共查询到20条相似文献,搜索用时 609 毫秒
1.
M. Capone M. Grilli W. Stephan 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(4):551-557
We investigate polaron formation in a many-electron system in the presence of a local repulsion sufficiently strong to prevent
local-bipolaron formation. Specifically, we consider a Hubbard-Holstein model of interacting electrons coupled to dispersionless
phonons of frequency . Numerically solving the model in a small one-dimensional cluster, we find that in the nearly adiabatic case , the necessary and sufficient condition for the polaronic regime to occur is that the energy gain in the atomic (i.e., extremely localized) regime overcomes the energy of the purely electronic system . In the antiadiabatic case, , polaron formation is instead driven by the condition of a large ionic displacement (g being the electron-phonon coupling). Dynamical properties of the model in the weak and moderately strong coupling regimes
are also analyzed.
Received 15 February 1999 相似文献
2.
L. Bányai H. Haug P. Gartner 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(2):209-213
We compute numerically the time dependent retarded Green function of the polaron within the self-consistent RPA approximation.
The results show an approximately Gaussian behaviour at t=0 changing at later times its concavity to an exponential decay, as it has been predicted in the approximate form of an inverse
hyperbolic cosine function. The result contrasts with the non-selfconsistent RPA, where the exponential decay is only a transitory
behaviour and the asymptotics is rather oscillatory. Our conclusions are significant in the context of the quantum kinetics
with LO-phonons, where the transition from an intitially coherent scattering kinetics to a Markov kinetics with energy conservation
is controlled by the time behaviour of the retarded Green function.
Received: 18 September 1997 / Revised: 24 October 1997 / Accepted: 6 November 1997 相似文献
3.
E. Rastelli A. Tassi 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):341-347
Ferromagnetic spin chains of a hexagonal lattice coupled by a weak antiferromagnetic interaction J1 develop a helix arrangement if the intrachain antiferromagnetic NNN exchange J2 is sufficiently large. We show that the classical minimum energy spin configuration is an umbrella when an external magnetic field is applied. The scenario is dramatically changed by quantum fluctuations. Indeed we find
that the zero point motion forces the spins in a plane containing the magnetic field so that classical expectation is deceptive
for our model. Our result is obtained by controlled expansion in the low field-long wavelength modulation limit.
Received: 9 September 1997 / Revised: 15 October 1997 / Accepted: 17 November 1997 相似文献
4.
M.-B. Lepetit M. Cousy G.M. Pastor 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(3):421-427
The half-filled Hubbard model on the Bethe lattice with coordination number z=3 is studied using the density-matrix renormalization group (DMRG) method. Ground-state properties such as the energy per
site E, average local magnetization , its fluctuations and various spin correlation functions are determined as a function of the Coulomb interaction strength U/t. The local magnetic moments increase monotonically with increasing Coulomb repulsion U/t showing antiferromagnetic order between nearest neighbors []. At large U/t, is strongly reduced with respect to the saturation value 1/2 due to exchange fluctuations between nearest neighbors (NN)
spins [ for ]. shows a maximum for U/t=2.4-2.9 that results from the interplay between the usual increase of with increasing U/t and the formation of important permanent moments at large U/t. While NN sites show antiferromagnetic spin correlations that increase with increasing Coulomb repulsion, the next NN sites
are very weakly correlated over the whole range of U/t. The DMRG results are discussed and compared with tight-binding calculations for U=0, independent DMRG studies for the Heisenberg model and simple first-order perturbation estimates.
Received 8 February 1999 and Received in final form 14 June 1999 相似文献
5.
A. Sundaresan J.L. Tholence A. Maignan C. Martin M. Hervieu B. Raveau E. Suard 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(3):431-438
Neutron and electron diffraction, electrical transport and magnetic measurements have been carried out on a newly synthesized
electron doped Sr1-xCe
x
MnO3 (x = 0.1, 0.2, 0.3 and 0.4) system. For x=0.1, while cooling, it undergoes a first-order metal-insulator transition at 315 K which is associated with a structural
transition from cubic (Pm3m) to tetragonal (I4/mcm) due to Jahn-Teller ordering () which stabilizes a chain like (C-type) antiferromagnetic ground state with . The antiferromagnetic insulator state is insensitive to an applied magnetic field of 7 T. With increase of x, while the nuclear structure at room temperature for x=0.2 and 0.3 remains tetragonal, for x=0.4 it becomes orthorhombic (Imma) where the doping electrons seem to occupy mainly the d
x2-y2 symmetry. Further, the JT distortion and the antiferromagnetic interactions decrease with doping and a small negative magnetoresistance
appears for . Magnetic measurements show that the dilution of antiferromagnetic interaction results into a spin glass like behaviour at
low temperature for the samples with x=0.3 and 0.4. This behaviour is in contrast with the CMR properties of calcium based electron doped systems and hole doped
manganites. The stability of C-type antiferromagnetic ordering in the electron doped system with large A-site cationic size
may be responsible for the absence of double exchange ferromagnetism and CMR effect.
Received 10 September 1999 相似文献
6.
7.
The two-dimensional extended Holstein-Hubbard model is investigated in the strong correlation regime to study the nature of self-trapping transition and the polaron phase diagram in the absence of superconductivity. Using a series of canonical transformations followed by zero-phonon averaging the extended Holstein-Hubbard model is converted into an effective extended Hubbard model which is subsequently transformed into an effective t-J model in the strong correlation limit. This effective t-J model is finally solved using the mean-field Hartree-Fock approximation to show that the self-trapping transition is continuous in the anti-adiabatic limit while it is discontinuous in the adiabatic limit. The phase diagrams for the localization-delocalization transition, namely the phase line and the phase surface separating the small polaron and large polaron states are also shown. 相似文献
8.
9.
The crossover of large to small radius polaron is studied in terms of the inverse-relaxation time and temperature.It is found that the small radius polaron exists at higher temperature than the large radius polaron.A formula which relates the inverse-relaxation time to the ratio of arbitrary temperature and Debye temperature of the crystal is derived.The polaron crossover temperatures in NaCl and KBr are found from plotted graphs.The straight line emerging at the Debye temperature T_d of a graph reflects the increase of the inverse relaxation time for increasing temperature up to the collapse of the small radius polaron.The relationship between the small and large radius polarons is found and known ratios of the effective and the bare masses of the electrons for the two substances are used to validate our results.The small radius polaron's mass is later compared with the mass obtained from the hopping formula and is found to be approximately equal.Finally,we point out that the crossover temperature is material-specific since it depends on the Debye and the effective dielectric function. 相似文献
10.
P. Bonville J. A. Hodges Z. Hossain R. Nagarajan S. K. Dhar L. C. Gupta E. Alleno C. Godart 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):377-383
We present magnetic properties of the three-band Hubbard model in the para- and antiferromagnetic phase on a hypercubic lattice
calculated with the Dynamical Mean-Field Theory (DMFT). To allow for solutions with broken spin-symmetry we extended the approach
to lattices with AB-like structure. Above a critical sublattice magnetization one can observe rich structures in the spectral-functions similar to the t-J model which can be related to the well known bound states for one hole in the Neél-background. In addition to the one-particle
properties we discuss the static spin-susceptibility in the paramagnetic state at the points and for different dopings . The -T-phase-diagram exhibits an enhanced stability of the antiferromagnetic state for electron-doped systems in comparison to hole-doped.
This asymmetry in the phase diagram is in qualitative agreement with experiments for high-Tc materials.
Received: 28 May 1998 / Revised and Accepted: 14 September 1998 相似文献
11.
Polaronic effects on the energy levels of a double donor impurity in quantum wells in the presence of a magnetic field 总被引:3,自引:0,他引:3
Zi-xin Liu Zhen-jiang Lai Yong-chang Huang Ya Liu Guo-jun Cheng 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(3):347-350
In the presence of a magnetic field the Hamiltonian of the single or double polaron bound to a helium-type donor impurity
in semiconductor quantum wells (QWs) are given in the case of positively charged donor center and neutral donor center. The
couplings of an electron and the impurity with various phonon modes are considered. The binding energy of the single and double
bound polaron in AlxlGa 1-xlAs/GaAs/AlxrGa 1-xrAs QWs are calculated. The results show that for a thin well the cumulative effects of the electron-phonon coupling and the
impurity-phonon coupling can contribute appreciably to the binding energy in the case of ionized donor. In the case of neutral
donor the contribution of polaronic effects are not very important, however the magnetic field significantly modifies the
binding energy of the double donor. The comparison between the binding energies in the case of the impurity placed at the
quantum well center and at the quantum well edge is also given.
Received 16 February 1999 相似文献
12.
X.-J. Fan W.-G. Yin X.-G. Li C.-D. Gong 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(1):5-10
The dispersion spectrum of single hole in a bilayer composed of plane and chain, each described by t-J model, coupled by - interactions between them, is calculated in terms of self-consistent Born approximation. It was found that for a weak interlayer
coupling the two different quasiparticle bands, plane-like and chain-like bands, show a minimum at (, ) and a maximum at (0, 0) or (, ). In plane-like dispersion we can find an anomalous “flat” region near Fermi surface along the antiferromagnetic Brillouin
zone boundary, which favors the formation of the van-Hove singularities. With increasing interlayer coupling, a large modification
of the dispersions is carried out, the minimum deviates from (, ) and the energy gap of the two bands decreases and finally disappears when the vertical coupling is larger enough. The shapes
of the QP bands are sensitive to the vertical hopping rather than the vertical exchange energy . As the interlayer coupling increases, the shapes of the two QP bands suggest that the chain-like band approaches to that
of quasi-one dimensional model, and the plane-like band undergoes the one layer t - t '-J models' band.
Received 17 March 1999 相似文献
13.
L. I. Korovin I. G. Lang S. T. Pavlov 《Journal of Experimental and Theoretical Physics》1999,89(4):764-774
Combined polaron states in a rectangular quantum well in a strong magnetic field perpendicular to the well plane are discussed.
These states are due to interaction between two discrete electron levels with different Landau quantum numbers (n and n
1) and different size-quantization quantum numbers (m and m
1) on the one hand and a confined LO phonon on the other under conditions of low temperature when the energy difference between
the electronic levelsis equal or close to the energy of the confined LO phonon. The expression for the resonant magnetic field
H
res at which a combined polaron is formed contains the energy difference between size-quantized levels, so it is a function of
quantum well parameters. The separation ΔE
res between branches in the energy spectrum of a combined polaron and H
res has been calculated as a function of the quantum well width d. The resonant field H
res can be reduced dramatically in comparison with the case m=m
1. The case of size-quantization with n=n
1 has been analyzed. The energy difference ΔE
res is in the range (1–5)· 10−3 eV. The damping of combined polaron states due to the effect of anharmonicity on the LO phonon has been studied. Interband
absorption and features in the reflection spectrum due to interband transitions have been calculated for an arbitrary ratio
between the radiative and “phonon” lifetime of a combined polaron have been investigated.
Zh. éksp. Teor. Fiz. 116, 1419–1439 (October 1999) 相似文献
14.
A. Paolone P. Roy A. Pimenov A. Loidl A.A. Mukhin A.M. Balbashov 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(2):245-250
The optical conductivity of La0.85Sr0.15MnO3 single crystals was studied by means of submillimeter and infrared spectroscopy for frequencies cm-1 and temperatures 10 K <
T
<300 K. The submillimeter conductivity follows the temperature dependence of the dc-data. The phonon spectrum of La0.85Sr0.15MnO3 changes considerably below K revealing a structural phase transition induced by charge or orbital order. At T
=10 K a number of phonon modes can be identified in addition to the room-temperature spectrum. The optical conductivity () in the mid-infrared reveals the characteristics of small polaron absorption. Below the magnetic ordering temperature the
polaron binding energy is highly reduced, but the onset of charge order interrupts the formation of free charge carriers with
a Drude-like behavior. The frequency and temperature dependence of in this regime qualitatively resembles the small polaron predictions by Millis et al. (Phys. Rev. B 54, 5405 (1996)).
Received 5 November 1999 相似文献
15.
A. Sherman M. Schreiber 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(2):203-214
Using the method which retains the rotation symmetry of spin components in the paramagnetic state and has no preset magnetic
ordering, spectral and magnetic properties of the two-dimensional t-J model in the normal state are investigated for the ranges of hole concentrations 0 ⩽ x ⩽ 0.16 and temperatures 0.01t ⩽ T ⩽ 0.2t. The used hopping t and exchange J parameters of the model correspond to hole-doped cuprates. The obtained solutions are homogeneous which indicates that stripes
and other types of phase separation are not connected with the strong electron correlations described by the model. A series
of nearly equidistant maxima in the hole spectral function calculated for low T and x is connected with hole vibrations in the region of the perturbed short-range antiferromagnetic order. The hole spectrum has
a pseudogap in the vicinity of (0,π) and (π, 0). For x ≈ 0.05 the shape of the hole Fermi surface is transformed from four small ellipses around (±π/2,±π/2) to two large rhombuses
centered at (0, 0) and (π,π). The calculated temperature and concentration dependencies of the spin correlation length and
the magnetic susceptibility are close to those observed in cuprate perovskites. These results offer explanations for the observed
scaling of the static uniform susceptibility and for the changes in the spin-lattice relaxation and spin-echo decay rates
in terms of the temperature and doping variations in the spin excitation spectrum of the model.
Received 14 November 2002 Published online 1st April 2003
RID="a"
ID="a"e-mail: alexei@fi.tartu.ee 相似文献
16.
We have studied the electronic and magnetic structures of the ternary iron arsenides AFe2As2 (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear
antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic
interactions bridged by As 4p orbitals. The correction from the spin-orbit interaction to the electronic band structure is
given. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based
on the calculations, we propose that the low energy dynamics of these materials can be described effectively by a t-J
H
-J
1-J
2-type model [2008, arXiv: 0806.3526v2]. 相似文献
17.
H. Razafimandimby M. Taguchi J. C. Parlebas 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(1):21-26
We investigate the competition between magnetic order and local Kondo effect in a Kondo lattice model (i.e. the Coqblin-Schrieffer Hamiltonian extended to a lattice) in a mean-field approximation, taking account of the spin-orbit
degeneracy of each localized f level. This leads to the definition of a dependent Kondo temperature. We study the Kondo phase and compare its energy with the energies of magnetic phases, when the
number of the conduction band electron per site is near one. We present a phase diagram which shows the occurrence of three
phases: Kondo, antiferromagnetic and paramagnetic phases. Our model in the mean-field approximation also shows a somewhat
flat Kondo temperature, for large values of , as a function of the exchange coupling J between conduction and localized f electrons. Finally we show some scaling effects between and J and we define a corresponding Kondo temperature.
Received 21 September 1998 and Received in final form 8 February 1999 相似文献
18.
A. N. Titov 《Physics of the Solid State》2008,50(5):945-948
The dependence of the character of the phase transition that accompanies the polaron band collapse in Ag x TiTe2 on the concentration of polaron centers determining the density of states of the polaron band was studied by differential thermal analysis and by measuring the magnetic susceptibility. In the low-concentration limit, the transition is found to be smooth. As the concentration of impurity centers increases, the transition changes to the first order. On the temperature-concentration diagram, the line along which the change in the phase transition order occurs is determined by percolation laws. It is shown that the thermal effect accompanying the transition can be related to the decrease in the electron free energy due to polaron localization. 相似文献
19.
Photoluminescence spectra associated with the donor bound exciton has been studied in Cd1-xMnxSe as a function of composition (x = .10, .05, and 0.0) and temperature in zero magnetic field. In comparison with the previously studied neutral donor, the bound exciton shows a large enhancement in its binding energy and broadening, attributed to the exchange interaction with the Mn++-ion 3d electrons. In particular, a transition from a bound polaron to a fluctuation dominated regime is in evidence for x = .10 in this multiparticle system. 相似文献
20.
Y. M. Quan L. J. Zou D. Y. Liu H. Q. Lin 《The European Physical Journal B - Condensed Matter and Complex Systems》2012,85(2):55
Competition between crystal field splitting and Hund’s rule coupling in magnetic metal-insulator transitions of half-filled
two-orbital Hubbard model is investigated by multi-orbital slave-boson mean field theory. We show that with the increase of
Coulomb interaction, the system firstly transits from a paramagnetic (PM) metal to a Néel antiferromagnetic (AFM) Mott insulator,
or to a nonmagnetic orbital insulator, depending on the competition of crystal field splitting and the Hund’s rule coupling.
The AFM Mott insulating, PM metallic and orbital insulating phases are not, partially and fully orbital polarized, respectively.
For a small J
H
and a finite crystal field, the orbital insulator is robust. These results demonstrate that large crystal field splitting
favors the formation of the orbital insulating phase, while large Hund’s rule coupling tends to destroy it, driving the low-spin
to high-spin transition. 相似文献