共查询到20条相似文献,搜索用时 15 毫秒
1.
H.-P. Breuer U. Dorner F. Petruccione 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,14(3):377-386
In a recent experiment the progressive decoherence of a mesoscopic superposition of two coherent field states in a high-Q cavity, known as Schr?dinger cat state, has been measured for the first time [Brune et al., Phys. Rev. Lett. 77, 4887 (1996)]. Here, the full master equation governing the coupled dissipative dynamics of the atom-field system studied
in the experiment is formulated and solved numerically for the experimental parameters. The model simulated avoids the approximations
underlying an analytically solvable model which is based on a harmonic expansion of the energies of the dressed atomic states
and on a treatment of their dynamics within the adiabatic approximation. In particular, the numerical simulations reveal that
the coupling of the cavity field mode to its environment causes important decoherence effects already during the initial preparation
phase of the Schr?dinger cat state. This phenomenon is investigated in detail with the help of a measure for the purity of
states. Moreover, the Hilbert-Schmidt distance of the intended target state, the Schr?dinger cat, to the state that is actually
prepared in the experiment is determined.
Received 13 September 2000 and Received in final form 22 December 2000 相似文献
2.
The usual semiclassical approximation for atom-field dynamics consists in substituting the field operators by complex numbers related to the (supposedly large enough) intensity of the field. We show that a semiclassical evolution for coupled systems can always be obtained by frequent Zeno-like measurements on the state of one subsystems, independently of the field intensity in the example given. We study the Jaynes-Cummings model from this perspective. 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.
R. D'Hulst G.J. Rodgers 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(4):619-625
We present a model describing the competition between information transmission and decision making in financial markets. The
solution of this simple model is recalled, and possible variations discussed. It is shown numerically that despite its simplicity,
it can mimic a size effect comparable to a crash localized in time. Two extensions of this model are presented that allow
to simulate the demand process. One of these extensions has a coherent stable equilibrium and is self-organized, while the
other has a bistable equilibrium, with a spontaneous segregation of the population of agents. A new model is introduced to
generate a transition between those two equilibriums. We show that the coherent state is dominant up to an equal mixing of
the two extensions. We focus our attention on the microscopic structure of the investment rate, which is the main parameter
of the original model. A constant investment rate seems to be a very good approximation.
Received 7 August 2000 and Received in final form 10 September 2000 相似文献
5.
Ll. Serra A. Puente 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,14(1):77-81
The magnetic extension of the Thomas-Fermi-Weizs?cker kinetic energy is used within density-functional-theory to numerically
obtain the ground state densities and energies of two-dimensional quantum dots. The results are thoroughly compared with the
microscopic Kohn-Sham ones in order to assess the validity of the semiclassical method. Circular as well as deformed systems
are considered.
Received 26 October 2000 and Received in final form 14 December 2000 相似文献
6.
E.G. Thrapsaniotis 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,15(1):19-23
In the present paper we study the dynamics of one-electron atoms in the presence of both a linearly polarized squeezed vacuum
and a magnetic field along the polarization vector of the photonic field. We adopt the dipole approximation and approach the
problem via path integral methods. After integrating over the light variables for certain initial and final squeezed vacuum states we
treat the path integral over the spatial variables via Monte-Carlo methods. As an application we calculate the survival probability of the ground state of a one-electron atom for
various values of the magnetic field.
Received 30 November 2000 and Received in final form 15 February 2001 相似文献
7.
J. Tempere J.T. Devreese 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(1):27-33
A sum rule for the first frequency moment of the optical absorption of a many-polaron system is derived, taking into account
many-body effects in the system of constituent charge carriers of the many-polaron system. In our expression for the sum rule,
the electron-phonon coupling and the many-body effects in the electron (or hole) system formally decouple, so that the many-body
effects can be treated to the desired level of approximation by the choice of the dynamical structure factor of the electron
(hole) gas. We calculate correction factors to take into account both low and high experimental cutoff frequencies.
Received 26 April 2000 and Received in final form 5 December 2000 相似文献
8.
P. Stein N. Aßfalg H. Finkelmann P. Martinoty 《The European physical journal. E, Soft matter》2001,4(3):255-262
We investigate the behavior of the complex shear modulus of a series of elastomers including mono-domain and poly-domain liquid
crystal samples, and a non-mesomorphic sample. We find that the dynamics of the glass transition are strongly modified by
the nematic order. This result explains why the truly elastic response of liquid crystal elastomers can only be observed in
the isotropic phase at very high temperatures and at very low frequencies. Between the elastic regime and the glassy state,
the elastomers have a visco-elastic regime, which is characterized by a Rouse-like behavior for mono-domain and poly-domain
samples, and by a Zimm-like behavior for the non-mesomorphic sample. We also show that the mono-domain sample exhibits marked
anisotropy of the shear-modulus G
′. This anisotropy, which is observed for the first time, is a function of frequency and is inverted between low and high frequencies,
due to relaxation effects of the orientational order.
Received 28 January 2000 and Received in final form 16 October 2000 相似文献
9.
J. Gemmer G. Mahler 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,17(3):385-393
For a bi-partite quantum system defined in a finite dimensional Hilbert-space we investigate in what sense entanglement change
and interactions imply each other. For this purpose we introduce an entanglement-operator, which is then shown to represent
a non-conserved property for any bi-partite system and any type of interaction. This general relation does not exclude the
existence of special initial product states, for which the entanglement remains small over some period of time, despite interactions.
For this case we derive an approximation to the full Schr?dinger-equation, which allows the treatment of the composite systems
in terms of product states. The induced error is estimated. In this factorization-approximation one subsystem appears as an
effective potential for the other. A pertinent example is the Jaynes-Cummings model, which then reduces to the semi-classical
rotating wave approximation.
Received 8 June 2001 相似文献
10.
W. Ihra M. Leadbeater J.L. Vega K. Richter 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(3):425-435
We study the effect on the density of states in mesoscopic ballistic billiards to which a superconducting lead is attached.
The expression for the density of states is derived in the semiclassical S-matrix formalism shedding light onto the origin of the differences between the semiclassical theory and the corresponding
result derived from random matrix models. Applications to a square billiard geometry and billiards with boundary roughness
are discussed. The saturation of the quasiparticle excitation spectrum is related to the classical dynamics of the billiard.
The influence of weak magnetic fields on the proximity effect in rough Andreev billiards is discussed and an analytical formula
is derived. The semiclassical theory provides an interpretation for the suppression of the proximity effect in the presence
of magnetic fields as a coherence effect of time reversed trajectories. It is shown to be in good agreement with quantum mechanical
calculations.
Received 21 August 1999 and Received in final form 21 March 2001 相似文献
11.
J. Berg S. Franz M. Sellitto 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(3):349-356
The Edwards hypothesis of ergodicity of blocked configurations for gently tapped granular materials is tested for aaabstract
models of spin systems on random graphs and spin chains with kinetic constraints. The tapping dynamics is modeled by considering
two distinct mechanisms of energy injection: thermal and random tapping. We find that ergodicity depends upon the tapping procedure (i.e. the way the blocked configurations are dynamically accessed): for thermal tapping ergodicity is a good approximation, while
it fails to describe the asymptotic stationary state reached by the random tapping dynamics.
Received 30 November 2001 相似文献
12.
13.
F. Thalmann 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(1):65-73
The mean field approach of glassy dynamics successfully describes systems which are out-of-equilibrium in their low temperature
phase. In some cases an aging behaviour is found, with no stationary regime ever reached. In the presence of dissipative forces
however, the dynamics is indeed stationary, but still out-of-equilibrium, as inferred by a significant violation of the fluctuation
dissipation theorem. The mean field dynamics of a particle in a random but short-range correlated environment, offers the
opportunity of observing both the aging and driven stationary regimes. Using a geometrical approach previously introduced
by the author, we study here the relation between these two situations, in the pure relaxational limit, i.e. the zero temperature case. In the stationary regime, the velocity (v)-force (F) characteristics is a power law v∼F
4, while the characteristic times scale like powers of v, in agreement with an early proposal by Horner. The cross-over between the aging, linear-response regime and the non-linear
stationary regime is smooth, and we propose a parametrization of the correlation functions valid in both cases, by means of
an “effective time”. We conclude that aging and non-linear response are dual manifestations of a single out-of-equilibrium
state, which might be a generic situation.
Received 7 May 2000 and Received in final form 22 August 2000 相似文献
14.
J. Barré F. Bouchet T. Dauxois S. Ruffo 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(4):577-591
We study an analytically tractable model with long-range interactions for which an out-of-equilibrium very long-lived coherent structure spontaneously appears. The dynamics of this model is indeed
very peculiar: a bicluster forms at low energy and is stable for very long time, contrary to statistical mechanics predictions. We first explain the
onset of the structure, by approximating the short time dynamics with a forced Burgers equation. The emergence of the bicluster
is the signature of the shock waves present in the associated hydrodynamical equations. The striking quantitative agreement
with the dynamics of the particles fully confirms this procedure. We then show that a very fast timescale can be singled out
from a slower motion. This enables us to use an adiabatic approximation to derive an effective Hamiltonian that describes
very well the long time dynamics. We then get an explanation of the very long time stability of the bicluster: this out-of-equilibrium
state corresponds to a statistical equilibrium of an effective mean-field dynamics.
Received 28 February 2002 / Received in final form 24 July 2002 Published online 31 October 2002
RID="a"
ID="a"e-mail: Thierry.Dauxois@ens-lyon.fr
RID="b"
ID="b"UMR-CNRS 5672
RID="c"
ID="c"UMR 5582 相似文献
15.
E. Del Gado L. de Arcangelis A. Coniglio 《The European physical journal. E, Soft matter》2000,2(4):359-365
Within a recently introduced model based on the bond-fluctuation dynamics, we study the viscoelastic behaviour of a polymer
solution at the gelation threshold. We here present the results of the numerical simulation of the model on a cubic lattice:
the percolation transition, the diffusion properties and the time autocorrelation functions have been studied. From both the
diffusion coefficients and the relaxation times critical behaviour a critical exponent k for the viscosity coefficient has been extracted: the two results are comparable within the errors giving , in close agreement with the Rouse model prediction and with some experimental results. In the critical region below the
transition threshold the time autocorrelation functions show a long-time tail which is well fitted by a stretched exponential
decay.
Received 20 December 1999 and Received in final form 18 February 2000 相似文献
16.
D.F. Underwood T. Kippeny S.J. Rosenthal 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):241-244
The dynamics of photo-generated electrons and holes in CdSe quantum dots have been studied using the femtosecond fluorescence
upconversion technique, permitting an unambiguous examination of the excited state. The band edge emission shows an expected
size dependence on the decay rate. We find that the deep trap emission is coupled to the band edge fluorescence, implicating
surface states as important factors in the excited state lifetime of the hole. As a factor of the overall efficiency of solar
cells, the rate of charge separation and the fate of the exciton are important considerations in the design of nanocrystal-based
photovoltaic devices.
Received 30 November 2000 相似文献
17.
S. Liehr K. Pawelzik 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(4):555-559
Assuming that financial markets behave similar to random walk processes we derive a trading strategy with variable investment
which is based on the equivalence of the period of bankruptcy risk and the risk to profit ratio. We define a state dependent
predictability measure which can be attributed to the deterministic and stochastic components of the price dynamics. The influence
of predictability variations and especially of short term inefficiency structures on the optimal amount of investment is analyzed
in the given context and a method for adaptation of a trading system to the proposed objective function is presented. Finally
we show the performance of our trading strategy on the DAX and S&P 500 as examples for real world data using different types of prediction models in comparison.
Received 15 September 2000 and Received in final form 2 October 2000 相似文献
18.
T.J. Davis 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,14(1):111-118
The diffraction of laser-cooled atoms from a spatially-periodic potential is modelled using rigorous coupled-wave analysis.
This numerical technique, normally applied to light-diffraction, is adapted for use with atomic de Broglie waves incident
on a reflecting diffraction grating. The technique approximates the potential by a large number of constant layers and successively
solves the complex eigenvalue problem in each layer, propagating the solution up to the surface of the grating. The method
enables the diffraction efficiencies to be calculated for any periodic potential. The results from the numerical model are
compared with the thin phase-grating approximation formulae for evanescent light-wave diffraction gratings and idealised magnetic
diffraction gratings. The model is applied to the problem of diffracting Rb atoms from a grating made from an array of permanent
magnets.
Received 13 June 2000 and Received in final form 15 December 2000 相似文献
19.
A.I. Vdovin A.N. Storozhenko 《The European Physical Journal A - Hadrons and Nuclei》1999,5(3):263-273
The renormalized random phase approximation for hot finite Fermi systems is evaluated with the use of the thermo field dynamics
formalism. This approximation treats vibrations of a hot finite Fermi system as harmonic ones but takes into account the Pauli
principle in a more proper way than the usual thermal RPA, thus incorporating a new type of correlations in a thermal ground
state. To demonstrate advantages of the approximation and to analyze a range of its validity, it is applied to the exactly
solvable Lipkin model. A comparison is made with the exact grand canonical ensemble calculations, results of the thermal Hartree
– Fock approximation and the thermal random phase approximation. The intrinsic energy of the system, the heat capacity, the
average value of the quasispin operator z-projection and the particle number variance are calculated as functions of temperature.
On the whole, the thermal renormalized RPA appears to be a better approximation than the other two. Its advantage is especially
evident in the vicinity of the phase transition point. It is found that within TRRPA the phase transition occurs at lower
temperature than in THFA and TRPA.
Received: 4 January 1999 / Revised version: 10 March 1999 相似文献
20.
Z.W. Shen T. Chen M. Heid W. Kiefer V. Engel 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,14(2):167-172
We present simulations on pump-dump-probe experiments performed on the potassium dimer. The interaction of two time-delayed
laser pulses prepares vibrational wave packets in the electronic ground state. The quantum calculations reveal to what extent
it is possible to prepare a ground state superposition of states with high versus low vibrational quantum numbers by changing the pump-dump delay time. It is shown that transient signals may exhibit interference
effects which are due to characteristics of ground state wave-packets composed of two components showing different vibrational
dynamics. In this way the signals are able to yield information about vibrational overtone motion.
Received 27 September 2000 and Received in final form 21 November 2000 相似文献