is especially important
for the layered crystal structures. The
exchange-correlation potential was trea-
ted within the local spin density appro-
ximation (LSDA, [17]) of the DFT.
The calculated basic features of electronic structure of LaFeAsO, FeSe and
FeTe are in a qualitative agreement with results of earlier calculations [12–14]. In
particular, the detailed density of states (DOS) N(E) of FeSe is presented in Fig. 3.
In the vicinity of the Fermi level EF the d-states of Fe provide the dominant con-
tribution to DOS in the range –2 eV and 2 eV around EF = 0. The p states of chal-
cogen/pnictogen atoms are predominantly extended in the interstitial region, and
their partial contributions to DOS in vicinity of EF are substantially smaller for
FeSe, FeTe and LaFeAsO. As seen in Fig. 3, in FeSe the Fermi level lies at the
steep slope of N(E) at the beginning of a pseudogap of about ≃ 0.7 eV. The
evaluated for LaFeAsO, FeSe and FeTe volume derivatives ( )d ln / d lnFN E V
are found to be positive and equal to 0.94, 1.25 and 1.42, respectively, suggesting
the decrease of N(EF) with uniform reduction of the unit cell volume.
The FP-LMTO-LSDA calculations of the field-induced spin and orbital (Van Vleck)
magnetic moments were carried out for LaFeAsO, FeSe and FeTe self-consistently
within the procedure described in Ref. [16] by means of the Zeeman operator
( )ˆˆμ 2Z BH = H s + I (2)
which was incorporated in the original FP-LMTO Hamiltonian. Here H is the external
magnetic field, ŝ and Î – the spin and orbital angular momentum operators, respec-
tively. For the tetragonal LaFeAsO, FeSe, and FeTe the paramagnetic contributions
χspin and χorb were derived from the field-induced spin and orbital magnetic moments
calculated in an external field of 10 T, which was applied parallel and perpendicular to
the c-axis. The evaluated magnetic anisotropy, which is determined by the orbital con-
Fig. 3. Total density of states of the par-
amagnetic FeSe around EF (solid line) and
the partial contribution of the iron d-states
(dashed line). The Fermi level position (at
0 eV) is marked by a vertical line
Физика и техника высоких давлений 2010, том 20, № 4
65
tribution, appeared to be negligible. The orbital Van Vleck contribution itself is sub-
stantially smaller (about 10%) than the strongly enhanced spin susceptibility χspin.
The calculated behavior of the field-induced magnetization in LaFeAsO is pre-
sented in Fig. 4. Due to the close proximity of the induced spin-polarized state to
the spontaneous magnetic ordering for the experimental lattice parameters, the
calculated total magnetic moment rises to ≃ 0.2µB, which in fact is close to the
experimentally observed magnetic moment of 0.36µB at Fe site in the AFM state
[5]. As seen in Fig. 4, the spontaneous spin polarization occurs in the field-induced
calculations at cell volumes above the theoretical value VLSDA, whereas for V ≤ aLSDA
(approximately 6% lower than the experimental value), a paramagnetic response
was obtained. For the theoretical equilibrium volume the contributions χspin and
χorb are found to be equal to 0.545·10−3 emu/mol and 0.046·10−3 emu/mol, re-
spectively.
For FeSe and FeTe the magnetic response to the external field appeared to be
very sensitive to the height Z of chalcogen species from the Fe plane. The corre-
sponding calculated dependences of magnetic susceptibility for FeSe and FeTe are
given in Figs. 5 and 6, respectively. We can estimate the contributions to χ of
FeSe as χspin = 0.55·10−3 emu/mol and χorb = 0.11·10−3 emu/mol for the experi-
mental values of lattice parameters and Z (Z = 0.26 [7]). Therefore the calculated
field-induced magnetic moments are in agreement with the experimental suscepti-
bility of FeSe in the normal state (Table).
The itinerant nature of the hybridized 3d-states of Fe is an essential condition for
the field- induced calculations of paramagnetic susceptibility. There is a strong ex-
perimental support of this itinerant picture for FeSe, which is expected to be in a
non-magnetic spin-degenerate state [5]. For FeTe, however, a validity of the field-
induced calculations of χ is questionable due to the expected more localized nature
of the 3d-states. Therefore, the calculations for FeTe are performed only for vol-
umes smaller than the experimental volume, and results of these calculations have to
be thoroughly verified by other methods, and compared with experimental data.
Fig. 4. Magnetic moment induced in the
undoped LaFeAsO in the field of H = 10 T
as a function of the cell volume with c/a
and Z parameters fixed to their experi-
mental ambient pressure values. Dashed
vertical lines denote the calculated LSDA
equilibrium volume and the experimental
volume of LaFeAsO (from left to right)
Физика и техника высоких давлений 2010, том 20, № 4
66
Fig. 5. Calculated paramagnetic susceptibility of FeSe as a function of the internal lattice
parameter Z. The unit cell volume and c/a ratio are fixed to their experimental ambient
pressure values (78.4 Å3 and 1.464, [7]). The dashed line is a guide for the eye. The
dashed-dotted line corresponds to the experimental value of Z
Fig. 6. Calculated paramagnetic susceptibility of FeTe as a function of the internal lattice
parameter Z for LSDA optimized (87 Å3) unit cell volume. The c/a ratio is fixed to the
experimental ambient pressure value (1.647 [7]). The dashed line is a guide for the eye
The calculated for FeSe magnetovolume effect d lnχ/d lnV is presented in Fig. 7.
It sharply increases with Z and reaches the values consistent with magnetovolume
effects in transition metals and compounds having substantially exchange-enhanced
paramagnetic susceptibility (e.g. Pd, Sc, TiCo, Ni3Al, YNi5 [16,18,19]). This means
that FeSe compound is on the verge of magnetic instability and close to a quantum
critical point, which can result in strong spin fluctuations.
Within the experimental errors, the obtained data in Table indicate a gradual
increase in magnetic susceptibility for FeSe1−xTex system with increasing of tellu-
rium content. For FeTe compound the paramagnetic state is unstable, and conver-
Fig. 7. Calculated volume dependence of the
paramagnetic susceptibility d ln χ / d lnV
of FeSe as a function of the internal lattice
parameter Z. The c/a ratio is fixed to the
experimental ambient pressure value
(1.464 [7]). The dashed line is a guide for
the eye
Физика и техника высоких давлений 2010, том 20, № 4
67
gence of the self-consistent field-induced LSDA calculations was actually ob-
tained only for reduced lattice parameters. This is especially relevant to parameter
Z, which had to be also reduced for about 10%. Therefore one should consider the
calculated paramagnetic susceptibility of FeTe in Fig. 6 as a rough estimation
which presumably allows to establish a trend for the effect of parameter Z. To
further address the question whether a qualitative agreement between the calcu-
lated χ and experimental data for FeTe in Table I might be fortuitous, the detailed
study of pressure effect on χ is highly desirable.
In conclusion, the superconducting transitions are detected at 8 and 13.6–14.2 K
in FeSe0.963 and FeSe0.5Te0.5 samples, respectively. Magnetic susceptibility of
FeSe1−xTex (x ≃ 0, 0.5 and 1.0) compounds was investigated in the temperature
range 4.2–300 K, and the intrinsic χ in the series of iron chalcogenides FeSe0.963,
FeSe0.5Te0.5 and FeTe is estimated to increase gradually ten times as much with
Te content.
Ab initio calculations of the electronic structure and paramagnetic contributions
to susceptibility of FeSe compound have revealed that this system is in close
proximity to a quantum critical point, and this nearness can result in strong spin
fluctuations. The paramagnetic susceptibility calculated in external magnetic field
appears to be close to the experimental value for FeSe. These results point out that
itinerant magnetism theory is relevant to describe magnetic properties of FeSe
system. For both FeSe and FeTe, the LSDA calculated paramagnetic susceptibili-
ties reveal a large magnetovolume effect and a drastic sensitivity to the structural
parameter Z. Therefore, the experimental study of pressure effect on χ would be
very useful to further address the question about a nature of paramagnetic state in
FeSe1−xTex.
This work has been supported by the Russian-Ukrainian RFBR-NASU project
43-02-10 and 10-02-90409.
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Г.Є. Гречнєв, В.А. Десненко, А.В. Федорченко, А.С. Панфілов, С.Л. Гнатченко,
І.П. Журавльова, О.С. Волкова, А.Н. Васильєв, В.В. Цуркан
ВПЛИВ ТИСКУ НА ЕЛЕКТРОННУ БУДОВУ ТА МАГНІТНІ
ВЛАСТИВОСТІ НАДПРОВІДНИКІВ НА ОСНОВІ ЗАЛІЗА
З ВИСОКИМИ ЗНАЧЕННЯМИ TC
Досліджено вплив тиску на електронну структуру та магнітні властивості над-
провідників на основі заліза FeSe1−xTex (x ≃ 0, 0.5 та 1.0) й LaFeAsO. Перехід в
надпровідний стан в сполуках FeSe0.963, FeSe0.5Te0.5 й LaFeAsO0.85F0.1 спо-
стерігався відповідно при TC ≃ 8, 13.6−14.2 та 26 K. Визначено, що притаманна
сполукам магнітна сприйнятливість χ в ряду FeSe0.963, FeSe0.5Te0.5 й FeTe поступо-
во збільшується приблизно на порядок величини зі зростанням концентрації телу-
ру. Для сполуки LaFeAsO0.85F0.1 на виміряній в слабкому полі залежності χ(T) ви-
явлена особливість у вигляді зламу поблизу температури TM ≃ 135 K. Для
досліджуваних сполук здійснено ab initio розрахунки електронної структури й па-
рамагнітної сприйнятливості в залежності від атомного об’єму і структурного па-
раметру Z, що визначає висоту атомів халькогену відносно площини атомів заліза.
Розрахунки індукованих полем магнітних моментів та їх об’ємних похідних вказу-
ють на близькість цих сполук до квантової критичної точки. Для сполук FeSe й
FeTe розраховані парамагнітні сприйнятливості виявляють значний магнітооб’єм-
ний ефект та екстремальну чутливість до параметру Z.
Ключові слова: FeAs-надпровідники, FeSe(Te)-сполуки, ефект тиску, намагніче-
ність, електронна структура
Физика и техника высоких давлений 2010, том 20, № 4
69
Г.Е. Гречнев, В.А. Десненко, А.В. Федорченко, А.С. Панфилов, С.Л. Гнатченко,
И.П. Журавлева, О.С. Волкова, А.Н. Васильев, В.В. Цуркан
ВЛИЯНИЕ ДАВЛЕНИЯ НА ЭЛЕКТРОННУЮ СТРУКТУРУ
И МАГНИТНЫЕ СВОЙСТВА ВЫСОКОТЕМПЕРАТУРНЫХ
СВЕРХПРОВОДНИКОВ НА ОСНОВЕ ЖЕЛЕЗА
Изучены эффекты давления на электронную структуру и магнитные свойства
сверхпроводников на основе железа FeSe1−xTex (x ≃ 0, 0.5 и 1.0) и LaFeAsO. Пере-
ход в сверхпроводящее состояние в соединениях FeSe0.963, FeSe0.5Te0.5 и
LaFeAsO0.85F0.1 наблюдался соответственно при TC ≃ 8, 13.6−14.2 и 26 K. Установ-
лено, что присущая соединениям магнитная восприимчивость χ в ряду FeSe0.963,
FeSe0.5Te0.5 и FeTe монотонно возрастает примерно на порядок величины с повыше-
нием концентрации теллура. Для соединения LaFeAsO0.85F0.1 в измеренной в слабом
поле зависимости χ(T) была обнаружена особенность в виде излома при TM ≃ 135 K.
Для исследуемых соединений выполнены ab initio расчеты электронной структуры
и парамагнитной восприимчивости в зависимости от атомного объема и структур-
ного параметра Z, который определяет относительную высоту атомов халькогена
над плоскостью железа. Расчеты индуцированных полем магнитных моментов и их
объемных производных указывают на близость этих соединений к квантовой кри-
тической точке. Для соединений FeSe и FeTe рассчитанные парамагнитные воспри-
имчивости свидетельствуют о значительном магнитообъемном эффекте и сильной
чувствительности к структурному параметру Z.
Ключевые слова: FeAs сверхпроводники, FeSe(Te)-соединения, эффект давления,
намагниченность, электронная структура
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