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1.
A. G. Volkov A. A. Povzner V. V. Kryuk P. V. Bayankin 《Physics of the Solid State》1999,41(10):1644-1648
The influence of spin fluctuations on the energy spectra of sp and d current carriers in almost ferromagnetic semiconductors based on compounds of d transition metals is examined. It is shown that because electron spectra split in the fluctuating exchange fields in almost
ferromagnetic systems, electronic transitions of the type semiconductor-metal are possible, accompanied by the disappearance
of energy gaps in the spectra of the sp and d electrons at various temperatures and by a shift of the chemical potential into the region of allowed energies. A specific
analysis of similar electronic transitions is presented, based on the almost ferromagnetic compound FeSi.
Fiz. Tverd. Tela (St. Petersburg) 41, 1792–1796 (October 1999) 相似文献
2.
In this note we consider long-range q-states Potts models on Z
d
, d≥ 2. For various families of non-summable ferromagnetic pair potentials φ(x)≥ 0, we show that there exists, for all inverse temperature β > 0, an integer N such that the truncated model, in which all interactions between spins at distance larger than N are suppressed, has at least q distinct infinite-volume Gibbs states. This holds, in particular, for all potentials whose asymptotic behaviour is of the
type φ(x)∼ ‖x‖−α, 0≤α≤ d. These results are obtained using simple percolation arguments.
Work supported by Swiss National Foundation for Science, Conselho Nacional de Desenvolvimento Cientìfico e Tecnològico, and
Programa de Auxìlio para Recèm Doutores PRPq-UFMG. 相似文献
3.
Based on the idea of a strong interaction within the same unit cell, the possible existence of a ferromagnetic instability
in a system with jumps from transition element cations to non-transition element anions and vice versa is established. A phase
diagram is constructed for the ferromagnetic ordering as a function of the degree of filling, n
p
and n
d
, of the p
6-and d
10-shells of non-transition and transition elements, respectively.
Zh. éksp. Teor. Fiz. 115, 1393–1410 (April 1999) 相似文献
4.
We consider the Q-state Potts model on Z
d
, Q≥ 3, d≥ 2, with Kac ferromagnetic interactions and scaling parameter γ. We prove the existence of a first order phase transition
for large but finite potential ranges. More precisely we prove that for γ small enough there is a value of the temperature
at which coexist Q+1 Gibbs states. The proof is obtained by a perturbation around mean-field using Pirogov-Sinai theory. The result is valid
in particular for d = 2, Q = 3, in contrast with the case of nearest-neighbor interactions for which available results indicate a second order phase
transition. Putting both results together provides an example of a system which undergoes a transition from second to first
order phase transition by changing only the finite range of the interaction. 相似文献
5.
We consider continuous-spin models on the d-dimensional hypercubic lattice with the spins σ
x
a priori uniformly distributed over the unit sphere in ℝ
n
(with n≥2) and the interaction energy having two parts: a short-range part, represented by a potential Φ, and a long-range antiferromagnetic
part λ|x−y|−s
σ
x
⋅σ
y
for some exponent s>d and λ≥0. We assume that Φ is twice continuously differentiable, finite range and invariant under rigid rotations of all spins.
For d≥1, s∈(d,d+2] and any λ>0, we then show that the expectation of each σ
x
vanishes in all translation-invariant Gibbs states. In particular, the spontaneous magnetization is zero and block-spin averages
vanish in all (translation invariant or not) Gibbs states. This contrasts the situation of λ=0 where the ferromagnetic nearest-neighbor systems in d≥3 exhibit strong magnetic order at sufficiently low temperatures. Our theorem extends an earlier result of A. van Enter ruling
out magnetized states with uniformly positive two-point correlation functions. 相似文献
6.
Z. Kovacevic R. Hayn N.M. Plakida 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):487-495
In the framework of the cell-perturbation method for the original p-d model an effective two-band Hubbard model for the CuO2 plane with Zn impurities is derived. Zn impurities are modelled by Wannir oxygen one-hole states at vacant Cu sites. The model is based on the results of band structure calculations carried out
within the local-density approximation. Further reduction to an extended t-J model shows a large ferromagnetic superexchange interaction between the Cu spin with the nearest virtual oxygen spin in the
Zn cell.
Received 17 November 1998 相似文献
7.
We consider a class of spin systems on ℤ
d
with vector valued spins (S
x
) that interact via the pair-potentials J
x,y
S
x
⋅S
y
. The interactions are generally spread-out in the sense that the J
x,y
's exhibit either exponential or power-law fall-off. Under the technical condition of reflection positivity and for sufficiently
spread out interactions, we prove that the model exhibits a first-order phase transition whenever the associated mean-field
theory signals such a transition. As a consequence, e.g., in dimensions d≥3, we can finally provide examples of the 3-state Potts model with spread-out, exponentially decaying interactions, which
undergoes a first-order phase transition as the temperature varies. Similar transitions are established in dimensions d = 1,2 for power-law decaying interactions and in high dimensions for next-nearest neighbor couplings. In addition, we also
investigate the limit of infinitely spread-out interactions. Specifically, we show that once the mean-field theory is in a
unique “state,” then in any sequence of translation-invariant Gibbs states various observables converge to their mean-field
values and the states themselves converge to a product measure. 相似文献
8.
D. Coupier 《Journal of statistical physics》2006,123(2):473-495
A d-dimensional Ising model on a lattice torus is considered. As the size n of the lattice tends to infinity, a Poisson approximation is given for the distribution of the number of copies in the lattice of any given local configuration, provided the magnetic field a = a(n) tends to −∞ and the pair potential b remains fixed. Using the Stein-Chen method, a bound is given for the total variation error in the ferromagnetic case.
AMS SUBJECT CLASSIFICATION: 60F05, 82B20. 相似文献
9.
A. N. Artemov 《JETP Letters》1999,69(9):682-687
The thermodynamics of a system of Pearl vortices in a superconducting thin film containing radiation defects is studied. It
is shown that three phase transition scenarios are possible, depending on the defect density. At low densities there is one
stable state of the system. When the first critical density n
d
1 is reached, there appears a temperature interval in which the system can be in two stable states. If the density exceeds
n
d
2, then the lower limit of stability of the metastable states shifts abruptly to zero.
Pis’ma Zh. éksp. Teor. Fiz. 69, No. 9, 643–648 (10 May 1999) 相似文献
10.
C.M. Teodorescu F. Chevrier R. Brochier C. Richter V. Ilakovac O. Heckmann P. De Padova K. Hricovini 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(3):305-313
Interface reaction and magnetism of epitaxially-grown Fe on InAs(100) are studied by core-level photoemission (As 3d and In 4d) and Fe 2p X-ray magnetic circular dichroism using synchrotron radiation. The reactivity of Fe/InAs(100) is relatively low compared
to that of other interfaces involving deposition of 3d metals on III-V semiconductors. As a consequence, we observe a magnetic signal at Fe L2, 3 edges for the lowest thicknesses studied (1 ML). The atomic magnetic moment reaches a value close to that of the bulk α-Fe
(2.2 μ
B) for Fe coverages exceeding 5 ML. A ferromagnetic compound with approximate stoichiometry of FeAs is formed at the interface.
The orbital magnetism represents between 12 and 20% of the total momentum, due to 3d density of states depletion and to crystal-field modification of the electronic levels. These properties make the Fe/InAs(100)
interface very promising for spin-tunneling devices.
Received 4 April 2002 / Received in final form 13 May 2002 Published online 31 July 2002 相似文献
11.
R. S. Iskhakov Zh. M. Moroz L. A. Chekanova N. A. Shepeta E. E. Shalygina 《JETP Letters》1997,66(7):517-521
It is found that the exchange interaction of Co (5 nm)ferromagnetic layers via Pd in Co/Pd multilayer films and of Co (5 nm)and
CoNi (5 nm) layers via the same nonmagnetic metal in Co/Pd/CoNi films can be both ferro-and antiferromagnetic. The period
of the AF-F-AF oscillations is of the order of 0.8 nm, and the amplitude of the oscillations decays as a power-law function
~d
Pd
−2
.
Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 487–491 (10 October 1997) 相似文献
12.
A theoretical model of a rough surface in a d-wave superconductor is studied for the general case of arbitrary strength of electron scattering by an impurity layer covering
the surface. Boundary conditions for quasiclassical Eilenberger equations are derived at the interface between the impurity
layer and the d-wave superconductor. The model is applied to the self-consistent calculation of the surface density of states and the critical
current in d-wave Josephson junctions.
Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 242–246 (10 February 1999)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
13.
X. Lan S. Kong W. Y. Zhang 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,84(3):357-364
In this paper, the electronic and magnetic properties of double perovskite
La2−x
Sr
x
MnCoO6
(x = 0,1,2) have been studied using the local-spin-density
approximation + U method. For the three compositions investigated, the
low symmetry P21/n structure yields
consistently lower energy than that of the high symmetry
\hbox{Fm[`3]mFm\bar{3}m}
Fm3̅m
structure. The strong electronic correlation and the orbital polarization of
Co-d electrons play crucial roles. In agreement with experiments, we
find that La2MnCoO6 is a ferromagnetic insulator with both Mn and Co
ions in their high-spin states. The tilting of oxygen octahedrons is most significant in
this case and is responsible for its insulating behavior; for LaSrMnCoO6, the
ground state remains a ferromagnetic insulator with Mn and Co ions in their high-spin
states. The optimized P21/n and
\hbox{Fm[`3]mFm\bar{3}m}Fm3̅m
crystal structures are nearly the same, and the
P21/n structure is stabilized by the
spontaneous layer-wise antiferro-orbital ordering of Co-d electrons. We
also predict that Sr2MnCoO6 is a ferromagnetic metal, and its
electronic structure can be viewed as a rigid band shifting from that of
LaSrMnCoO6. Due to the strong covalency between transition metal and oxygen
ions, the valences of Mn and Co ions differ considerably from those derived from purely
ionic model. Also, doping induced holes mainly go to oxygen sites though the density of
states near the Fermi energy has strong mixed character. This feature, together with the
orbital ordering phenomenon, should be observable via the X-ray near-edge absorption
spectroscopy and the polarized X-ray diffraction spectra. 相似文献
14.
E. R. Likhachev S. I. Kurganskii O. I. Dubrovskii A. M. Bugakov 《Physics of the Solid State》1997,39(3):378-380
The results of calculations of the energy band structure of thin films are used to obtain theoretical photoelectron spectra
of the valence band of the compound YBa2Cu3O7−δ
for δ=0 and δ=1 at various photon energies; the spectra are averaged over the photoelectron emission angles. The principal structural features
of the spectra are determined by the d states of copper atoms, while the variation of the shape of the spectra with increasing photon energy is attributed to a
relative decrease in the contribution from the p states of oxygen atoms. The density of d states around the Fermi level for YBa2Cu3O6 films is observed to increase relative to YBa2Cu3O7 films.
Fiz. Tverd. Tela (St. Petersburg) 39, 437–440 (March 1997) 相似文献
15.
E. A. Shapoval 《JETP Letters》1998,67(5):376-381
A model of strongly coupled electrons on a square lattice with attraction of the electrons to nearest-neighbor and next-nearest
neighbor lattice sites is studied. For this model, the phase diagrams containing d
x
2−y
2, d
xy, and (d
x
2−y
2+id
xy) states are constructed in the variables temperature versus chemical potential for different ratios of the corresponding
potentials.
Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 356–360 (10 March 1998) 相似文献
16.
N. V. Baranov V. G. Pleshchev E. M. Sherokalova N. V. Selezneva A. S. Volegov 《Physics of the Solid State》2011,53(4):701-706
The crystal structure and magnetic properties of layered Fe0.5TiS2 − x
Se
x
(0 ≤ x ≤ 2) compounds intercalated by iron atoms have been investigated. It has been shown that the substitution of selenium for
sulfur is accompanied by an increase in the unit cell volume, a transition from the ferromagnetic to antiferromagnetic behavior,
and a nonmonotonic variation in the paramagnetic Curie temperature. The intercalated iron atoms are characterized by lower
values of the effective moment (3.4–4.0μB) as compared to the predicted value (4.89μB) for the Fe2+ ion at g = 2. The results obtained have been discussed under the assumption that there are the hybridization of 3d electronic states of intercalated Fe atoms with the electronic states of the TiS2 − x
Se
x
host compounds and the competition of exchange interactions of different types. 相似文献
17.
A. N. Artemov 《Journal of Experimental and Theoretical Physics》1999,89(3):577-582
The effect of radiation defects on the thermodynamics of a system of Pearl vortices in a thin superconducting film is examined.
The scenario for a Kosterlitz-Thouless transition in this system is shown to depend on the defect concentration n
d
. At low concentrations, the transition takes place continuously, while at high concentrations, a range of temperatures exists
in which there are two metastable states. The concentrations of free vortices and of vortices captured by defects are calculated
as functions of temperature for different defect concentrations n
d
. A phase diagram is constructed for the vortex system in the n
d
−T plane.
Zh. éksp. Teor. Fiz. 116, 1081–1090 (September 1999) 相似文献
18.
S. A. Karamov 《Physics of the Solid State》1998,40(2):176-179
A study is made of some characteristics of phonon-free pairing of hybridized p and d electrons in planar NiB complexes in the presence of strong short-range Hubbard repulsion. A generalized Hubbard model is
used to calculate the superconductivity phase diagram as a function of the degree of underfilling of the 2p
6 and 3d
10 shells in NiB complexes. The phase region of states having the highest superconducting transition temperatures is established.
Fiz. Tverd. Tela (St. Petersburg) 40, 198–201 (February 1998) 相似文献
19.
A. E. Patrick 《Journal of statistical physics》2007,128(5):1211-1235
We investigate the properties of the Gibbs states and thermodynamic observables of the spherical model in a random field.
We show that on the low-temperature critical line the magnetization of the model is not a self-averaging observable, but it
self-averages conditionally. We also show that an arbitrarily weak homogeneous boundary field dominates over fluctuations
of the random field once the model transits into a ferromagnetic phase. As a result, a homogeneous boundary field restores
the conventional self-averaging of thermodynamic observables, like the magnetization and the susceptibility. We also investigate
the effective field created at the sites of the lattice by the random field, and show that at the critical temperature of
the spherical model the effective field undergoes a transition into a phase with long-range correlations ∼r
4−d
. 相似文献
20.
We study zero-temperature, stochastic Ising models σ
t
on Z
d
with (disordered) nearest-neighbor couplings independently chosen from a distribution μ on R and an initial spin configuration chosen uniformly at random. Given d, call μ type ℐ (resp., type ℱ) if, for every
x in Z
d
, σ
x
t
flips infinitely (resp., only finitely) many times as t→∞ (with probability one) – or else mixed type ℳ. Models of type ℒ and ℳ exhibit a zero-temperature version of “local non-equilibration”.
For d=1, all types occur and the type of any μ is easy to determine. The main result of this paper is a proof that for d=2, ±J models (where μ=αδ
J
+(1-α)δ-
J
) are type ℳ, unlike homogeneous models (type ℐ) or continuous (finite mean) μ's (type ℳ). We also prove that all other noncontinuous
disordered systems are type ℳ for any d≥ 2. The ±J proof is noteworthy in that it is much less “local” than the other (simpler) proof. Homogeneous and ±J models for d≥ 3 remain an open problem.
Received: 3 November 1999 / Accepted: 10 April 2000 相似文献