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1.
J.E. Espinosa O. Navarro M. Avignon 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(1):9-16
A real-space method has been introduced to study the pairing problem within the generalized Hubbard Hamiltonian. This method
includes the bond-charge interaction term as an extension of the previously proposed mapping method [1] for the Hubbard model.
The generalization of the method is based on mapping the correlated many-body problem onto an equivalent site- and bond-impurity
tight-binding one in a higher dimensional space, where the problem can be solved exactly. In a one-dimensional lattice, we
analyzed the three particle correlation by calculating the binding energy at the ground state, using different values of the
bond-charge, the on-site (U) and the nearest-neighbor (V) interactions. A pairing asymmetry is found between electrons and holes for the generalized hopping amplitude, where the
hole pairing is not always easier than the electron case. For some special values of the hopping parameters and for all kinds
of interactions in the Hubbard Hamiltonian, an analytical solution is obtained.
Received 21 January 2000 and Received in final form 18 July 2000 相似文献
2.
3.
J. Kurzyk W. Wójcik J. Spałek 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(3):385-398
The method used earlier for analysis of correlated nanoscopic systems is extended to infinite (periodic) s-band-like systems
described by the Hubbard model. The optimized single-particle Wannier wave functions contained in the parameters of the extended
Hubbard model (in the nearest-neghbor hopping (-t), in the magnitude of the intraatomic interaction U, and in other parameters) are determined explicitly in the correlated state for the electronic systems of various symmetries
and dimensions: Hubbard chain, square and triangular planar lattices, and the three cubic lattices (SC, BCC, FCC). In effect,
the evolution of the electronic properties as a function of interatomic distance R is obtained. The model parameters in most
cases do not scale linearly with the lattice spacing and hence, their solution as a function of microscopic parameters reflects
only qualitatively the system evolution. Also, the atomic energy changes with R and therefore should be included in the model
analysis. The solutions in one dimension (D = 1) can be analyzed both rigorously (by making use of the Lieb–Wu solution) and compared with the approximate Gutzwiller
treatments. In higher dimensions (D = 2 and 3) only the latter approach is possible to implement within the scheme. The renormalized single particle wave functions
are almost independent of the choice of the scheme selected to diagonalize the Hamiltonian in the Fock space in D = 1 case.
For dimensions D > 1 the qualitative behavior is independent of the structure considered. The wave-function size increases
above the Mott-Hubbard localization threshold and gradually reaches the atomic limit value. The method can be extended to
other approximation schemes, as stressed at the end. 相似文献
4.
P. Richmond 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(4):523-526
A simple agent model is introduced by analogy with the mean field approach to the Ising model for a magnetic system. Our model
is characterised by a generalised Langevin equation = F
ϕ + G
ϕ
t
where
t
is the usual Gaussian white noise, i.e.:
t
t
′
= 2Dδ
t-t
′
and
t
= 0. Both the associated Fokker Planck equation and the long time probability distribution function can be obtained analytically.
A steady state solution may be expressed as P
ϕ =
exp{ - Ψϕ - ln G(ϕ)} where Ψϕ = -
F/
G
dϕ and Z is a normalization factor. This is explored for the simple case where F
ϕ = Jϕ + bϕ2 - cϕ3 and fluctuations characterised by the amplitude G
ϕ = ϕ + ɛ when it readily yields for ϕ≫ɛ, a distribution function with power law tails, viz: P
ϕ =
exp{2bϕ-cϕ2
/D}. The parameter c ensures convergence of the distribution function for large values of ϕ. It might be loosely associated with the activity
of so-called value traders. The parameter J may be associated with the activity of noise traders. Output for the associated time series show all the characteristics
of familiar financial time series providing J < 0 and D≈ | J|.
Received 25 July 2000 相似文献
5.
D. Lee 《The European Physical Journal A - Hadrons and Nuclei》2008,35(2):171-187
The symmetric heavy-light ansatz is a method for finding the ground state of any dilute unpolarized system of attractive two-component
fermions. Operationally it can be viewed as a generalization of the Kohn-Sham equations in density functional theory applied
to N -body density correlations. While the original Hamiltonian has an exact Z2 symmetry, the heavy-light ansatz breaks this symmetry by skewing the mass ratio of the two components. In the limit where
one component is infinitely heavy, the many-body problem can be solved in terms of single-particle orbitals. The original
Z2 symmetry is recovered by enforcing Z2 symmetry as a constraint on N -body density correlations for the two components. For the 1D, 2D, and 3D attractive Hubbard models the method is in very
good agreement with exact Lanczos calculations for few-body systems at arbitrary coupling. For the 3D attractive Hubbard model
there is very good agreement with lattice Monte Carlo results for many-body systems in the limit of infinite scattering length. 相似文献
6.
R.I. Eglitis E.A. Kotomin G. Borstel 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(4):483-486
The origin of the intrinsic excitonic (“green”) luminescence in ABO3 perovskites remains a hot topic over the last quarter of a century. We suggest as a theoretical interpretation for the “green”
luminescence in these crystals, the recombination of electron and hole polarons forming a charge transfer vibronic exciton.
In order to check quantitatively the proposed model, we performed quantum chemical calculations using the Intermediate Neglect
of Differential Overlap (INDO) method combined with the periodic defect model. The luminescence energies calculated for four
perovskite crystals are found to be in good agreement with experimental data.
Received 19 December 2001 and Received in final form 14 March 2002 Published online 25 June 2002 相似文献
7.
T. Gottwald P. G.J. van Dongen 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(3):277-285
In order to describe unbalanced ultracold fermionic quantum gases on optical lattices in a harmonic trap, we investigate an
attractive (U < 0) asymmetric (t↑≠t↓) Hubbard model with a Zeeman-like magnetic field. In view of the model's spatial inhomogeneity, we focus in this paper on
the solution at Hartree-Fock level. The Hartree-Fock Hamiltonian is diagonalized with particular emphasis on superfluid phases.
For the special case of spin-independent hopping we analytically determine the number of solutions of the resulting self-consistency
equations and the nature of the possible ground states at weak coupling. We present the phase diagram of the homogeneous system
and numerical results for unbalanced Fermi-mixtures obtained within the local density approximation. In particular, we find
a fascinating shell structure, involving normal and superfluid phases. For the general case of spin-dependent hopping we calculate
the density of states and the possible superfluid phases in the ground state. In particular, we find a new magnetized superfluid
phase. 相似文献
8.
E. Perfetto G. Stefanucci M. Cini 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(2):139-142
We show that different non-conventional superconductors have one fundamental feature in common: pair eigenstates of the Hamiltonian
are repulsion-free, the W = 0 pairs. In extended Hubbard models, pairing can occur for reasonable parameter values. For (N, N) nanotubes the binding energy of the pair depends strongly on the filling and decreases towards a reduced but nonzero value
for the graphite sheet N → ∞.
Received 13 July 2002 Published online 29 November 2002 相似文献
9.
We discuss the excess conductivity at nonzero frequencies in a superconductor above Tc within the Gaussian approximation. We focus the attention on the temperature range not too close to Tc: within a time-dependent Ginzburg-Landau formulation, we phenomenologically introduce a short wavelength cutoff (of the order
of the inverse coherence length) in the fluctuational spectrum to suppress high momentum modes. We treat the general cases
of thin wires, anisotropic thin films and anisotropic bulk samples. We obtain in all cases explicit expressions for the finite
frequency fluctuational conductivity. The dc case directly follows. Close to Tc the cutoff has no effect, and the known results for Gaussian fluctuations are recovered. Above Tc, and already for ε = ln(T/T
c) > 10-2, we find strong suppression of the paraconductivity as compared to the Gaussian prediction, in particular in the real part
of the paraconductivity. At high ε the cutoff effects are dominant. We discuss our results in comparison with data on high-Tc superconductors.
Received 19 March 2002 Published online 25 June 2002 相似文献
10.
A. Malakis P. Kalozoumis N. Tyraskis 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,50(1-2):63-67
We apply a new entropic scheme to study the critical behavior of
the square-lattice Ising model with nearest- and
next-nearest-neighbor antiferromagnetic interactions. Estimates of
the present scheme are compared with those of the Metropolis
algorithm. We consider interactions in the range where
superantiferromagnetic (SAF) order appears at low temperatures. A
recent prediction of a first-order transition along a certain
range (0.5–1.2) of the interaction ratio (R=Jnnn/Jnn) is
examined by generating accurate data for large lattices at a
particular value of the ratio (R=1). Our study does not support
a first-order transition and a convincing finite-size scaling
analysis of the model is presented, yielding accurate estimates
for all critical exponents for R=1. The magnetic exponents are
found to obey “weak universality” in accordance with a previous
conjecture. 相似文献
11.
Electronic structures in coupled two quantum dots by 3D-mesh Hartree-Fock-Kohn-Sham calculation 总被引:2,自引:0,他引:2
T. Matsuse T. Hama H. Kaihatsu N. Toyoda T. Takizawa 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):391-394
To study the electronic structures of quantum dots in the framework of self-interaction-free including three dimensional effects,
we adopt the theory of nonlocal effective potential introduced by Kohn and Sham [#!ks65!#]. For utilizing the advantageous
point of the real space (3D) mesh method to solve the original nonlinear and nonlocal Hartree-Fock-Kohn-Sham (HFKS)-equation,
we introduce a linearization of the equation in the local form by introducing the local Coulomb potentials which depend on
explicitly the two single particle states. In practice, for solving the local form HFKS-equation, we use the Car-Parrinello-like
relaxation method and the Coulomb potentials are obtained by solving the Poisson equation under proper boundary conditions.
Firstly the observed energy gap between triplet- and singlet-states of N = 4 in DBS [#!tarucha96!#] is discussed to reproduce the addition energies and chemical potentials depending the magnetic
field. Next the coupling between two-quantum dots in TBS [#!aht97!#] is studied by adding the square barrier between two dots.
The spin-degeneracy [#!aht97!#] measured in gate-voltage depending on magnetic field is well reproduced in the limit of small
mismatch. Finally, the electronic states in the ring structure are calculated and discussed how the ring size and magnetic
field affect to the structures.
Received 30 November 2000 相似文献
12.
Summary The recently introduced Connected-Moments Expansion (CMX) is compared to a variational Lanczos scheme for estimating ground-state
energies of many-body systems. A systematic approach is given for three quantum-mechanical systems: harmonic oscillator, anharmonic
oscillator and the Kondo model. A second-order analysis is given in terms of particular ratios of moments of the Hamiltonian. 相似文献
13.
14.
A. Chakraborti B.K. Chakrabarti 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(4):677-680
If one places N cities randomly on a lattice of size L, we find that and vary with the city concentration p=N/L
2, where is the average optimal travel distance per city in the Euclidean metric and is the same in the Manhattan metric. We have studied such optimum tours for visiting all the cities using a branch and bound
algorithm, giving the exact optimized tours for small system sizes () and near-optimal tours for bigger system sizes (). Extrapolating the results for , we find that for p=1, and and with as . Although the problem is trivial for p=1, for it certainly reduces to the standard travelling salesman problem on continuum which is NP-hard. We did not observe any irregular
behaviour at any intermediate point. The crossover from the triviality to the NP-hard problem presumably occurs at p=1.
Received 15 April 2000 相似文献
15.
Y. Benmimoun A. Bouhemadou R. Khenata A. H. Reshak B. Amrani M. Ameri H. Baltache 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(2):165-171
The results of first-principles theoretical study of the structural,
electronic and optical properties of SrCl2 in its cubic structure, have
been performed using the full-potential linear augmented plane-wave method
plus local orbitals (FP-APW+lo) as implemented in the WIEN2k code. In this
approach both the local density approximation (LDA) and the generalized
gradient approximation (GGA) are used for the exchange-correlation (XC)
potential. Also we have used the Engel-Vosko GGA formalism, which optimizes
the corresponding potential for band structure calculations. We performed
these calculations with and without spin-orbit interactions. Including
spin-orbit coupling cause to lifts the triple degeneracy at Γ point
and a double degeneracy at X point. Results are given for structural
properties. The pressure dependence of elastic constants and band gaps are
investigated. The dielectric function, reflectivity spectra and refractive
index are calculated up to 30 eV. Also we calculated the pressure and volume
dependence of the static optical dielectric constant. 相似文献
16.
M. A. Jafarizadeh Y. Akbari N. Behzadi 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,47(2):283-293
The Gell-Mann λ matrices for Lie algebra su(3) are the
natural basis for the Hilbert space of Hermitian operators acting
on the states of a three-level system(qutrit). So the
construction of EWs for two-qutrit states by using these matrices
may be an interesting problem. In this paper, several two-qutrit
EWs are constructed based on the Gell-Mann matrices by using the
linear programming (LP) method exactly or approximately. The
decomposability and non-decomposability of constructed EWs are
also discussed and it is shown that the λ-diagonal EWs
presented in this paper are all decomposable but by adding
λ-non-diagonal terms, one can obtain various
non-decomposable EWs. 相似文献
17.
D. Stauffer P.M.C. de Oliveira 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):587-592
The density of never changed opinions during the Sznajd consensus-finding process decays with time t as 1/t
θ. We find θ ≃ 3/8 for a chain, compatible with the exact Ising result of Derrida et al. In higher dimensions, however, the exponent differs from the Ising θ. With simultaneous updating of sublattices instead of
the usual random sequential updating, the number of persistent opinions decays roughly exponentially. Some of the simulations
used multi-spin coding.
Received 22 August 2002 / Received in final form 12 November 2002 Published online 31 December 2002 相似文献
18.
J. Uhlig J.C. Lemm A. Weiguny 《The European Physical Journal A - Hadrons and Nuclei》1998,2(4):343-354
Abstact: Three variants of mean field methods for atomic and nuclear reactions are compared with respect to both conception and applicability:
The time–dependent Hartree–Fock method solves the equation of motion for a Hermitian density operator as initial value problem,
with the colliding fragments in a continuum state of relative motion. With no specification of the final state, the method
is restricted to inclusive reactions. The time–dependent mean field method, as developed by Kerman, Levit and Negele as well
as by Reinhardt, calculates the density for specific transitions and thus applies to exclusive reactions. It uses the Hubbard–Stratonovich
transformation to express the full time-development operator with two–body interactions as functional integral over one–body
densities. In stationary phase approximation and with Slater determinants as initial and final states, it defines non–Hermitian,
time–dependent mean field equations to be solved self–consistently as boundary value problem in time. The time–independent
mean field method of Giraud and Nagarajan is based on a Schwinger–type variational principle for the resolvent. It leads to
a set of inhomogeneous, non-Hermitian equations of Hartree–Fock type to be solved for given total energy. All information
about initial and final channels is contained in the inhomogeneities, hence the method is designed for exclusive reactions.
A direct link is established between the time–dependent and time–independent versions. Their relation is non–trivial due to
the non–linear nature of mean field methods.
Received: 7 January 1998 / Revised version: 20 April 1998 相似文献
19.
C. Brunhuber F. G. Mertens Y. Gaididei 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,57(1):57-65
Studies on thermal diffusion of lattice solitons in Fermi-Pasta-Ulam (FPU)-like lattices
were recently generalized to the
case of dispersive long-range interactions (LRI) of the Kac-Baker form.
The variance of the soliton position shows a stronger than linear
time-dependence (superdiffusion) as found earlier for lattice solitons on FPU chains with
nearest-neighbour interactions (NNI). Since the superdiffusion seems to be generic for nontopological solitons, we want to
illuminate the role of the soliton shape on the superdiffusive mechanism.
Therefore, we concentrate on an FPU-like lattice with a certain class of power-law long-range interactions where the solitons
have algebraic tails instead of
the exponential tails in the case of FPU-type interactions (with or without Kac-Baker LRI).Despite of structurally similar
Langevin equations which hold for the soliton position and width of the two soliton types, the
algebraic solitons reach the superdiffusive long-time limit with a
characteristic t3/2 time-dependence much faster than
exponential solitons. The soliton shape determines the diffusion constant in the long-time limit that is approximately a factor
of π smaller for algebraic
solitons. Our results appear to be generic for nonlinear excitaitons in
FPU-chains, because the same superdiffusive time-dependence was also observed in
simulations with discrete breathers. 相似文献
20.
U. Köbler A. Hoser 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,60(2):151-159
The basic issues of renormalization group (RG) theory, i.e. universality, crossover phenomena, relevant interactions etc.
are verified experimentally on magnetic materials. Universality is demonstrated on account of the saturation of the magnetic
order parameter for T ↦ 0. Universal means that the deviations with respect to saturation at T = 0 can perfectly be described
by a power function of absolute temperature with an exponent ε that is independent of spin structure and lattice symmetry.
Normally the Tε function holds up to ~0.85Tc where crossover to the critical power function occurs.
Universality for T ↦ 0 cannot be explained on the basis of the material specific magnon dispersions that are due to atomistic
symmetry. Instead, continuous dynamic symmetry has to be assumed. The quasi particles of the continuous symmetry can be described
by plane waves and have linear dispersion in all solids. This then explains universality. However, those quasi particles cannot
be observed using inelastic neutron scattering. The principle of relevance is demonstrated using the competition between crystal
field interaction and exchange interaction as an example. If the ratio of crystal field interaction to exchange interaction
is below some threshold value the local crystal field is not relevant under the continuous symmetry of the ordered state and
the saturation moment of the free ion is observed for T ↦ 0. Crossover phenomena either between different exponents or between
discrete changes of the pre-factor of the Tε function are demonstrated for the spontaneous magnetization and for the heat capacity. 相似文献