共查询到20条相似文献,搜索用时 359 毫秒
1.
A.S.T. Pires 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(1):65-71
We calculate the excitation gap, the soliton energy, and the soliton density at finite temperature, of the spin-1/2 one dimensional
antiferromagnet coupled to phonons, using a self-consistent harmonic approximation, and the thermal-Green function technique.
The spin degrees of freedom are represented by the phase Hamiltonian with the help of the boson representation of the spinless
fermions. We estimate the critical field Hc above which begins the incommensurate phase. We also present a theoretical calculation for the specific heat in this phase.
We use CuGeO3 as an example of a compound where our theory could be applied.
Received 22 October 2002 / Received in final form 21 January 2003 Published online 14 March 2003 相似文献
2.
B.S. Kandemir T. Altanhan 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(4):517-526
We investigate the stability condition of large bipolarons confined in a parabolic potential containing certain parameters
and a uniform magnetic field. The variational wave function is constructed as a product form of electronic parts, consisting
of center of mass and internal motion, and a part of coherent phonons generated by Lee-Low-Pines transformation from the vacuum.
An analytical expression for the bipolaron energy is found, from which the ground and excited-state energies are obtained
numerically by minimization procedure. The bipolaron stability region is determined by comparing the bipolaron energy with
those of two separate polarons, which is already calculated within the same approximation. It is shown that the results obtained
for the ground state energy of bipolarons reduce to the existing works in zero magnetic field. In the presence of a magnetic
field, the stability of bipolarons is examined, for three types of low-dimensional system, as function of certain parameters,
such as the magnetic-field, the electron-phonon coupling constant, Coulomb repulsion and the confinement strength. Numerical
solutions for the energy levels of the ground and first excited states are examined as functions of the same parameters.
Received 7 March 2002 and Received in final form 22 April 2002 Published online 25 June 2002 相似文献
3.
C. Rummel J. Ankerhold 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(1):105-115
Based on the path integral approach the partition function of a many body system with separable two body interaction is calculated
in the sense of a semiclassical approximation. The commonly used Gaussian type of approximation, known as the perturbed static
path approximation (PSPA), breaks down near a crossover temperature due to instabilities of the classical mean field solution.
It is shown how the PSPA is systematically improved within the crossover region by taking into account large non-Gaussian
fluctuations and an approximation applicable down to very low temperatures is carried out. These findings are tested against
exact results for the archetypical cases of a particle moving in a one dimensional double well and the exactly solvable Lipkin-Meshkov-Glick
model. The extensions should have applications in finite systems at low temperatures as in nuclear physics and mesoscopic
systems, e.g. for gap fluctuations in nanoscale superconducting devices previously studied within a PSPA type of approximation.
Received 28 March 2002 Published online 17 September 2002 相似文献
4.
D.U. Matrasulov F.C. Khanna Kh.Yu. Rakhimov Kh.T. Butanov 《The European Physical Journal A - Hadrons and Nuclei》2002,14(1):81-86
Doubly heavy baryons, i.e., the baryons containing two heavy quarks are treated in the adiabatic approximation, considering the motion of the light
quark as a relativistic motion. The binding energy and mass spectra of doubly heavy baryons are calculated solving the two-center
Dirac equation the one-centre Schr?dinger equation for Coulomb plus linear potential. The binding energy of the light quark
as a function of the distance between heavy quarks is found.
Received: 16 January 2002 / Accepted: 7 February 2002 相似文献
5.
M. Tamine 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(1):109-118
A Heisenberg model is employed to study the spin fluctuation dynamics on a (001) ferromagnetic surface using a new theoretical
formalism. The solution of the full magnetic problem arising from the absence of magnetic translation symmetry in one dimension
due to the presence of a magnetic surface is presented. The calculations are described using simultaneously a closed form
of the spin-wave Green's function and the matching procedure in the random-phase approximation. Analytic expressions for the
Green's functions are also derived in a low-temperature spin-wave approximation. The theoretical approach determines the bulk
and evanescent spin fluctuation fields in the two dimensional plane normal to the surface. The results are used to calculate
the localised modes of magnons associated with the surface. Numerical examples of the modes are given and they are found to
exhibit various effects due to the interplay between the bulk and surface modes. It is shown that there may be surface spin-waves
that decay in amplitude with distance into the bulk domain. Also the bulk spin fluctuations field as well as the magnons localised
at the surface depend on the nature of the bulk-surface coupling exchange. The unstable surface magnetic configurations are
illustrated and discussed. The results derived from the dynamic correlation functions between a pair of spin operators at
any two sites are employed to evaluate the spin deviation in the ferromagnet due to localised surface modes obtained by the
matching procedure as a function of temperature.
Received 21 April 2002 / Received in final form 25 October 2002 Published online 14 March 2003 相似文献
6.
R. Currat E. Kats I. Luk'yanchuk 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(3):339-347
We propose a simple phenomenological model describing composite crystals, constructed from two parallel sets of periodic inter-penetrating
chains. In the harmonic approximation and neglecting thermal fluctuations we find the eigenmodes of the system. It is shown
that at high frequencies there are two longitudinal sound modes with standard attenuation, while in the low frequency region
there is one propagating sound mode and an over-damped phase mode. The crossover between these two regions is analyzed numerically
and the dynamical structure factor is calculated. It is shown that the qualitative features of the experimentally observed
spectra can be consistently described by our model.
Received 28 November 2001 and Received in final form 23 January 2002 相似文献
7.
M. Modugno L. Pricoupenko Y. Castin 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,22(2):235-257
We consider a 3D dilute Bose-Einstein condensate at thermal equilibrium in a rotating harmonic trap. The condensate wavefunction
is a local minimum of the Gross-Pitaevskii energy functional and we determine it numerically with the very efficient conjugate
gradient method. For single vortex configurations in a cigar-shaped harmonic trap we find that the vortex line is bent, in
agreement with the numerical prediction of Garcia-Ripoll and Perez-Garcia [Phys. Rev. A 63, 041603 (2001)]. We derive a simple energy functional for the vortex line in a cigar-shaped condensate which allows to understand
physically why the vortex line bends and to predict analytically the minimal rotation frequency required to stabilize the
bent vortex line. This analytical prediction is in excellent agreement with the numerical results. It also allows to find
in a simple way a saddle point of the energy, where the vortex line is in a stationary configuration in the rotating frame
but not a local minimum of energy. Finally we investigate numerically the effect of thermal fluctuations on the vortex line
for a condensate with a straight vortex: we can predict what happens in a single realization of the experiment by a Monte
Carlo sampling of an atomic field quasi-distribution function of the density operator of the gas at thermal equilibrium in
the Bogoliubov approximation.
Received 28 March 2002 / Received in final form 13 September 2002 Published online 21 January 2003
RID="a"
ID="a"e-mail: yvan.castin@lkb.ens.fr 相似文献
8.
In this paper, the dynamics of heuristic algorithms for constructing small vertex covers (or independent sets) of finite-connectivity
random graphs is analysed. In every algorithmic step, a vertex is chosen with respect to its vertex degree. This vertex, and
some environment of it, is covered and removed from the graph. This graph reduction process can be described as a Markovian
dynamics in the space of random graphs of arbitrary degree distribution. We discuss some solvable cases, including algorithms
already analysed using different techniques, and develop approximation schemes for more complicated cases. The approximations
are corroborated by numerical simulations.
Received 14 March 2002 Published online 31 July 2002 相似文献
9.
T.R. Hovhannisyan A.G. Markossian G.F. Mkrtchian 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,20(1):17-25
On the base of relativistic generalized eikonal approximation wave function the multiphoton cross-sections of a Dirac particle
bremsstrahlung on an arbitrary electrostatic potential and strong laser radiation field are presented. In the limit of the
Born approximation the ultimate analytical formulas for arbitrary polarization of electromagnetic wave have been obtained
and numerically analyzed.
Received 5 April 2001 / Received in final form 18 March 2002 Published online 28 June 2002 相似文献
10.
T. Janssen O. Radulescu A.N. Rubtsov 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(1):85-95
Aperiodic crystals may have additional low frequency modes related to the possibility to describe them in a higher-dimensional
space. Dynamics associated with these degrees of freedom is called phasonic, but there are very different phenomena of this
type. A discussion is given of the use of the term. The relation between phason modes, the crystal structure, and the modulation
and sliding modes is discussed. Finally a relation with frictionless motion is studied.
Received 4 April 2002 / Received in final form 22 July 2002 Published online 17 September 2002 相似文献
11.
L. Zhang W. Kleemann J. Dec R. Wang M. Itoh 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(2):163-171
The dielectric permittivity ε′ - i
of SrTi 18O 3 (STO18) is studied under a dc electric field E as a function of the temperature, T. In ε′
vs.
T, a double-peak is found when 0 < E < 30 KV/m. While the peak at high-T is attributed to the smeared ferroelectric phase transition, the low-T one is induced by domain wall motion. The transverse Ising model including an external homogeneous and quenched random-fields
is successfully used to describe both the smeared phase transition and the domain wall response in the low-T domain state. The calculations are in good agreement with the experimental results.
Received 4 January 2002 / Received in final form 25 March 2002 Published online 19 July 2002 相似文献
12.
G. Michałek B.R. Bułka 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(1):121-128
The currents and their fluctuations in two capacitively coupled single electron transistors are determined in the limit of
sequential tunnelling. Our considerations are restricted to the case when the islands (dots) of the transistors are atomic-sized,
which means each of them has only one single electronic level available for the tunnelling processes. The Coulomb interactions
of accumulated charges on the both single electron transistors lead to the effect of the negative differential resistance.
An enhancement of the current shot-noise was also found. Spectral decomposition analysis indicated the two main contributions
to the shot-noise: low- and high-frequency fluctuations. It was found that the low frequency fluctuations (polarization noise)
are responsible for a strong enhancement of the current noise.
Received 9 October 2001 / Received in final form 8 March 2002 Published online 9 July 2002 相似文献
13.
A.V. Volotka V.M. Shabaev G. Plunien G. Soff 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,23(1):51-56
The finite nuclear size effect on the hyperfine splitting of low-Z hydrogen-like atoms is studied in the external field approximation. A simple non-relativistic formula is proposed which expresses
the nuclear size correction to the hyperfine splitting in terms of moments of the nuclear charge and magnetization distribution.
The numerical results obtained via this formula are compared with corresponding results derived by means of the Zemach formula. A relativistic formula for the
nuclear size correction to the hyperfine splitting is also derived.
Received 18 March 2002 / Received in final form 14 November 2002 Published online 28 January 2003
RID="a"
ID="a"e-mail: plunien@physik.tu-dresden.de 相似文献
14.
A.V. Shtoff M. Rérat S.I. Gusarov 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,15(2):199-208
Floquet theory is used to describe the response of a molecule to applied radiation electric field. The method of ab initio calculation of frequency dependent (hyper)polarizabilities based on combination of perturbation theory with the finite field
method has been developed. Electron correlation is taken into account by means of the CIPSI algorithm. The total wave function
expansion involves spectral, pseudo-spectral states and polynomial terms. The developed approach is applied to the calculation
of the second hyperpolarizability of the lithium hydride molecule subjected to the superposition of harmonic and static electric
fields. The method can be used to calculate the higher order nonlinear properties of molecules.
Received 23 November 2000 and Received in final form 25 March 2001 相似文献
15.
We study the effect of electrostatic interactions on the distribution function of the end-to-end distance of a single polyelectrolyte
chain in the rod-like limit. The extent to which the radial distribution function of a polyelectrolyte is reproduced by that
of a wormlike chain with an adjusted effective persistence length is investigated. Strong evidence is found for a universal
scaling formula connecting the effective persistence length of a polyelectrolyte with the strength of the electrostatic interaction
and the Debye screening length.
Received 4 March 2002 and Received in final form 1 July 2002
RID="a"
ID="a"e-mail: jrudnick@physics.ucla.edu 相似文献
16.
S. Fratini P. Quémerais 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(1):41-49
We consider a three dimensional Wigner crystal of electrons lying in a host ionic dielectric. Owing to their interaction with
the lattice polarization, each localized electron forms a polaron. We study the collective excitations of such a polaronic
Wigner crystal at zero temperature, taking into account the quantum fluctuations of the polarization within the Feynman harmonic
approximation. We show that, contrary to the ordinary electron crystal, the system undergoes a polarization catastrophe when
the density is increased. An optical signature of this instability is derived, whose trend agrees with the experiments carried
out in Nd-based cuprates.
Received 4 July 2002 Published online 17 September 2002 相似文献
17.
Z.P. Karkuszewski K. Sacha A. Smerzi 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,21(3):251-254
We study a Bose-Einstein condensate trapped in an asymmetric double well potential. Solutions of the time-independent Gross-Pitaevskii
equation reveal intrinsic loops in the energy (or chemical potential) level behavior when the shape of the potential is varied.
We investigate the corresponding behavior of the quantum (many-body) energy levels. Applying the two-mode approximation to
the bosonic field operators, we show that the quantum energy levels create an anti-crossing net inside the region bounded
by the loop of the mean field solution.
Received 6 March 2002 / Received in final form 19 September 2002 Published online 15 November 2002
RID="a"
ID="a"e-mail: smerzi@cnls.lanl.gov 相似文献
18.
Y. Li Voskoboynikov C.P. Lee S.M. Sze O. Tretyak 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(4):475-481
In this article we study the impact of the spin-orbit interaction on the electron quantum confinement for narrow gap semiconductor
quantum dots. The model formulation includes: (1) the effective one-band Hamiltonian approximation; (2) the position- and
energy-dependent quasi-particle effective mass approximation; (3) the finite hard wall confinement potential; and (4) the
spin-dependent Ben Daniel-Duke boundary conditions. The Hartree-Fock approximation is also utilized for evaluating the characteristics
of a two-electron quantum dot system. In our calculation, we describe the spin-orbit interaction which comes from both the
spin-dependent boundary conditions and the Rashba term (for two-electron quantum dot system). It can significantly modify
the electron energy spectrum for InAs semiconductor quantum dots built in the GaAs matrix. The energy state spin-splitting
is strongly dependent on the dot size and reaches an experimentally measurable magnitude for relatively small dots. In addition,
we have found the Coulomb interaction and the spin-splitting are suppressed in quantum dots with small height.
Received 15 May 2001 / Received in final form 14 May 2002 Published online 13 August 2002 相似文献
19.
K. Vetter 《The European Physical Journal A - Hadrons and Nuclei》2002,15(1-2):265-269
Gamma-ray tracking in a closed array of highly segmented HPGe detectors is a new concept for the detection of γ-radiation.
Each of the interacting γ-rays is identified and separated by measuring the energies and positions of individual interactions
and by applying tracking algorithms to reconstruct the scattering sequences, even if many γ-rays hit the array at the same
time. The three-dimensional position and the energy of interactions are determined by using two-dimensionally segmented Ge
detectors along with pulse-shape analysis of the signals. Such a detector will have new and much improved capabilities compared
to current γ-ray spectrometer. One implementation of this concept, called GRETA (Gamma-Ray Energy Tracking Array), is currently
being under development at LBNL.
Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002
RID="a"
ID="a"e-mail: kvetter@lbl.gov 相似文献
20.
A. Minguzzi G. Ferrari Y. Castin 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,17(1):49-55
We describe the excitation spectrum of a two-component neutral Fermi gas with attractive interactions in the superfluid phase
at finite temperature by deriving a suitable Random-Phase approximation in the collisionless regime with the technique of
functional derivatives. The obtained spectrum for the homogeneous gas at small wavevectors contains the Bogoliubov-Anderson
phonon and is essentially different from the spectrum predicted by the static Bogoliubov theory, which instead shows an unphysically
large response. We adapt the results for the homogeneous system to obtain the dynamic structure factor of a harmonically confined
superfluid and we identify in the spectrum a unique feature of the superfluid phase.
Received 28 March 2001 相似文献