5,219 research outputs found
A microscopic mechanism for rejuvenation and memory effects in spin glasses
Aging in spin glasses (and in some other systems) reveals astonishing effects
of `rejuvenation and memory' upon temperature changes. In this paper, we
propose microscopic mechanisms (at the scale of spin-spin interactions) which
can be at the origin of such phenomena. Firstly, we recall that, in a
frustrated system, the effective average interaction between two spins may take
different values (possibly with opposite signs) at different temperatures. We
give simple examples of such situations, which we compute exactly. Such
mechanisms can explain why new ordering processes (rejuvenation) seem to take
place in spin glasses when the temperature is lowered. Secondly, we emphasize
the fact that inhomogeneous interactions do naturally lead to a wide
distribution of relaxation times for thermally activated flips. `Memory spots'
spontaneously appear, in the sense that the flipping time of some spin clusters
becomes extremely long when the temperature is decreased. Such memory spots are
capable of keeping the memory of previous ordering at a higher temperature
while new ordering processes occur at a lower temperature. After a qualitative
discussion of these mechanisms, we show in the numerical simulation of a
simplified example that this may indeed work. Our conclusion is that certain
chaos-like phenomena may show up spontaneously in any frustrated and
inhomogeneous magnetic system, without impeding the occurrence of memory
effects.Comment: 9 pages (11 figures) - revised version, to appear in Eur. Phys. J. B
(2001
Quantum Tunneling in Half-Integer Spin Systems
Motivated by the experimental observations of resonant tunnelings in the
systems with half-integer spin, such as V and Mn, we study the
mechanism of adiabatic change of the magnetization in systems with the
time-reversal symmetry. Within the time-reversal symmetric models, effects of
several types of perturbations are investigated. Although tunneling between the
ground states is suppressed in a simple Kramers doublet, we show that the
nonadiabatic transition governed by the Landau-Zener-St\"uckelberg mechanism
occurs in many cases due to the additional degeneracy of the ground state. We
also found more general cases where LZS mechanism can not be applied directly
even the system shows a kind of adiabatic change of the magnetization
Slow Relaxation Process in Ising like Heisenberg Kagome Antiferromagnets due to Macroscopic Degeneracy in the Ordered State
We study relaxation phenomena in the ferromagnetically ordered state of the
Ising-like Heisenberg kagome antiferromagnets. We introduce the "weathervane
loop" in order to characterize macroscopic degenerate ordered states and study
the microscopic mechanism of the slow relaxation from a view point of the
dynamics of the weathervane loop configuration. This mechanism may give a
possible origin of the slow relaxation reported in recent experiments.Comment: 6pages, 4figures, HFM2006 proceeding
Quantum Annealing in the Transverse Ising Model
We introduce quantum fluctuations into the simulated annealing process of
optimization problems, aiming at faster convergence to the optimal state.
Quantum fluctuations cause transitions between states and thus play the same
role as thermal fluctuations in the conventional approach. The idea is tested
by the transverse Ising model, in which the transverse field is a function of
time similar to the temperature in the conventional method. The goal is to find
the ground state of the diagonal part of the Hamiltonian with high accuracy as
quickly as possible. We have solved the time-dependent Schr\"odinger equation
numerically for small size systems with various exchange interactions.
Comparison with the results of the corresponding classical (thermal) method
reveals that the quantum annealing leads to the ground state with much larger
probability in almost all cases if we use the same annealing schedule.Comment: 15 pages, RevTeX, 8 figure
Deceptive Apparent Nonadiabatic Magnetization Process
We discuss the effect of the thermal environment on the low-temperature
response of the magnetization of uniaxial magnets to a time-dependent applied
magnetic field. At sufficiently low temperatures the staircase magnetization
curves observed in molecular magnets such as Mn_{12} and Fe_8 display little
temperature dependence. However the changes of the magnetization at each step
do not seem to be directly related to the probability for a quantum mechanical
nonadiabatic transition. In order to explain this deceptive apparent
nonadiabatic behavior, we study the quantum dynamics of the system in a thermal
environment and propose a relation between the observed magnetization steps and
the quantum mechanical transition probability due to the nonadiabatic
transition.Comment: 4 pages, 7 eps figure
Retrieval Properties of Hopfield and Correlated Attractors in an Associative Memory Model
We examine a previouly introduced attractor neural network model that
explains the persistent activities of neurons in the anterior ventral temporal
cortex of the brain. In this model, the coexistence of several attractors
including correlated attractors was reported in the cases of finite and
infinite loading. In this paper, by means of a statistical mechanical method,
we study the statics and dynamics of the model in both finite and extensive
loading, mainly focusing on the retrieval properties of the Hopfield and
correlated attractors. In the extensive loading case, we derive the evolution
equations by the dynamical replica theory. We found several characteristic
temporal behaviours, both in the finite and extensive loading cases. The
theoretical results were confirmed by numerical simulations.Comment: 12 pages, 7 figure
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