687 research outputs found
Nonequilibrium dynamics of the Ising chain in a fluctuating transverse field
We study nonequilibrium dynamics of the quantum Ising chain at zero
temperature when the transverse field is varied stochastically. In the
equivalent fermion representation, the equation of motion of Majorana operators
is derived in the form of a one-dimensional, continuous-time quantum random
walk with stochastic, time-dependent transition amplitudes. This type of
external noise gives rise to decoherence in the associated quantum walk and the
semiclassical wave-packet generally has a diffusive behavior. As a consequence,
in the quantum Ising chain, the average entanglement entropy grows in time as
and the logarithmic average magnetization decays in the same form. In
the case of a dichotomous noise, when the transverse-field is changed in
discrete time-steps, , there can be excitation modes, for which coherence
is maintained, provided their energy satisfies
with a positive integer . If the dispersion of is quadratic,
the long-time behavior of the entanglement entropy and the logarithmic
magnetization is dominated by these ballistically traveling coherent modes and
both will have a time-dependence.Comment: 12 pages, 10 figure
Reducing defect production in random transverse-field Ising chains by inhomogeneous driving fields
In transverse-field Ising models, disorder in the couplings gives rise to a
drastic reduction of the critical energy gap and, accordingly, to an
unfavorable, slower-than-algebraic scaling of the density of defects produced
when the system is driven through its quantum critical point. By applying
Kibble-Zurek theory and numerical calculations, we demonstrate in the
one-dimensional model that the scaling of defect density with annealing time
can be made algebraic by balancing the coupling disorder with suitably chosen
inhomogeneous driving fields. Depending on the tail of the coupling
distribution at zero, balancing can be either perfect, leading to the
well-known inverse-square law of the homogeneous system, or partial, still
resulting in an algebraic decrease but with a smaller, non-universal exponent.
We also study defect production during an environment-temperature quench of the
open variant of the model in which the system is slowly cooled down to its
quantum critical point. According to our scaling and numerical results,
balanced disorder leads again to an algebraic temporal decrease of the defect
density.Comment: 11 pages, 6 figure
Scaling behavior of the contact process in networks with long-range connections
We present simulation results for the contact process on regular, cubic
networks that are composed of a one-dimensional lattice and a set of long edges
with unbounded length. Networks with different sets of long edges are
considered, that are characterized by different shortest-path dimensions and
random-walk dimensions. We provide numerical evidence that an absorbing phase
transition occurs at some finite value of the infection rate and the
corresponding dynamical critical exponents depend on the underlying network.
Furthermore, the time-dependent quantities exhibit log-periodic oscillations in
agreement with the discrete scale invariance of the networks. In case of
spreading from an initial active seed, the critical exponents are found to
depend on the location of the initial seed and break the hyper-scaling law of
the directed percolation universality class due to the inhomogeneity of the
networks. However, if the cluster spreading quantities are averaged over
initial sites the hyper-scaling law is restored.Comment: 9 pages, 10 figure
Correlation amplitude and entanglement entropy in random spin chains
Using strong-disorder renormalization group, numerical exact diagonalization,
and quantum Monte Carlo methods, we revisit the random antiferromagnetic XXZ
spin-1/2 chain focusing on the long-length and ground-state behavior of the
average time-independent spin-spin correlation function C(l)=\upsilon
l^{-\eta}. In addition to the well-known universal (disorder-independent)
power-law exponent \eta=2, we find interesting universal features displayed by
the prefactor \upsilon=\upsilon_o/3, if l is odd, and \upsilon=\upsilon_e/3,
otherwise. Although \upsilon_o and \upsilon_e are nonuniversal (disorder
dependent) and distinct in magnitude, the combination \upsilon_o + \upsilon_e =
-1/4 is universal if C is computed along the symmetric (longitudinal) axis. The
origin of the nonuniversalities of the prefactors is discussed in the
renormalization-group framework where a solvable toy model is considered.
Moreover, we relate the average correlation function with the average
entanglement entropy, whose amplitude has been recently shown to be universal.
The nonuniversalities of the prefactors are shown to contribute only to surface
terms of the entropy. Finally, we discuss the experimental relevance of our
results by computing the structure factor whose scaling properties,
interestingly, depend on the correlation prefactors.Comment: v1: 16 pages, 15 figures; v2: 17 pages, improved discussions and
statistics, references added, published versio
Asymmetric simple exclusion process in one-dimensional chains with long-range links
We study the boundary-driven asymmetric simple exclusion process (ASEP) in a
one-dimensional chain with long-range links. Shortcuts are added to a chain by
connecting different pairs of sites selected randomly where and
denote the chain length and the shortcut density, respectively. Particles flow
into a chain at one boundary at rate and out of a chain at the other
boundary at rate , while they hop inside a chain via nearest-neighbor
bonds and long-range shortcuts. Without shortcuts, the model reduces to the
boundary-driven ASEP in a one-dimensional chain which displays the low density,
high density, and maximal current phases. Shortcuts lead to a drastic change.
Numerical simulation studies suggest that there emerge three phases; an empty
phase with , a jammed phase with , and a shock phase
with where is the mean particle density. The shock phase is
characterized with a phase separation between an empty region and a jammed
region with a localized shock between them. The mechanism for the shock
formation and the non-equilibrium phase transition is explained by an analytic
theory based on a mean-field approximation and an annealed approximation.Comment: revised version (16 pages and 6 eps figures
Generalization of seizures parallels the formation of "dark" neurons in the hippocampus and pontine reticular formation after focal-cortical application of 4-aminopyridine (4-AP) in the rat.
Distribution and time course of the occurrence of "dark" neurons were compared with the EEG activity and behavior of rats during 4-aminopyridine (4-AP) induced epileptic seizures. A crystal of the K(+) channel blocker 4-AP (0.5 mg/kg) was placed onto the exposed parieto-occipital cortex of Halothane-anesthetized rats for 40 min. Thereafter, the anesthesia was discontinued and the behavioral signs of the epileptic seizure activity were observed. The presence of "dark" neurons was demonstrated by the sensitive silver method of Gallyas in rats sacrificed at 0, 3 and 6 h after the end of the 4-AP crystal application. The EEG activity was recorded in the rats with longer survival times. The EEG analysis revealed the generalization of the epileptic seizures. We found that the formation of "dark" neurons in the hippocampus and the pontine reticular formation paralleled the generalization of the seizures
- …