51 research outputs found
Effective potential approach to quantum dissipation in condensed matter systems
The effects of dissipation on the thermodynamic properties of nonlinear
quantum systems are approached by the path-integral method in order to
construct approximate classical-like formulas for evaluating thermal averages
of thermodynamic quantities. Explicit calculations are presented for
one-particle and many-body systems. The effects of the dissipation mechanism on
the phase diagram of two-dimensional Josephson arrays is discussed.Comment: 7 pages, 5 figures, to appear in the Proceedings of Nonlinearity,
Integrability And All That 20 Years After Needs 7
Entanglement and factorized ground states in two-dimensional quantum antiferromagnets
Making use of exact results and quantum Monte Carlo data for the entanglement
of formation, we show that the ground state of anisotropic two-dimensional
S=1/2 antiferromagnets in a uniform field takes the classical-like form of a
product state for a particular value and orientation of the field, at which the
purely quantum correlations due to entanglement disappear. Analytical
expressions for the energy and the form of such states are given, and a novel
type of exactly solvable two-dimensional quantum models is therefore singled
out. Moreover, we show that the field-induced quantum phase transition present
in the models is unambiguously characterized by a cusp minimum in the
pairwise-to-global entanglement ratio R, marking the quantum-critical
enhancement of \emph{multipartite} entanglement. A detailed discussion is
provided on the universality of the cusp in R as a signature of quantum
critical behavior entirely based on entanglement.Comment: 4 pages, 3 figure
Staggered magnetization and entanglement enhancement by magnetic impurities in spin chain
We study the effects of a magnetic impurity on the behavior of a spin
chain. At T=0, both with and without an applied uniform magnetic field, an
oscillating magnetization appears, whose decay with the distance from the
impurity is ruled by a power law. As a consequence, pairwise entanglement is
either enhanced or quenched, depending on the distance of the spin pair with
respect to the impurity and on the values of the magnetic field and the
intensity of the impurity itself. This leads us to suggest that acting on such
control parameters, an adiabatic manipulation of the entanglement distribution
can be performed. The robustness of our results against temperature is checked,
and suggestions about possible experimental applications are put forward.Comment: 4 pages, 8 figure
Reading entanglement in terms of spin configurations in quantum magnets
We consider a quantum many-body system made of interacting
spins on a lattice, and develop a formalism which allows to extract, out of
conventional magnetic observables, the quantum probabilities for any selected
spin pair to be in maximally entangled or factorized two-spin states. This
result is used in order to capture the meaning of entanglement properties in
terms of magnetic behavior. In particular, we consider the concurrence between
two spins and show how its expression extracts information on the presence of
bipartite entanglement out of the probability distributions relative to
specific sets of two-spin quantum states. We apply the above findings to the
antiferromagnetic Heisenberg model in a uniform magnetic field, both on a chain
and on a two-leg ladder. Using Quantum Monte Carlo simulations, we obtain the
above probability distributions and the associated entanglement, discussing
their evolution under application of the field.Comment: Final version, to appear in European Physical Journal
Simulating Quantum Dissipation in Many-Body Systems
An efficient Path Integral Monte Carlo procedure is proposed to simulate the
behavior of quantum many-body dissipative systems described within the
framework of the influence functional. Thermodynamic observables are obtained
by Monte Carlo sampling of the partition function after discretization and
Fourier transformation in imaginary time of the dynamical variables. The method
is tested extensively for model systems, using realistic dissipative kernels.
Results are also compared with the predictions of a recently proposed
semiclassical approximation, thus testing the reliability of the latter
approach for weak quantum coupling. Our numerical method opens the possibility
to quantitatively describe real quantum dissipative systems as, e.g., Josephson
junction arrays.Comment: 10 pages, 4 figure
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