366 research outputs found
Effective Hamiltonian with holomorphic variables
The pure-quantum self-consistent harmonic approximation (PQSCHA) permits to
study a quantum system by means of an effective classical Hamiltonian. In this
work the PQSCHA is reformulated in terms of the holomorphic variables connected
to a set of bosonic operators. The holomorphic formulation, based on the
olomorphic path integral for the Weyl symbol of the density matrix, makes it
possible to directly approach general Hamiltonians given in terms of bosonic
creation and annihilation operators.Comment: Proceedings of the Conference "Path Integrals from peV to TeV - 50
Years from Feynman's paper" (Florence, August 1998) -- 2 pages, ReVTe
Effective Hamiltonian with holomorphic variables
The pure-quantum self-consistent harmonic approximation (PQSCHA) permits to
study a quantum system by means of an effective classical Hamiltonian -
depending on quantum coupling and temperature - and classical-like expressions
for the averages of observables. In this work the PQSCHA is derived in terms of
the holomorphic variables connected to a set of bosonic operators. The
holomorphic formulation, based on the path integral for the Weyl symbol of the
density matrix, makes it possible to approach directly general Hamiltonians
given in terms of bosonic creation and annihilation operators.Comment: 11 pages, no figures (2nd version: few mistakes fixed in Sects. IV-V
Effective Potential and Quantum Dynamical Correlators
The approach to the calculation of quantum dynamical correlation functions is
presented in the framework of the Mori theory. An unified treatment of classic
and quantum dynamics is given in terms of Weyl representation of operators and
holomorphic variables. The range of validity of an approximate molucular
dynamics is discussedComment: 8 pages, Latex fil
Quantum fluctuations in one-dimensional arrays of condensates
The effects of quantum and thermal fluctuations upon the fringe structure
predicted to be observable in the momentum distribution of coupled
Bose-Einstein condensates are studied by the effective-potential method. For a
double-well trap, the coherence factor recently introduced by Pitaevskii and
Stringari [Phys. Rev. Lett. 87, 180402 (2001)] is calculated using the
effective potential approach and is found in good agreement with their result.
The calculations are extended to the case of a one-dimensional array of
condensates, showing that quantum effects are essentially described through a
simple renormalization of the energy scale in the classical analytical
expression for the fringe structure. The consequences for the experimental
observability are discussed.Comment: RevTeX, 4 pages, 5 eps figures (published version with updated
references
Minimal coupling in oscillator models of quantum dissipation
The dissipative harmonic oscillator has two representations. In the first
representation the central oscillator couples with its position to an
oscillator bath. In the second one it couples with its momentum to the bath.
Both representations are related by a unitary transformation. If the central
oscillator couples with its position and momentum to two independent baths, no
such unitary transformation exists. We discuss two possible models of this type
and their physical relevance
Heisenberg antiferromagnet on the square lattice for S>=1
Theoretical predictions of a semiclassical method - the pure-quantum
self-consistent harmonic approximation - for the correlation length and
staggered susceptibility of the Heisenberg antiferromagnet on the square
lattice (2DQHAF) agree very well with recent quantum Monte Carlo data for S=1,
as well as with experimental data for the S=5/2 compounds Rb2MnF4 and KFeF4.
The theory is parameter-free and can be used to estimate the exchange coupling:
for KFeF4 we find J=2.33 +- 0.33 meV, matching with previous determinations. On
this basis, the adequacy of the quantum nonlinear sigma model approach in
describing the 2DQHAF when S>=1 is discussed.Comment: 4 pages RevTeX file with 5 figures included by psfi
Thermodynamics of the quantum easy-plane antiferromagnet on the triangular lattice
The classical XXZ triangular-lattice antiferromagnet (TAF) shows both an
Ising and a BKT transition, related to the chirality and the in-plane spin
components, respectively. In this paper the quantum effects on the
thermodynamic quantities are evaluated by means of the pure-quantum
self-consistent harmonic approximation (PQSCHA), that allows one to deal with
any spin value through classical MC simulations. We report the internal energy,
the specific heat, and the in-plane correlation length of the quantum XX0 TAF,
for S=1/2, 1, 5/2. The quantum transition temperatures turn out to be smaller
the smaller the spin, and agree with the few available theoretical and
numerical estimates.Comment: 4 pages,3 postscript figure
Quantum thermodynamics of systems with anomalous dissipative coupling
The standard {\em system-plus-reservoir} approach used in the study of
dissipative systems can be meaningfully generalized to a dissipative coupling
involving the momentum, instead of the coordinate: the corresponding equation
of motion differs from the Langevin equation, so this is called {\em anomalous}
dissipation. It occurs for systems where such coupling can indeed be derived
from the physical analysis of the degrees of freedom which can be treated as a
dissipation bath. Starting from the influence functional corresponding to
anomalous dissipation, it is shown how to derive the effective classical
potential that gives the quantum thermal averages for the dissipative system in
terms of classical-like calculations; the generalization to many degrees of
freedom is given. The formalism is applied to a single particle in a
double-well and to the discrete model. At variance with the standard
case, the fluctuations of the coordinate are enhanced by anomalous dissipative
coupling.Comment: 12 pages, 5 figures, to be published in Phys. Rev.
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