403 research outputs found
Vapor phase interaction of trimethylaluminum with graphite during OMVPE
Journal ArticleWe have used a molecular beam mass spectrometer to study the interaction of trimethylaluminum (TMA) with graphite surfaces. Our results indicate that a strong adsorption effect occurs which explains phenomena occuring during OMVPE growth of AlxGa1-xAs, such as long A1 transients, oxygen gettering, and reduced incorporation of selenium
Simulation Studies on the Stability of the Vortex-Glass Order
The stability of the three-dimensional vortex-glass order in random type-II
superconductors with point disorder is investigated by equilibrium Monte Carlo
simulations based on a lattice XY model with a uniform field threading the
system. It is found that the vortex-glass order, which stably exists in the
absence of screening, is destroyed by the screenng effect, corroborating the
previous finding based on the spatially isotropic gauge-glass model. Estimated
critical exponents, however, deviate considerably from the values reported for
the gauge-glass model.Comment: Minor modifications made, a few referenced added; to appear in J.
Phys. Soc. Jpn. Vol.69 No.1 (2000
Geometric Aspects of the Dipolar Interaction in Lattices of Small Particles
The hysteresis curves of systems composed of small interacting magnetic
particles, regularly placed on stacked layers, are obtained with Monte Carlo
simulations. The remanence as a function of temperature, in interacting
systems, presents a peak that separates two different magnetic states. At low
temperatures, small values of remanence are a consequence of antiferromagnetic
order due to the dipolar interaction. At higher values of temperature the
increase of the component normal to the lattice plane is responsible for the
small values of remanence. The effect of the number of layers, coordination
number and distance between particles are investigated.Comment: 5 pages, 7 figure
Superconductor-to-Normal Phase Transition in a Vortex Glass Model: Numerical Evidence for a New Percolation Universality Class
The three-dimensional strongly screened vortex-glass model is studied
numerically using methods from combinatorial optimization. We focus on the
effect of disorder strength on the ground state and found the existence of a
disorder-driven normal-to-superconducting phase transition. The transition
turns out to be a geometrical phase transition with percolating vortex loops in
the ground state configuration. We determine the critical exponents and provide
evidence for a new universality class of correlated percolation.Comment: 11 pages LaTeX using IOPART.cls, 11 eps-figures include
Direct Mott Insulator-to-Superfluid Transition in the Presence of Disorder
We introduce a new renormalization group theory to examine the quantum phase
transitions upon exiting the insulating phase of a disordered, strongly
interacting boson system. For weak disorder we find a direct transition from
this Mott insulator to the Superfluid phase. In d > 4 a finite region around
the particle-hole symmetric point supports this direct transition, whereas for
2=< d <4 perturbative arguments suggest that the direct transition survives
only precisely at commensurate filling. For strong disorder the renormalization
trajectories pass next to two fixed points, describing a pair of distinct
transitions; first from the Mott insulator to the Bose glass, and then from the
Bose glass to the Superfluid. The latter fixed point possesses statistical
particle-hole symmetry and a dynamical exponent z, equal to the dimension d.Comment: 4 pages, Latex, submitted to Physical Review Letter
Destruction of diagonal and off-diagonal long range order by disorder in two-dimensional hard core boson systems
We use quantum Monte Carlo simulations to study the effect of disorder, in
the form of a disordered chemical potential, on the phase diagram of the hard
core bosonic Hubbard model in two dimensions. We find numerical evidence that
in two dimensions, no matter how weak the disorder, it will always destroy the
long range density wave order (checkerboard solid) present at half filling and
strong nearest neighbor repulsion and replace it with a bose glass phase. We
study the properties of this glassy phase including the superfluid density,
energy gaps and the full Green's function. We also study the possibility of
other localized phases at weak nearest neighbor repulsion, i.e. Anderson
localization. We find that such a phase does not truly exist: The disorder must
exceed a threshold before the bosons (at weak nn repulsion) are localized. The
phase diagram for hard core bosons with disorder cannot be obtained easily from
the soft core phase diagram discussed in the literature.Comment: 7 pages, 10 eps figures include
Cluster Monte Carlo Algorithm for the Quantum Rotor Model
We propose a highly efficient "worm" like cluster Monte Carlo algorithm for
the quantum rotor model in the link-current representation. We explicitly prove
detailed balance for the new algorithm even in the presence of disorder. For
the pure quantum rotor model with the new algorithm yields high
precision estimates for the critical point and the correlation
length exponent . For the disordered case, , we
find .Comment: 5 pages, 3 figure
Dynamics and Transport in Random Antiferromagnetic Spin Chains
We present the first results on the low-frequency dynamical and transport
properties of random antiferromagnetic spin chains at low temperature (). We
obtain the momentum and frequency dependent dynamic structure factor in the
Random Singlet (RS) phases of both spin-1/2 and spin-1 chains, as well as in
the Random Dimer phase of spin-1/2 chains. We also show that the RS phases are
unusual `spin-metals' with divergent low-frequency conductivity at T=0, and
follow the spin conductivity through `metal-insulator' transitions tuned by the
strength of dimerization or Ising anisotropy in the spin-1/2 case, and by the
strength of disorder in the spin-1 case.Comment: 4 pages (two-column format). Presentation substantially revised to
accomodate new result
Numerical Study on Aging Dynamics in the 3D Ising Spin-Glass Model. II. Quasi-Equilibrium Regime of Spin Auto-Correlation Function
Using Monte Carlo simulations, we have studied isothermal aging of
three-dimensional Ising spin-glass model focusing on quasi-equilibrium behavior
of the spin auto-correlation function. Weak violation of the time translational
invariance in the quasi-equilibrium regime is analyzed in terms of {\it
effective stiffness} for droplet excitations in the presence of domain walls.
Within the range of computational time window, we have confirmed that the
effective stiffness follows the expected scaling behavior with respect to the
characteristic length scales associated with droplet excitations and domain
walls, whose growth law has been extracted from our simulated data. Implication
of the results are discussed in relation to experimental works on ac
susceptibilities.Comment: 18 pages, 6 figure
Chaos in the Random Field Ising Model
The sensitivity of the random field Ising model to small random perturbations
of the quenched disorder is studied via exact ground states obtained with a
maximum-flow algorithm. In one and two space dimensions we find a mild form of
chaos, meaning that the overlap of the old, unperturbed ground state and the
new one is smaller than one, but extensive. In three dimensions the
rearrangements are marginal (concentrated in the well defined domain walls).
Implications for finite temperature variations and experiments are discussed.Comment: 4 pages RevTeX, 6 eps-figures include
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