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1.
We present results of LDA calculations (band structure, densities of states, Fermi surfaces) for possible iron based superconductor
BaFe2Se3 (Ba123) in normal (paramagnetic) phase. Results are briefly compared with similar data on prototype BaFe2As2 and (K,Cs)Fe2Se2 superconductors. Without doping this system is anti-ferromagnetic with T
Nexp ∼ 250 K and rather complicated magnetic structure. Neutron diffraction experiments indicated the possibility of two possible
spin structures (antiferromagnetically ordered “plaquettes” or “zigzags”), indistinguishable by neutron scattering. Using
LSDA calculated exchange parameters we estimate Neel temperatures for both spin structures within the molecular field approximation
and show τ1 (plaquettes) spin configuration to be more favorable than τ2 (zigzags). 相似文献
2.
Early work on the iron-arsenide compounds supported the view, that a reduced dimensionality might be a necessary prerequisite
for high-T
c superconductivity. Later, however, it was found that the zero-temperature upper critical magnetic field, H
c2(0), for the 122 iron pnictides is in fact rather isotropic. Here, we report measurements of the temperature dependence of
the electrical resistivity, ρ(T), in Ba0.5K0.5Fe2As2 and Ba0.68K0.32Fe2As2 single crystals in zero magnetic field and in Ba0.68K0.32Fe2As2 in static and pulsed magnetic fields up to 60 T. We find that the resistivity of both compounds in zero field is well described
by an exponential term due to inter-sheet umklapp electron-phonon scattering between light electrons around the M point to heavy hole sheets at the Γ point in reciprocal space.
From our data, we construct an H-T phase diagram for the inter-plane (H | c) and in-plane (H | ab) directions for Ba0.68K0.32Fe2As2. Contrary to published data for 122 underdoped FeAs compounds, we find that H
c2(T) is in fact anisotropic in optimally doped samples down to low temperatures. The anisotropy parameter, γ = H
c2
ab
/H
c2
c
, is about 2.2 at T
c
. For both field orientations we find a concave curvature of the H
c2 lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism,
we perfectly can describe H
c2 and its anisotropy. 相似文献
3.
Y. Xiao Y. Su C. M. N. Kumar C. Ritter R. Mittal S. Price J. Perßon Th. Brückel 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,82(2):113-121
The physical and structural properties of Fe1.11Te and
Fe1.11Te0.5Se0.5 have been investigated by means of
X-ray and neutron diffraction as well as physical property
measurements. For the Fe1.11Te compound, the structure
distortion from a tetragonal to monoclinic phase takes place at 64 K
accompanied with the onset of antiferromagnetic order upon cooling.
The magnetic structure of the monoclinic phase was confirmed to be
of antiferromagnetic configuration with a propagation vector
k = (1/2, 0, 1/2) based on Rietveld refinement of neutron
powder diffraction data. The structural/magnetic transitions are
also clearly visible in magnetic, electronic and thermodynamic
measurements. For superconducting Fe1.11Te0.5Se0.5
compound, the superconducting transition with T
c
= 13.4 K
is observed in the resistivity and ac susceptibility measurements.
The upper critical field H
c2 is obtained by measuring
the resistivity under different magnetic fields. The Kim’s critical
state model is adopted to analyze the temperature dependence of the
ac susceptibility and the intergranular critical current density is
calculated as a function of both field amplitude and temperature.
Neutron diffraction results show that
Fe1.11Te0.5Se0.5 crystalizes in tetragonal structure
at 300 K as in the parent compound Fe1.11Te and no structural
distortion is detected upon cooling to 2 K. However an anisotropic
thermal expansion anomaly is observed around 100 K. 相似文献
4.
Yu. A. Aleshchenko A. V. Muratov V. M. Pudalov E. S. Zhukova B. P. Gorshunov F. Kurth K. Iida 《JETP Letters》2012,94(9):719-722
The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(Fe0.9Co0.1)2As2 are reported. The reflectivity is found to be close to unity at frequencies ω lower than 2Δ/h (2Δ is the superconducting gap and h is Planck’s constant). This is evidence for the s
+/− or s
+/+ symmetry of the superconducting order parameter in the studied compound. The infrared reflectivity spectra of Ba(Fe0.9Co0.1)2As2 manifest opening of several superconducting gaps at temperatures lower than critical T
c
. 相似文献
5.
We report measurements of the temperature dependence of the electrical resistivity, ρ(T), and magnetic pen-etration depth, λ(T), for polycrystalline samples of Eu0.5K0.5Fe2As2 with T
c
= 31 K. ρ(T) follows a linear temperature dependence above T
c
and bends over to a weaker temperature dependence around 150 K. The magnetic penetration depth, determined by radio frequency
technique displays an unusual minimum around 4 K which is associated with short-range ordering of localized Eu3+ moments.
The article is published in the original. 相似文献
6.
The electronic structure of the recently discovered superconductor SrPt2As2 with T
c
= 5.2 K has been calculated in the local-density approximation. Despite its chemical composition and crystal structure are
somehow similar to FeAs-based high-temperature superconductors, the electronic structure of SrPt2As2 is very much different. The crystal structure is orthorhombic (or tetragonal if idealized) and has layered nature with alternating
PtAs4 and AsPt4 tetrahedra slabs sandwiched with Sr ions. The Fermi level is crossed by Pt-5d states with rather strong admixture of As-4p states. Fermi surface of SrPt2As2 is essentially three-dimensional, with complicated sheets corresponding to multiple bands. We compare SrPt2As2 with 1111 and 122 representatives of FeAs-class of superconductors, as well as with isovalent (Ba,Sr)Ni2As2 superconductors. Brief discussion of superconductivity in SrPt2As2 is also presented. 相似文献
7.
E. Z. Kuchinskii I. A. Nekrasov Z. V. Pchelkina M. V. Sadovskii 《Journal of Experimental and Theoretical Physics》2007,104(5):792-804
Pseudogap phenomena are observed for the normal underdoped phase of different high-T
c cuprates. Among others, the Bi2Sr2CaCu2O8 − δ (Bi2212) compound is one of the most studied experimentally. To describe the pseudogap regime in Bi2212, we use a novel generalized
ab initio LDA + DMFT + Σk hybrid scheme. This scheme is based on the strategy of one of the most powerful computational tools for real correlated materials:
the local density approximation (LDA) + dynamical mean-field theory (DMFT). Conventional LDA + DMFT equations are here supplied
with an additional (momentum-dependent) self-energy Σk in the spirit of our recently proposed DMFT + Σk approach taking into account pseudogap fluctuations. In the present model, Σk describes nonlocal correlations induced by short-range collective Heisenberg-like antiferromagnetic spin fluctuations. The
effective single-impurity problem of the DMFT is solved by the numerical renormalization group (NRG) method. Material-specific
model parameters for the effective x
2 − y
2 orbital of Cu-3d shell of the Bi2212 compound, e.g., the values of intra-and interlayer hopping integrals between different Cu sites, the
local Coulomb interaction U, and the pseudogap potential Δ were obtained within the LDA and LDA + DMFT schemes. Here, we report
on the theoretical LDA + DMFT + Σk quasiparticle band dispersion and damping, Fermi surface renormalization, momentum anisotropy of (quasi)static scattering,
densities of states, spectral densities, and angular-resolved photoemission (ARPES) spectra, taking into account pseudogap
and bilayer splitting effects for normal (slightly) underdoped Bi2212 (δ = 0.15). We show that LDA + DMFT + Σk successfully describes strong (pseudogap) scattering close to Brillouin zone boundaries. Our calculated LDA + DMFT + Σk Fermi surfaces and ARPES spectra in the presence of pseudogap fluctuations are almost insensitive to the bilayer splitting
strength. However, our LDA-calculated value of bilayer splitting is rather small to describe the experimentally observed peak-dip-hump
structure. The results obtained are in good semiquantitative agreement with various recent ARPES experiments.
The article was submitted by the authors in English. 相似文献
8.
Ruiting Sun Xiaoli Wang Jing Shi Le Wang 《Applied Physics A: Materials Science & Processing》2011,104(1):129-133
New perovskite solid solution ceramics of (1−x)BaTiO3-xBi(Mg1/2Ti1/2)O3 ((1−x)BT-xBMT, x≤0.09) were synthesized by the solid-state reaction technique. X-ray diffraction studies have revealed a stable single perovskite
structure for all samples. Dielectric measurements were carried out at different frequencies and temperatures. The polarization
evolutions with temperatures were measured to investigate the ferroelectric properties. All the compositions show features
of ferroelectrics with diffuse phase transition, though the temperature T
m
of their dielectric constant maximum ε
m
is frequency dependent. The dielectric constant peak ε(T) of (1−x)BT-xBMT ceramics become broad with increasing BMT content. During the temperature range of ε(T) peak summit, (1−x)BT-xBMT ceramics present quasi-linear dielectric phenomenon under high electric field with very high dielectric constant. 相似文献
9.
Pramod Bhatt E. Carlegrim A. Kanciurzewska M. P. de Jong M. Fahlman 《Applied Physics A: Materials Science & Processing》2009,95(1):131-138
Thin film iron-tetracyanoethylene Fe(TCNE)
x
, x∼2, as determined by photoelectron spectroscopy, was grown in situ under ultra-high vacuum conditions using a recently developed
physical vapor deposition-based technique for fabrication of oxygen- and precursor-free organic-based molecular magnets. Photoelectron
spectroscopy results show no spurious trace elements in the films, and the iron is of Fe2+ valency. The highest occupied molecular orbital of Fe(TCNE)
x
is located at ∼1.7 eV vs. Fermi level and is derived mainly from the TCNE− singly occupied molecular orbital according to photoelectron spectroscopy and resonant photoelectron spectroscopy results.
The Fe(3d)-derived states appear at higher binding energy, ∼4.5 eV, which is in contrast to V(TCNE)2 where the highest occupied molecular orbital is mainly derived from V(3d) states. Fitting ligand field multiplet and charge
transfer multiplet calculations to the Fe L-edge near edge X-ray absorption fine structure spectrum yields a high-spin Fe2+ (3d6) configuration with a crystal field parameter 10Dq∼0.6 eV for the Fe(TCNE)
x
system. We propose that the significantly weaker Fe-TCNE ligand interaction as compared to the room temperature magnet V(TCNE)2 (10Dq∼2.3 eV) is a strongly contributing factor to the substantially lower magnetic ordering temperature (T
C
) seen for Fe(TCNE)
x
-type magnets. 相似文献
10.
We have studied photoluminescence and thermoluminescence (PL and TL) in CaGa2Se4:Eu crystals in the temperature range 77–400 K. We have established that broadband photoluminescence with maximum at 571 nm
is due to intracenter transitions 4f6 5d–4f7 (8S7/2) of the Eu2+ ions. From the temperature dependence of the intensity (log I–103/T), we determined the activation energy (E
a = 0.04 eV) for thermal quenching of photoluminescence. From the thermoluminescence spectra, we determined the trap depths:
0.31, 0.44, 0.53, 0.59 eV. The lifetime of the excited state 4f6 5d of the Eu2+ ions in the CaGa2Se4 crystal found from the luminescence decay kinetics is 3.8 μsec.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 112–116, January–February, 2009. 相似文献
11.
Influence of an external magnetic field on the reluctance of the YBa
2
Cu
3
O
x ceramics is investigated. A significant reluctance of the oxygen-deficient ceramics (with critical temperature Tc < 77 K) is established for a sample in the normal state at T < 160 K. It is demonstrated that after ceramics annealing that
restores the oxygen content to a nearly optimum value, the magnetic field has essentially no effect on the sample reluctance
at temperatures exceeding Tc. To explain the revealed mechanisms, a model involving ferromagnetic clusters effectively decreasing the free carrier density
is used.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 68–71, April, 2007. 相似文献
12.
The heavy-fermion metal CePd1−x
Rh
x
can be tuned from ferromagnetism at x = 0 to the nonmagnetic state at some critical concentration x
c
. The non-Fermi liquid behavior (NFL) at x ≃ x
c
is recognized by the power-law dependence of the specific heat C(T) given by the electronic contribution susceptibility X(T) and volume expansion coefficient α(T) at low temperatures: C/T ∝ X(T) ∝ α(T)/T∝ 1/ √T. We also demonstrate that the behavior of the normalized effective mass M
N
*
observed in CePd1−x
Rh
x
at x ≃ 0.8 agrees with that of M
N
*
observed in paramagnetic CeRu2Si2 and conclude that these alloys exhibit the universal NFL thermodynamic behavior at their quantum critical points. We show
that the NFL behavior of CePd1−x
Rh
x
can be accounted for within the frameworks of the quasiparticle picture and fermion condensation quantum phase transition,
while this alloy exhibits a universal thermodynamic NFL behavior that is independent of the characteristic features of the
given alloy such as its lattice structure, magnetic ground state, dimension, etc.
The text was submitted by the authors in English. 相似文献
13.
S. Wongsaenmai W. Qu S. Ananta R. Yimnirun X. Tan 《Applied Physics A: Materials Science & Processing》2007,88(4):757-761
Ferroelectric ceramics with formula Pb0.8Ba0.2[(In1/2Nb1/2)1-xTix]O3 (PBINT) (x=0.0,0.1,0.2,0.3,0.4 and 0.5) were prepared via a two-step solid state reaction method. It was found that ceramics
with compositions in the range of x=0.0∼0.3 showed a pseudo-cubic structure, whereas the ceramic with x=0.5 displayed a tetragonal
structure. All compositions showed significant frequency dispersion in their dielectric properties. The remanent polarization
Pr as well as the coercive field Ec, measured at room temperature, increases with the Ti content. The experimental results obtained in this system are summarized
into a phase diagram, with the morphotropic phase boundary (MPB) located at x=0.4. Compared with the Pb[(In1/2Nb1/2)1-xTix]O3 solid solution system, incorporating Ba in the A-site leads to a significant decrease in the dielectric maximum temperature
Tmax, a suppression of the dielectric relaxation parameter γ, and a shift of the MPB composition to a higher Ti content.
PACS 77.84.Dy; 77.80.Bh; 77.22.Ch 相似文献
14.
N. Seeburrun H. H. Abdallah E. F. Archibong P. Ramasami 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2011,63(3):351-358
We hereby report a theoretical study on the equilibrium geometries, electronic structures and harmonic vibrational frequencies
of Ga2Se3, Ga3Se2 and their anions. The ground and low-lying excited states of Ga2Se3−, Ga2Se3, Ga3Se2− and Ga3Se2 are studied at the B3LYP and/or MP2 and CCSD(T) levels in conjunction with 6-311+G(d) and 6-311+G(2df) one particle basis
sets. Ga2Se2− adopts the C2v
kite geometry while Ga2Se3 has a ‘V’ geometry. Ga3Se2− has a three-dimensional ‘D3h
’ geometry and Ga3Se2 prefers the three-dimensional ‘C2v
’ structure. Electron detachment energies from the ground electronic states of the anions to several neutral states are reported
and discussed. At CCSD(T)//MP2 level, the adiabatic electron affinity (AEA) of Ga2Se3 is calculated to be 3.23 eV when using the 6-311+G(2df) basis set and that of Ga3Se2 is 2.77 eV with the 6-311+G(d) basis set. The findings of this research are analyzed and compared with gallium oxide and
sulfide analogues. 相似文献
15.
G. O. Andrianov S. A. Nemov R. V. Parfen’ev D. V. Shamshur A. V. Chernyaev 《Physics of the Solid State》2010,52(9):1810-1814
This paper reports on a study of the low-temperature conductivity and parameters of the superconducting state, namely, the
critical temperature T
c
and the second critical magnetic field Hc2, in the (Pb0.3Sn0.7)0.95In0.05Te solid solution under hydrostatic pressure P ≤ 9 kbar at T = 4.2 K. The choice of this material has been motivated by the fact that, according to earlier observations, it undergoes
a superconducting transition at T
c
∼ 2.3 K, i.e., close to the maximum value T
c
∼ 2.9 K found for the (Pb
z
Sn1 − z
)0.95In0.05Te solid solutions with a lead content z ∼ 0.15–0.25. It has been demonstrated that an increase in the pressure to P ≤ 9 kbar leads to a bell-shaped dependence T
c
(P). The observed dependences are assigned to the effect of hydrostatic compression on the band structure of the solid solution
and indicate a shift in the position of the Fermi level E
F with increasing pressure within the impurity band of the In quasi-local states. In this case, E
F passes through a maximum in the density of impurity states at P = 3–5 kbar. 相似文献
16.
V. A. Ryzhov P. L. Molkanov A. V. Lazuta V. V. Runov V. P. Khavronin I. O. Troyanchuk 《Bulletin of the Russian Academy of Sciences: Physics》2010,74(10):1477-1480
We present the results from studying the magnetic properties (linear and nonlinear susceptibilities and the depolarization
of polarized neutrons) of Nd1 − x
Ba
x
MnO3 manganite, x = 0.3, with Curie temperature T
C ≈ 140 K and dielectric-to-metal transition temperature T
DM ≈ 129 K. Its critical behavior corresponds to that of an isotropic 3-D ferromagnet at temperatures above T*≈ 144 K, but a strong nonlinear response in weak magnetic fields and depolarization are observed at temperatures below T*. It is shown that this nontraditional behavior is related to the generation of ferromagnetic clusters in the paramagnetic
matrix that form a conducting percolative network at temperatures near T
DM. 相似文献
17.
A. Yu. Mollaev I. K. Kamilov R. K. Arslanov T. R. Arslanov U. Z. Zalibekov V. M. Novotortsev S. F. Marenkin 《JETP Letters》2010,91(9):478-480
The magnetic susceptibility χ/χ0 and the longitudinal Δρ
zz
/ρ0 and transverse Δρ
xx
/ρ0 magnetoresistances have been measured as functions of the hydrostatic pressure P ≤ 7 GPa at room temperature in the high-temperature ferromagnetic semiconductor Cd0.7Mn0.3GeAs2 with a chalcopyrite structure and the Curie temperature T
c = 355 K. A pressure-induced metamagnetic transition from the low-magnetization state to the high-magnetization state has
been observed in Cd0.7Mn0.3GeAs2 near the magnetic ordering temperature. This transition is accompanied by the hysteresis of the magnetic susceptibility and
magnetoresistance. 相似文献
18.
Alexander G. Andreev Stefka G. Tsintsarska Maria Dimitrova-Ivanovich Ivaylo Polyanski Mladen Georgiev Alexander D. Gochev 《Central European Journal of Physics》2004,2(2):329-356
The temperatureT dependencies ρ(T) of normal state electric resistivitiesρ
c
(axial) andρ
ab
(in plane) of single-layer high-T
c
superconductors show common trends: AsT is raised, the resistivity first drops steeply before it starts rising αT above an apparent semiconductor-to-metal crossoverT
cross
. To analyze ρ(T) we plottT/ρ againstT at various dopingsx for bothρ
c
andρ
ab
.T/ρ is inversely proportional to the traversal time across a potential barrier as an ionic particle drifts in an electric field.
We findT/ρ in good agreement with theT dependence of the quantum rate of migrating particles: AsT is raised, a zero-point rate at the lowestT is extended to a nearly flat plateau before a thermally activated branch sets in. We also find evidence for the admixture
of 1- & 2-phonon absorptions below the Arrhenius range. These features shape the semiconductor-like branch below T
cross
. AboveT
cross
a metallic-like branch sets in, its αT character deriving from the field coupling of the migrating particle. Our analysis suggests that metal physics may not suffice
if ionic features play a role in transport. We attribute our conclusions to the drift of strong-coupling polarons along Cu−O
bonds. These “bond polarons” originate from carrier scattering by double-well potentials associated with the bonds. A bond
polaron dissociates to a free hole as it passes onto a neighboring O-O link. 相似文献
19.
The crystal structure and magnetic properties of Bi1 − x
A
x
FeO3 − x/2 (A = Ca, Sr, Pb, Ba), Bi1 − x
A
x
(Fe1 − x
Ti
x
)O3, and Bi1 − x
A
x
(Fe1 − x/2Nb
x/2)O3 solid solutions have been studied. It is shown that the homogeneous polar weak ferromagnetic state occurs in the vicinity
of a morphotropic phase boundary in the systems where dopant ions lead to the reduction of the unit cell volume in the polar
phase. In the case of A = Ca, the non-polar phase also exhibits weak ferromagnetism and the spontaneous magnetizations in
the polar and nonpolar phases differ only slightly. 相似文献
20.
We present the results a study of structure by neutron diffraction and data on the magnetic properties (linear and nonlinear
(second and third order) susceptibilities) of polycrystalline La0.88MnO2.95. This compound exhibits an insulator-metal (IM) phase transition at T
IM
≈ 253 K (above the Curie temperature, T
C
≈ 244 K) and reveals colossal magnetoresistance. The crystal structure is found to be rhombohedral, and the space group is
R3c. Analysis of magnetic properties shows that at T* ≈ 258 K > T
C
, isolated paramagnetic clusters occur in the paramagnetic matrix; their concentration increases upon cooling. We observed
no noticeable differences between the temperature evolution of the clustered state of this manganite with its insulator-metal
transition and in the insulator La0.88MnO2.91. Possible scenarios of the paramagnet-ferromagnet and I-M transitions in a self-organized clustered structure are discussed. 相似文献