245 research outputs found
Diabolical points in the magnetic spectrum of Fe_8 molecules
The magnetic molecule Fe_8 has been predicted and observed to have a rich
pattern of degeneracies in its spectrum as an external magnetic field is
varied. These degeneracies have now been recognized to be diabolical points.
This paper analyzes the diabolicity and all essential properties of this system
using elementary perturbation theory. A variety of arguments is gievn to
suggest that an earlier semiclassical result for a subset of these points may
be exactly true for arbitrary spinComment: uses europhys.sty package; 3 embedded ps figure
Instanton picture of the spin tunneling in the Lipkin model
A consistent theory of the ground state energy and its splitting due to the
process of tunneling for the Lipkin model is presented. For the functional
integral in terms of the spin coherent states for the partition function of the
model we accurately calculate the trivial and the instanton saddle point
contributions. We show that such calculation has to be perfomed very accurately
taking into account the discrete nature of the functional integral. Such
accurate consideration leads to finite corrections to a naive continous
consideration. We present comparison with numerical calculation of the ground
state energy and the tunneling splitting and with the results obtained by the
quasiclassical method and get excellent agreement.Comment: REVTEX, 32 pages, 3 figure
Static magnetization induced by time-periodic fields with zero mean
We consider a single spin in a constant magnetic field or an anisotropy
field. We show that additional external time-periodic fields with zero mean may
generate nonzero time-averaged spin components which are vanishing for the
time-averaged Hamiltonian. The reason is a lowering of the dynamical symmetry
of the system. A harmonic signal with proper orientation is enough to display
the effect. We analyze the problem both with and without dissipation, both for
quantum spins (s=1/2,1) and classical spins. The results are of importance for
controlling the system's state using high or low frequency fields and for using
new resonance techniques which probe internal system parameters, to name a few.Comment: 11 pages, 2 figure
Strong Coupling Theory of Two Level Atoms in Periodic Fields
We present a new convergent strong coupling expansion for two-level atoms in
external periodic fields, free of secular terms. As a first application, we
show that the coherent destruction of tunnelling is a third-order effect. We
also present an exact treatment of the high-frequency region, and compare it
with the theory of averaging. The qualitative frequency spectrum of the
transition probability amplitude contains an effective Rabi frequency.Comment: 4 pages with 3 figure
Quantum-Classical Phase Transition of Escape rate in Biaxial Spin Particles
The escape rates of the biaxial single domain spin particles with and without
an applied magnetic field are investigated. Using the strict potential field
description of spin systems developed by Ulyanov and Zaslavskii we obtain new
effective Hamiltonians which are considered to be in exact spin-coordinate
correspondence unlike the well studied effective Hamiltonians with the
approximate correspondence. The sharp first-order transition is found in both
cases. The phase diagram of the transitions depending on the anisotropy
constant and the external field is also given.Comment: 15 pages, 8 figure
Ising spin glass models versus Ising models: an effective mapping at high temperature III. Rigorous formulation and detailed proof for general graphs
Recently, it has been shown that, when the dimension of a graph turns out to
be infinite dimensional in a broad sense, the upper critical surface and the
corresponding critical behavior of an arbitrary Ising spin glass model defined
over such a graph, can be exactly mapped on the critical surface and behavior
of a non random Ising model. A graph can be infinite dimensional in a strict
sense, like the fully connected graph, or in a broad sense, as happens on a
Bethe lattice and in many random graphs. In this paper, we firstly introduce
our definition of dimensionality which is compared to the standard definition
and readily applied to test the infinite dimensionality of a large class of
graphs which, remarkably enough, includes even graphs where the tree-like
approximation (or, in other words, the Bethe-Peierls approach), in general, may
be wrong. Then, we derive a detailed proof of the mapping for all the graphs
satisfying this condition. As a byproduct, the mapping provides immediately a
very general Nishimori law.Comment: 25 pages, 5 figures, made statements in Sec. 10 cleare
Quantum dynamics of crystals of molecular nanomagnets inside a resonant cavity
It is shown that crystals of molecular nanomagnets exhibit enhanced magnetic
relaxation when placed inside a resonant cavity. Strong dependence of the
magnetization curve on the geometry of the cavity has been observed, providing
evidence of the coherent microwave radiation by the crystals. A similar
dependence has been found for a crystal placed between Fabry-Perot
superconducting mirrors. These observations open the possibility of building a
nanomagnetic microwave laser pumped by the magnetic field
Nonequilibrium dynamics of a simple stochastic model
We investigate the low-temperature dynamics of a simple stochastic model,
introduced recently in the context of the physics of glasses. The slowest
characteristic time at equilibrium diverges exponentially at low temperature.
On smaller time scales, the nonequilibrium dynamics of the system exhibits an
aging regime. We present an analytical study of the scaling behaviour of the
mean energy, of its local correlation and response functions, and of the
associated fluctuation-dissipation ratio throughout the regime of low
temperature and long times. This analysis includes the aging regime, the
convergence to equilibrium, and the crossover behaviour between them.Comment: 36 pages, plain tex, 7 figures, to be published by Journal of Physics
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