519 research outputs found
Many-body localization phase transition
We use exact diagonalization to explore the many-body localization transition in a random-field spin-1/2
chain. We examine the correlations within each many-body eigenstate, looking at all high-energy states and
thus effectively working at infinite temperature. For weak random field the eigenstates are thermal, as expected
in this nonlocalized, âergodicâ phase. For strong random field the eigenstates are localized with only shortrange entanglement. We roughly locate the localization transition and examine some of its finite-size scaling,
finding that this quantum phase transition at nonzero temperature might be showing infinite-randomness scaling with a dynamic critical exponent zâïżœ
Energy transport in disordered classical spin chains
We present a numerical study of the diffusion of energy at high temperature in strongly disordered chains of
interacting classical spins evolving deterministically. We find that quenched randomness strongly suppresses
transport with the diffusion constant becoming reduced by several orders of magnitude upon the introduction
of moderate disorder. We have also looked for but not found signs of a classical many-body localization
transition at any nonzero strength of the spin-spin interactions
Localization-protected quantum order
Closed quantum systems with quenched randomness exhibit many-body localized regimes wherein they do not equilibrate, even though prepared with macroscopic amounts of energy above their ground states. We show that such localized systems can order, in that individual many-body eigenstates can break symmetries or display topological order in the infinite-volume limit. Indeed, isolated localized quantum systems can order even at energy densities where the corresponding thermally equilibrated system is disordered, i.e., localization protects order. In addition, localized systems can move between ordered and disordered localized phases via nonthermodynamic transitions in the properties of the many-body eigenstates. We give evidence that such transitions may proceed via localized critical points. We note that localization provides protection against decoherence that may allow experimental manipulation of macroscopic quantum states. We also identify a âspectral transitionâ involving a sharp change in the spectral statistics of the many-body Hamiltonian
An alternative order parameter for the 4-state Potts model
We have investigated the dynamic critical behavior of the two-dimensional
4-state Potts model using an alternative order parameter first used by
Vanderzande [J. Phys. A: Math. Gen. \textbf{20}, L549 (1987)] in the study of
the Z(5) model. We have estimated the global persistence exponent by
following the time evolution of the probability that the considered
order parameter does not change its sign up to time . We have also obtained
the critical exponents , , , and using this alternative
definition of the order parameter and our results are in complete agreement
with available values found in literature.Comment: 6 pages, 6 figure
A comparative study of the dynamic critical behavior of the four-state Potts like models
We investigate the short-time critical dynamics of the Baxter-Wu (BW) and
Turban (3TU) models to estimate their global persistence exponent . We conclude that this new dynamical exponent can be useful in detecting
differences between the critical behavior of these models which are very
difficult to obtain in usual simulations. In addition, we estimate again the
dynamical exponents of the four-state Potts (FSP) model in order to compare
them with results previously obtained for the BW and 3TU models and to decide
between two sets of estimates presented in the current literature. We also
revisit the short-time dynamics of the 3TU model in order to check if, as
already found for the FSP model, the anomalous dimension of the initial
magnetization could be equal to zero
Global persistence exponent of the two-dimensional Blume-Capel model
The global persistence exponent is calculated for the
two-dimensional Blume-Capel model following a quench to the critical point from
both disordered states and such with small initial magnetizations.
Estimates are obtained for the nonequilibrium critical dynamics on the
critical line and at the tricritical point.
Ising-like universality is observed along the critical line and a different
value is found at the tricritical point.Comment: 7 pages with 3 figure
Mixed initial conditions to estimate the dynamic critical exponent in short-time Monte Carlo simulation
We explore the initial conditions in short-time critical dynamics to propose
a new method to evaluate the dynamic exponent z. Estimates are obtained with
high precision for 2D Ising model and 2D Potts model for three and four states
by performing heat-bath Monte Carlo simulations.Comment: Latex paper, 2 eps figure
Stretched Polymers in Random Environment
We survey recent results and open questions on the ballistic phase of
stretched polymers in both annealed and quenched random environments.Comment: Dedicated to Erwin Bolthausen on the occasion of his 65th birthda
Exciton Condensation and Perfect Coulomb Drag
Coulomb drag is a process whereby the repulsive interactions between
electrons in spatially separated conductors enable a current flowing in one of
the conductors to induce a voltage drop in the other. If the second conductor
is part of a closed circuit, a net current will flow in that circuit. The drag
current is typically much smaller than the drive current owing to the heavy
screening of the Coulomb interaction. There are, however, rare situations in
which strong electronic correlations exist between the two conductors. For
example, bilayer two-dimensional electron systems can support an exciton
condensate consisting of electrons in one layer tightly bound to holes in the
other. One thus expects "perfect" drag; a transport current of electrons driven
through one layer is accompanied by an equal one of holes in the other. (The
electrical currents are therefore opposite in sign.) Here we demonstrate just
this effect, taking care to ensure that the electron-hole pairs dominate the
transport and that tunneling of charge between the layers is negligible.Comment: 12 pages, 4 figure
Dynamical frustration in ANNNI model and annealing
Zero temperature quench in the Axial Next Nearest Neighbour Ising (ANNNI)
model fails to bring it to its ground state for a certain range of values of
the frustration parameter , the ratio of the next nearest neighbour
antiferromagnetic interaction strength to the nearest neighbour one. We apply
several annealing methods, both classical and quantum, and observe that the
behaviour of the residual energy and the order parameter depends on the value
of strongly. Classical or thermal annealing is found to be adequate
for small values of .
However, neither classical nor quantum annealing is effective at values of
close to the fully frustrated point , where the residual
energy shows a very slow algebraic decay with the number of MCS.Comment: 6 pages,10 figures, to be published in Proceedings of " The
International Workshop on Quantum annealing and other Optimization Methods
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