4,246 research outputs found
Crossover Behavior from Decoupled Criticality
We study the thermodynamic phase transition of a spin Hamiltonian comprising
two 3D magnetic sublattices. Each sublattice contains XY spins coupled by the
usual bilinear exchange, while spins in different sublattices only interact via
biquadratic exchange. This Hamiltonian is an effective model for XY magnets on
certain frustrated lattices such as body centered tetragonal. By performing a
cluster Monte Carlo simulation, we investigate the crossover from the 3D-XY
fixed point (decoupled sublattices) and find a systematic flow toward a
first-order transition without a separatrix or a new fixed point. This strongly
suggests that the correct asymptotic behavior is a first-order transition.Comment: 10 pages, 3 figures; added reference
Numerical renormalization group study of random transverse Ising models in one and two space dimensions
The quantum critical behavior and the Griffiths-McCoy singularities of random
quantum Ising ferromagnets are studied by applying a numerical implementation
of the Ma-Dasgupta-Hu renormalization group scheme. We check the procedure for
the analytically tractable one-dimensional case and apply our code to the
quasi-one-dimensional double chain. For the latter we obtain identical critical
exponents as for the simple chain implying the same universality class. Then we
apply the method to the two-dimensional case for which we get estimates for the
exponents that are compatible with a recent study in the same spirit.Comment: 10 pages LaTeX, eps-figures and PTP-macros included. Proceedings of
the ICCP5, Kanazawa (Japan), 199
Labour Power Matters and Capitalist Racism - An Interview with Ken C. Kawashima
Raju Das and Robert Latham interview Ken C. Kawashima about his work and lessons for workers involved in the struggle today
Scalings of domain wall energies in two dimensional Ising spin glasses
We study domain wall energies of two dimensional spin glasses. The scaling of
these energies depends on the model's distribution of quenched random
couplings, falling into three different classes. The first class is associated
with the exponent theta =-0.28, the other two classes have theta = 0, as can be
justified theoretically. In contrast to previous claims, we find that theta=0
does not indicate d=d_l but rather d <= d_l, where d_l is the lower critical
dimension.Comment: Clarifications and extra reference
Random quantum magnets with broad disorder distribution
We study the critical behavior of Ising quantum magnets with broadly
distributed random couplings (J), such that , , for large (L\'evy flight statistics).
For sufficiently broad distributions, , the critical behavior
is controlled by a line of fixed points, where the critical exponents vary with
the L\'evy index, . In one dimension, with , we obtaind
several exact results through a mapping to surviving Riemann walks. In two
dimensions the varying critical exponents have been calculated by a numerical
implementation of the Ma-Dasgupta-Hu renormalization group method leading to
. Thus in the region , where the
central limit theorem holds for the broadness of the distribution is
relevant for the 2d quantum Ising model.Comment: 10pages, 13figures, final for
Generalization of the Fortuin-Kasteleyn transformation and its application to quantum spin simulations,
We generalize the Fortuin-Kasteleyn (FK) cluster representation of the
partition function of the Ising model to represent the partition function of
quantum spin models with an arbitrary spin magnitude in arbitrary dimensions.
This generalized representation enables us to develop a new cluster algorithm
for the simulation of quantum spin systems by the worldline Monte Carlo method.
Because the Swendsen-Wang algorithm is based on the FK representation, the new
cluster algorithm naturally includes it as a special case. As well as the
general description of the new representation, we present an illustration of
our new algorithm for some special interesting cases: the Ising model, the
antiferromagnetic Heisenberg model with , and a general Heisenberg model.
The new algorithm is applicable to models with any range of the exchange
interaction, any lattice geometry, and any dimensions.Comment: 46 pages, 10 figures, to appear in J.Stat.Phy
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Rocking isolation of a typical bridge pier on spread foundation
It has been observed that after some earthquakes a number of structures resting on spread footings responded to seismic excitation by rocking on their foundation and in some cases this enabled them to avoid failure. Through application to a standard bridge supported by direct foundations, this paper discusses the major differences in response when foundation uplift is taken into consideration. Special focus is given on the modifications of rocking response under biaxial and tri-axial excitation with respect to uniaxial excitation. It is found that inelastic rocking has a significant isolation effect. It is also shown that this effect increases under biaxial excitation while it is less sensitive to the vertical component of the earthquake. Finally, parametric analyses show that the isolation effect of foundation rocking increases as the size of the footing and the yield strength of the underlying soil decreases
Transition matrix Monte Carlo method for quantum systems
We propose an efficient method for Monte Carlo simulation of quantum lattice
models. Unlike most other quantum Monte Carlo methods, a single run of the
proposed method yields the free energy and the entropy with high precision for
the whole range of temperature. The method is based on several recent findings
in Monte Carlo techniques, such as the loop algorithm and the transition matrix
Monte Carlo method. In particular, we derive an exact relation between the DOS
and the expectation value of the transition probability for quantum systems,
which turns out to be useful in reducing the statistical errors in various
estimates.Comment: 6 pages, 4 figure
Absence of an equilibrium ferromagnetic spin glass phase in three dimensions
Using ground state computations, we study the transition from a spin glass to
a ferromagnet in 3-d spin glasses when changing the mean value of the spin-spin
interaction. We find good evidence for replica symmetry breaking up till the
critical value where ferromagnetic ordering sets in, and no ferromagnetic spin
glass phase. This phase diagram is in conflict with the droplet/scaling and
mean field theories of spin glasses. We also find that the exponents of the
second order ferromagnetic transition do not depend on the microscopic
Hamiltonian, suggesting universality of this transition.Comment: 4 pages, 5 figures, revte
On the Use of Finite-Size Scaling to Measure Spin-Glass Exponents
Finite-size scaling (FSS) is a standard technique for measuring scaling
exponents in spin glasses. Here we present a critique of this approach,
emphasizing the need for all length scales to be large compared to microscopic
scales. In particular we show that the replacement, in FSS analyses, of the
correlation length by its asymptotic scaling form can lead to apparently good
scaling collapses with the wrong values of the scaling exponents.Comment: RevTeX, 5 page
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