287 research outputs found
Random Field and Random Anisotropy Effects in Defect-Free Three-Dimensional XY Models
Monte Carlo simulations have been used to study a vortex-free XY ferromagnet
with a random field or a random anisotropy on simple cubic lattices. In the
random field case, which can be related to a charge-density wave pinned by
random point defects, it is found that long-range order is destroyed even for
weak randomness. In the random anisotropy case, which can be related to a
randomly pinned spin-density wave, the long-range order is not destroyed and
the correlation length is finite. In both cases there are many local minima of
the free energy separated by high entropy barriers. Our results for the random
field case are consistent with the existence of a Bragg glass phase of the type
discussed by Emig, Bogner and Nattermann.Comment: 10 pages, including 2 figures, extensively revise
Lower Neutrino Mass Bound from SN1987A Data and Quantum Geometry
A lower bound on the light neutrino mass is derived in the framework
of a geometrical interpretation of quantum mechanics. Using this model and the
time of flight delay data for neutrinos coming from SN1987A, we find that the
neutrino masses are bounded from below by eV, in
agreement with the upper bound
eV currently available. When the model is applied to photons with effective
mass, we obtain a lower limit on the electron density in intergalactic space
that is compatible with recent baryon density measurements.Comment: 22 pages, 3 figure
CAFF CBMP Report No. 9: Community-based Monitoring – a discussion paper
Community-based Monitoring – a discussion paper. Supporting publication to the CAFF Circumpolar Biodiversity Monitoring Program – Framework Document. CAFF CBMP Report No.
Power-law correlations and orientational glass in random-field Heisenberg models
Monte Carlo simulations have been used to study a discretized Heisenberg
ferromagnet (FM) in a random field on simple cubic lattices. The spin variable
on each site is chosen from the twelve [110] directions. The random field has
infinite strength and a random direction on a fraction x of the sites of the
lattice, and is zero on the remaining sites. For x = 0 there are two phase
transitions. At low temperatures there is a [110] FM phase, and at intermediate
temperature there is a [111] FM phase. For x > 0 there is an intermediate phase
between the paramagnet and the ferromagnet, which is characterized by a
|k|^(-3) decay of two-spin correlations, but no true FM order. The [111] FM
phase becomes unstable at a small value of x. At x = 1/8 the [110] FM phase has
disappeared, but the power-law correlated phase survives.Comment: 8 pages, 12 Postscript figure
New algorithm and results for the three-dimensional random field Ising Model
The random field Ising model with Gaussian disorder is studied using a new
Monte Carlo algorithm. The algorithm combines the advantanges of the replica
exchange method and the two-replica cluster method and is much more efficient
than the Metropolis algorithm for some disorder realizations. Three-dimensional
sytems of size are studied. Each realization of disorder is simulated at
a value of temperature and uniform field that is adjusted to the phase
transition region for that disorder realization. Energy and magnetization
distributions show large variations from one realization of disorder to
another. For some realizations of disorder there are three well separated peaks
in the magnetization distribution and two well separated peaks in the energy
distribution suggesting a first-order transition.Comment: 24 pages, 23 figure
Tricritical Points in the Sherrington-Kirkpatrick Model in the Presence of Discrete Random Fields
The infinite-range-interaction Ising spin glass is considered in the presence
of an external random magnetic field following a trimodal (three-peak)
distribution. The model is studied through the replica method and phase
diagrams are obtained within the replica-symmetry approximation. It is shown
that the border of the ferromagnetic phase may present first-order phase
transitions, as well as tricritical points at finite temperatures. Analogous to
what happens for the Ising ferromagnet under a trimodal random field, it is
verified that the first-order phase transitions are directly related to the
dilution in the fields (represented by ). The ferromagnetic boundary at
zero temperature also exhibits an interesting behavior: for , a single tricritical point occurs, whereas if
the critical frontier is completely continuous; however, for
, a fourth-order critical point appears. The stability
analysis of the replica-symmetric solution is performed and the regions of
validity of such a solution are identified; in particular, the Almeida-Thouless
line in the plane field versus temperature is shown to depend on the weight
.Comment: 23pages, 7 ps figure
Equilibrium random-field Ising critical scattering in the antiferromagnet Fe(0.93)Zn(0.07)F2
It has long been believed that equilibrium random-field Ising model (RFIM)
critical scattering studies are not feasible in dilute antiferromagnets close
to and below Tc(H) because of severe non-equilibrium effects. The high magnetic
concentration Ising antiferromagnet Fe(0.93)Zn(0.07)F2, however, does provide
equilibrium behavior. We have employed scaling techniques to extract the
universal equilibrium scattering line shape, critical exponents nu = 0.87 +-
0.07 and eta = 0.20 +- 0.05, and amplitude ratios of this RFIM system.Comment: 4 pages, 1 figure, minor revision
Anderson-Mott transition as a quantum glass problem
We combine a recent mapping of the Anderson-Mott metal-insulator transition
on a random-field problem with scaling concepts for random-field magnets to
argue that disordered electrons near an Anderson-Mott transition show
glass-like behavior. We first discuss attempts to interpret experimental
results in terms of a conventional scaling picture, and argue that some of the
difficulties encountered point towards a glassy nature of the electrons. We
then develop a general scaling theory for a quantum glass, and discuss critical
properties of both thermodynamic and transport variables in terms of it. Our
most important conclusions are that for a correct interpretation of experiments
one must distinguish between self-averaging and non-self averaging observables,
and that dynamical or temperature scaling is not of power-law type but rather
activated, i.e. given by a generalized Vogel-Fulcher law. Recent mutually
contradicting experimental results on Si:P are discussed in the light of this,
and new experiments are proposed to test the predictions of our quantum glass
scaling theory.Comment: 25pp, REVTeX, 5 ps figs, final version as publishe
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