655 research outputs found
The quantum Heisenberg antiferromagnet on the square lattice
The pure-quantum self-consistent harmonic approximation, a semiclassical
method based on the path-integral formulation of quantum statistical mechanics,
is applied to the study of the thermodynamic behaviour of the quantum
Heisenberg antiferromagnet on the square lattice (QHAF). Results for various
properties are obtained for different values of the spin and successfully
compared with experimental data.Comment: Proceedings of the Conference "Path Integrals from peV to TeV - 50
Years from Feynman's paper" (Florence, August 1998) -- 2 pages, ReVTeX, 2
figure
High-Resolution Analytical Approach to Describe the Sensitivity of Tree–Environment Dependences through Stem Radial Variation
Spectral shapes of solid neon
We present a Path Integral Monte Carlo calculation of the first three moments
of the displacement-displacement correlation functions of solid neon at
different temperatures for longitudinal and transverse phonon modes. The
Lennard-Jones potential is considered. The relevance of the quantum effects on
the frequency position of the peak and principally on the line-width of the
spectral shape is clearly pointed out. The spectrum is reconstructed via a
continued fraction expansion; the approximations introduced using the effective
potential quantum molecular dynamics are discussed.Comment: 3 pages, 2 figures, 3 table
Two-spin entanglement distribution near factorized states
We study the two-spin entanglement distribution along the infinite
chain described by the XY model in a transverse field; closed analytical
expressions are derived for the one-tangle and the concurrences ,
being the distance between the two possibly entangled spins, for values of the
Hamiltonian parameters close to those corresponding to factorized ground
states. The total amount of entanglement, the fraction of such entanglement
which is stored in pairwise entanglement, and the way such fraction distributes
along the chain is discussed, with attention focused on the dependence on the
anisotropy of the exchange interaction. Near factorization a characteristic
length-scale naturally emerges in the system, which is specifically related
with entanglement properties and diverges at the critical point of the fully
isotropic model. In general, we find that anisotropy rule a complex behavior of
the entanglement properties, which results in the fact that more isotropic
models, despite being characterized by a larger amount of total entanglement,
present a smaller fraction of pairwise entanglement: the latter, in turn, is
more evenly distributed along the chain, to the extent that, in the fully
isotropic model at the critical field, the concurrences do not depend on .Comment: 14 pages, 6 figures. Final versio
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
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.
Thermodynamics of quantum dissipative many-body systems
We consider quantum nonlinear many-body systems with dissipation described
within the Caldeira-Leggett model, i.e., by a nonlocal action in the path
integral for the density matrix. Approximate classical-like formulas for
thermodynamic quantities are derived for the case of many degrees of freedom,
with general kinetic and dissipative quadratic forms. The underlying scheme is
the pure-quantum self-consistent harmonic approximation (PQSCHA), equivalent to
the variational approach by the Feynman-Jensen inequality with a suitable
quadratic nonlocal trial action. A low-coupling approximation permits to get
manageable PQSCHA expressions for quantum thermal averages with a classical
Boltzmann factor involving an effective potential and an inner Gaussian average
that describes the fluctuations originating from the interplay of quanticity
and dissipation. The application of the PQSCHA to a quantum phi4-chain with
Drude-like dissipation shows nontrivial effects of dissipation, depending upon
its strength and bandwidth.Comment: ReVTeX, 12 pages, 9 embedded figures (vers.2: typo mistake fixed
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