889 research outputs found
Ground states and thermal states of the random field Ising model
The random field Ising model is studied numerically at both zero and positive
temperature. Ground states are mapped out in a region of random and external
field strength. Thermal states and thermodynamic properties are obtained for
all temperatures using the the Wang-Landau algorithm. The specific heat and
susceptibility typically display sharp peaks in the critical region for large
systems and strong disorder. These sharp peaks result from large domains
flipping. For a given realization of disorder, ground states and thermal states
near the critical line are found to be strongly correlated--a concrete
manifestation of the zero temperature fixed point scenario.Comment: 5 pages, 5 figures; new material added in this versio
Monte Carlo study of the two-dimensional site-diluted dipolar Ising model
By tempered Monte Carlo simulations, we study 2D site-diluted dipolar Ising
systems. Dipoles are randomly placed on a fraction x of all L^2 sites in a
square lattice, and point along a common crystalline axis. For x_c< x<=1, where
x_c = 0.79(5), we find an antiferromagnetic phase below a temperature which
vanishes as x approaches x_c from above. At lower values of x, we study (i)
distributions of the spin--glass (SG) overlap q, (ii) their relative mean
square deviation Delta_q^2 and kurtosis and (iii) xi_L/L, where xi_L is a SG
correlation length. From their variation with temperature and system size, we
find that the paramagnetic phase covers the entire T>0 range. Our results
enable us to obtain an estimate of the critical exponent associated to the
correlation length at T=0, 1/nu=0.35(10).Comment: 10 LaTeX pages, 10 figures, 1 table
Renormalization of modular invariant Coulomb gas and Sine-Gordon theories, and quantum Hall flow diagram
Using the renormalisation group (RG) we study two dimensional electromagnetic
coulomb gas and extended Sine-Gordon theories invariant under the modular group
SL(2,Z). The flow diagram is established from the scaling equations, and we
derive the critical behaviour at the various transition points of the diagram.
Following proposal for a SL(2,Z) duality between different quantum Hall fluids,
we discuss the analogy between this flow and the global quantum Hall phase
diagram.Comment: 10 pages, 1 EPS figure include
Island Density in Homoepitaxial Growth:Improved Monte Carlo Results
We reexamine the density of two dimensional islands in the submonolayer
regime of a homoepitaxially growing surface using the coarse grained Monte
Carlo simulation with random sequential updating rather than parallel updating.
It turns out that the power law dependence of the density of islands on the
deposition rate agrees much better with the theoretical prediction than
previous data obtained by other methods if random sequential instead of
parallel updating is used.Comment: Latex with 2 PS figure file
Floating Phase in 1D Transverse ANNNI Model
To study the ground state of ANNNI chain under transverse field as a function
of frustration parameter and field strength , we present here
two different perturbative analyses. In one, we consider the (known) ground
state at and as the unperturbed state and treat an
increase of the field from 0 to coupled with an increase of
from 0.5 to as perturbation. The first order perturbation
correction to eigenvalue can be calculated exactly and we could conclude that
there are only two phase transition lines emanating from the point
, . In the second perturbation scheme, we consider the
number of domains of length 1 as the perturbation and obtain the zero-th order
eigenfunction for the perturbed ground state. From the longitudinal spin-spin
correlation, we conclude that floating phase exists for small values of
transverse field over the entire region intermediate between the ferromagnetic
phase and antiphase.Comment: 11 pages, 11 figure
Antiferromagnetic Quantum Spins on the Pyrochlore Lattice
The ground state of the S=1/2 Heisenberg antiferromagnet on the pyrochlore
lattice is theoretically investigated. Starting from the limit of isolated
tetrahedra, I include interactions between the tetrahedra and obtain an
effective model for the spin-singlet ground state multiplet by third-order
perturbation. I determine its ground state using the mean-field approximation
and found a dimerized state with a four-sublattice structure, which agrees with
the proposal by Harris et al. I also discuss chirality correlations and spin
correlations for this state.Comment: 4 pages in 2-column format, 5 figures; To appear in J. Phys. Soc.
Jpn. (Mar, 2001
Determination of step--edge barriers to interlayer transport from surface morphology during the initial stages of homoepitaxial growth
We use analytic formulae obtained from a simple model of crystal growth by
molecular--beam epitaxy to determine step--edge barriers to interlayer
transport. The method is based on information about the surface morphology at
the onset of nucleation on top of first--layer islands in the submonolayer
coverage regime of homoepitaxial growth. The formulae are tested using kinetic
Monte Carlo simulations of a solid--on--solid model and applied to estimate
step--edge barriers from scanning--tunneling microscopy data on initial stages
of Fe(001), Pt(111), and Ag(111) homoepitaxy.Comment: 4 pages, a Postscript file, uuencoded and compressed. Physical Review
B, Rapid Communications, in press
The Kagome Antiferromagnet with Defects: Satisfaction, Frustration, and Spin Folding in a Random Spin System
It is shown that site disorder induces noncoplanar states, competing with the
thermal selection of coplanar states, in the nearest neighbor, classical kagome
Heisenberg antiferromagnet (AFM). For weak disorder, it is found that the
ground state energy is the sum of energies of separately satisfied triangles of
spins. This implies that disorder does not induce conventional spin glass
behavior. A transformation is presented, mapping ground state spin
configurations onto a folded triangular sheet (a new kind of ``spin origami'')
which has conformations similar to those of tethered membranes.Comment: REVTEX, 11 pages + 3 pictures upon reques
The Generic, Incommensurate Transition in the two-dimensional Boson Hubbard Model
The generic transition in the boson Hubbard model, occurring at an
incommensurate chemical potential, is studied in the link-current
representation using the recently developed directed geometrical worm
algorithm. We find clear evidence for a multi-peak structure in the energy
distribution for finite lattices, usually indicative of a first order phase
transition. However, this multi-peak structure is shown to disappear in the
thermodynamic limit revealing that the true phase transition is second order.
These findings cast doubts over the conclusion drawn in a number of previous
works considering the relevance of disorder at this transition.Comment: 13 pages, 10 figure
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