9,430 research outputs found
Entanglement in the Quantum Heisenberg XY model
We study the entanglement in the quantum Heisenberg XY model in which the
so-called W entangled states can be generated for 3 or 4 qubits. By the concept
of concurrence, we study the entanglement in the time evolution of the XY
model. We investigate the thermal entanglement in the two-qubit isotropic XY
model with a magnetic field and in the anisotropic XY model, and find that the
thermal entanglement exists for both ferromagnetic and antiferromagnetic cases.
Some evidences of the quantum phase transition also appear in these simple
models.Comment: 7 pages, 6 figs, revised version submitted to Phys. Rev.
Soluble `Supersymmetric' Quantum XY Model
We present a `supersymmetric' modification of the -dimensional quantum
rotor model whose ground state is exactly soluble. The model undergoes a
vortex-binding transition from insulator to metal as the rotor coupling is
varied. The Hamiltonian contains three-site terms which are relevant: they
change the universality class of the transition from that of the ()--- to
the -dimensional classical XY model. The metallic phase has algebraic ODLRO
but the superfluid density is identically zero. Variational wave functions for
single-particle and collective excitations are presented.Comment: 12 pages, REVTEX 3.0, IUCM93-00
Invaded cluster simulations of the XY model in two and three dimensions
The invaded cluster algorithm is used to study the XY model in two and three
dimensions up to sizes 2000^2 and 120^3 respectively. A soft spin O(2) model,
in the same universality class as the 3D XY model, is also studied. The static
critical properties of the model and the dynamical properties of the algorithm
are reported. The results are K_c=0.45412(2) for the 3D XY model and
eta=0.037(2) for the 3D XY universality class. For the 2D XY model the results
are K_c=1.120(1) and eta=0.251(5). The invaded cluster algorithm does not show
any critical slowing for the magnetization or critical temperature estimator
for the 2D or 3D XY models.Comment: 30 pages, 11 figures, problem viewing figures corrected in v
First-Order Phase Transition by XY Model of Particle Dynamics
A gas-liquid type of phase transition is found based on the particle dynamics
on radius- circle in which the coordinate appears as the angle-variable of
1D XY-model. Due to the specific appearance of compact-space radius (volume) in
the present interpretation of XY-model, the ground-state develops a minimum at
some critical radius, leading to the multi-valued Gibbs energy similar to
systems with first-order phase transition.Comment: v1: LaTeX, 7 pages, 3 figs; v2: 9 pages, 5 figs, detailed comparison
with magnetic system interpretation of XY-model is presented. Accepted for
EP
Entanglement renormalization of anisotropic XY model
The renormalization group flows of the one-dimensional anisotropic XY model
and quantum Ising model under a transverse field are obtained by different
multiscale entanglement renormalization ansatz schemes. It is shown that the
optimized disentangler removes the short-range entanglement by rotating the
system in the parameter space spanned by the anisotropy and the magnetic field.
It is understood from the study that the disentangler reduces the entanglement
by mapping the system to another one in the same universality class but with
smaller short range entanglement. The phase boundary and corresponding critical
exponents are calculated using different schemes with different block sizes,
look-ahead steps and truncation dimensions. It is shown that larger truncation
dimension leads to more accurate results and that using larger block size or
look-ahead step improve the overall calculation consistency.Comment: 5 pages, 3 figure
Bound entanglement in the XY model
We study the multi-spin entanglement for the 1D anisotropic XY model
concentrating on the simplest case of three-spin entanglement. As compared to
the pairwise entanglement, three-party quantum correlations have a longer range
and they are more robust on increasing the temperature.
We find regions of the phase diagram of the system where bound entanglement
occurs, both at zero and finite temperature. Bound entanglement in the ground
state can be obtained by tuning the magnetic field. Thermal bound entanglement
emerges naturally due to the effect of temperature on the free ground state
entanglement.Comment: 7 pages, 3 figures; some typos corrected, references adde
Magnetic screening in proximity effect Josephson-junction arrays
The modulation with magnetic field of the sheet inductance measured on
proximity effect Josephson-junction arrays (JJAs) is progressively vanishing on
lowering the temperature, leading to a low temperature field-independent
response. This behaviour is consistent with the decrease of the two-dimensional
penetration length below the lattice parameter. Low temperature data are
quantitatively compared with theoretical predictions based on the XY model in
absence of thermal fluctuations. The results show that the description of a JJA
within the XY model is incomplete and the system is put well beyond the weak
screening limit which is usually assumed in order to invoke the well known
frustrated XY model describing classical Josephson-junction arrays.Comment: 6 pages, 5 figure
XY model in small-world networks
The phase transition in the XY model on one-dimensional small-world networks
is investigated by means of Monte-Carlo simulations. It is found that
long-range order is present at finite temperatures, even for very small values
of the rewiring probability, suggesting a finite-temperature transition for any
nonzero rewiring probability. Nature of the phase transition is discussed in
comparison with the globally-coupled XY model.Comment: 5 pages, accepted in PR
- …