67,624 research outputs found
Stability and ensemble inequivalence in a globally coupled system
We consider a system of globally coupled rotors, described by a set of
Langevin equations, and examine stability of the incoherent phase. The
corresponding Fokker-Planck equation, providing a unified description of
microcanonical and canonical ensembles, bears a few solutions, depending upon
the ensemble. It is found that the stability of each solution varies
differently with the temperature, revealing the inequivalence between the two
ensembles. This also suggests a physical explanation of the quasi-stationarity
observed in recent numerical results.Comment: 4 pages, no figur
On the dynamics of traveling phase-oscillators with positive and negative couplings
We investigate numerically the dynamics of traveling clusters in systems of
phase oscillators, some of which possess positive couplings and others negative
couplings. The phase distribution, speed of traveling, and average separation
between clusters as well as order parameters for positive and negative
oscillators are computed, as the ratio of the two coupling constants and/or the
fraction of positive oscillators are varied. The traveling speed depending on
these parameters is obtained and observed to fit well with the numerical data
of the systems. With the help of this, we describe the conditions for the
traveling state to appear in the systems with or without periodic driving.Comment: 5 pages, 7 figure
Interaction Effects on the Size Distribution in a Growth Model
We study, both analytically and numerically, the interaction effects on the
skewness of the size distribution of elements in a growth model. We incorporate
two types of global interaction into the growth model, and develop analytic
expressions for the first few moments from which the skewness of the size
distribution is calculated. It is found that depending on the sign of coupling,
interactions may suppress or enhance the size growth, which in turn leads to
the decrease or increase of the skewness. The amount of change tends to
increase with the coupling strength, rather irrespectively of the details of
the model.Comment: 8 pages, 9 figure
Quantum Phase Transitions and Persistent Currents in Josephson-Junction Ladders
In this work we study quantum phase transitions and persistent currents in
capacitively coupled one-dimensional Josephson-junction arrays. We will focus
particularly on the roles of exciton-like pairs in the strong coupling limit in
the presence of external gate charges and magnetic fluxes. We use the numerical
density-matrix renormalization group method for the study in the full range of
values of gate charge and magnetic flux. To clarify the various effects, we
report the pair correlation functions and the exciton densities as welll as the
persistent current.Comment: To appear in Phys. Rev. B; title has been changed; a few parts of the
text have been change; 12 pages, 17 figure
Quantum Dissipative Dynamics of Entanglement in the Spin-Boson Model
We study quantum dissipative dynamics of entanglement in the spin-boson
model, described by the generalized master equation. We consider the two
opposite limits of pure-dephasing and relaxation models, measuring the degree
of entanglement with the concurrence. When the Markovian approximation is
employed, entanglement is shown to decay exponentially in both cases. On the
other hand, non-Markovian contributions alter the analytic structure of the
master equation, resulting in logarithmic decay in the pure dephasing model
Quantum Phase Transitions and Particle-Hole Pair Transport in Capacitively Coupled Josephson-Junction Chains
We consider two chains of ultrasmall Josephson junctions, coupled
capacitively with each other, and investigate the transport of particle-hole
pairs and the quantum phase transitions at zero temperature. For appropriate
parameter ranges, the particle-hole pairs are found to play major roles in
transport phenomena; condensation of such pairs leads to the superconducting
state, displaying perfect drag of supercurrents along the two chains.Comment: REVTeX, EPS figures. Submitted to PR
Dissipative Dynamics of Quantum Vortices in Superconducting Arrays
We consider a two-dimensional array of ultra-small superconducting grains,
weakly coupled by Josephson junctions with large charging energy. We start from
an effective action based on a microscopic tunneling Hamiltonian, which
includes quasiparticle degrees of freedom, and study the resulting dissipative
dynamics of quantum vortices. The equation of motion for a single vortex is
deduced, and compared with a commonly adopted phenomenological model.Comment: REVTeX, 1 EPS figure, To appear in PR
Strong ferromagnetic-dielectric coupling in multiferroic Lu2CoMnO6 single crystals
We have grown single crystals of multiferroic double-perovskite Lu2CoMnO6 and
studied the directional dependence of their magnetic and dielectric properties.
The ferromagnetic order emerges below TC ~ 48K along the crystallographic c
axis. Dielectric anomaly arises along the b axis at TC, contrary to the
polycrystalline work suggesting ferroelectricity along the c axis. Through the
strongly coupled ferromagnetic and dielectric states, the highly non-linear
variation of both dielectric constant and magnetization was achieved in
application of magnetic fields. This concurrent tunability provides an
efficient route to manipulation of multiple order parameters in multiferroics
Optimal storage capacity of neural networks at finite temperatures
Gardner's analysis of the optimal storage capacity of neural networks is
extended to study finite-temperature effects. The typical volume of the space
of interactions is calculated for strongly-diluted networks as a function of
the storage ratio , temperature , and the tolerance parameter ,
from which the optimal storage capacity is obtained as a function of
and . At zero temperature it is found that regardless of
while in general increases with the tolerance at finite
temperatures. We show how the best performance for given and is
obtained, which reveals a first-order transition from high-quality performance
to low-quality one at low temperatures. An approximate criterion for recalling,
which is valid near , is also discussed.Comment: 22 pages in LaTex, 4 figures upon request, SNUTP-93-2
Traveling cluster pairs in a system of phase oscillators with positive and negative couplings under a periodic driving field
We investigate numerically the clustering behavior of a system of phase
oscillators with positive and negative couplings under a periodic external
driving field with a bimodal distribution of driving phases. The phase
distribution and the mean speed of the traveling state, as well as the order
parameter for synchronization, are computed as the driving amplitude is varied.
We observe that the periodically-driven system can also host traveling states
for parameters in the same range as those for the case of a system without a
driving field. The traveling speed is found to depend non-monotonically on the
driving amplitude. In particular, oscillators divide into four clusters and
move in pairs. Further, depending on the driving amplitude, two kinds of
traveling mode arise: pairs of clusters traveling in the same direction
(symmetric mode) and in opposite directions (antisymmetric mode). In the latter
case (antisymmetric traveling mode), the average phase speed of the whole
system apparently vanishes. A phenomenological argument for such behavior is
given.Comment: 5 pages, 7 figures. arXiv admin note: text overlap with
arXiv:1408.2894 by other author
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