3,103 research outputs found
Generalised Shastry-Sutherland Models in three and higher dimensions
We construct Heisenberg anti-ferromagnetic models in arbitrary dimensions
that have isotropic valence bond crystals (VBC) as their exact ground states.
The d=2 model is the Shastry-Sutherland model. In the 3-d case we show that it
is possible to have a lattice structure, analogous to that of SrCu_2(BO_3)_2,
where the stronger bonds are associated with shorter bond lengths. A dimer mean
field theory becomes exact at d -> infinity and a systematic 1/d expansion can
be developed about it. We study the Neel-VBC transition at large d and find
that the transition is first order in even but second order in odd dimensions.Comment: Published version; slightly expande
Statistical Properties of Functionals of the Paths of a Particle Diffusing in a One-Dimensional Random Potential
We present a formalism for obtaining the statistical properties of
functionals and inverse functionals of the paths of a particle diffusing in a
one-dimensional quenched random potential. We demonstrate the implementation of
the formalism in two specific examples: (1) where the functional corresponds to
the local time spent by the particle around the origin and (2) where the
functional corresponds to the occupation time spent by the particle on the
positive side of the origin, within an observation time window of size . We
compute the disorder average distributions of the local time, the inverse local
time, the occupation time and the inverse occupation time, and show that in
many cases disorder modifies the behavior drastically.Comment: Revtex two column 27 pages, 10 figures, 3 table
Black hole mass and variability in quasars
We report on a study that finds a positive correlation between black hole
mass and variability amplitude in quasars. Roughly 100 quasars at z<0.75 were
selected by matching objects from the QUEST1 Variability Survey with
broad-lined objects from the Sloan Digital Sky Survey. Black hole masses were
estimated with the virial method using the broad Hbeta line, and variability
was characterized from the QUEST1 light curves. The correlation between black
hole mass and variability amplitude is significant at the 99% level or better
and does not appear to be caused by obvious selection effects inherent to
flux-limited samples. It is most evident for rest frame time lags of the order
a few months up to the QUEST1 maximum temporal resolution of about 2 years. The
correlation between black hole mass and variability amplitude means that the
more massive black holes have larger percentage flux variations. Over 2-3
orders of magnitude in black hole mass, the amplitude increases by
approximately 0.2 mag. A likely explanation for the correlation is that the
more massive black holes are starving and produce larger flux variations
because they do not have a steady inflow of gaseous fuel. Assuming that the
variability arises from changes in the accretion rate Li & Cao [8] show that
flux variations similar to those observed are expected as a consequence of the
more massive black holes having cooler accretion disks.Comment: 4 pages, to be published in the proceedings of the "2nd Kolkata
conference on observational evidence for black holes in the Universe", Feb
10-15, 2008, Kolkata, Indi
Persistence of a Continuous Stochastic Process with Discrete-Time Sampling: Non-Markov Processes
We consider the problem of `discrete-time persistence', which deals with the
zero-crossings of a continuous stochastic process, X(T), measured at discrete
times, T = n(\Delta T). For a Gaussian Stationary Process the persistence (no
crossing) probability decays as exp(-\theta_D T) = [\rho(a)]^n for large n,
where a = \exp[-(\Delta T)/2], and the discrete persistence exponent, \theta_D,
is given by \theta_D = \ln(\rho)/2\ln(a). Using the `Independent Interval
Approximation', we show how \theta_D varies with (\Delta T) for small (\Delta
T) and conclude that experimental measurements of persistence for smooth
processes, such as diffusion, are less sensitive to the effects of discrete
sampling than measurements of a randomly accelerated particle or random walker.
We extend the matrix method developed by us previously [Phys. Rev. E 64,
015151(R) (2001)] to determine \rho(a) for a two-dimensional random walk and
the one-dimensional random acceleration problem. We also consider `alternating
persistence', which corresponds to a < 0, and calculate \rho(a) for this case.Comment: 14 pages plus 8 figure
Phase diagram and hidden order for generalized spin ladders
We investigate the phase diagram of antiferromagnetic spin ladders with
additional exchange interactions on diagonal bonds by variational and numerical
methods. These generalized spin ladders interpolate smoothly between the
chain with competing nn and nnn interactions, the chain with
alternating exchange and the antiferromagnetic chain. The Majumdar-Ghosh
ground states are formulated as matrix product states and are shown to exhibit
the same type of hidden order as the af chain. Generalized matrix product
states are used for a variational calculation of the ground state energy and
the spin and string correlation functions. Numerical (Lanczos) calculations of
the energies of the ground state and of the low-lying excited states are
performed, and compare reasonably with the variational approach. Our results
support the hypothesis that the dimer and Majumdar-Ghosh points are in the same
phase as the af chain.Comment: 23 pages, REVTEX, 7 figure
Microscopic Non-Universality versus Macroscopic Universality in Algorithms for Critical Dynamics
We study relaxation processes in spin systems near criticality after a quench
from a high-temperature initial state. Special attention is paid to the stage
where universal behavior, with increasing order parameter emerges from an early
non-universal period. We compare various algorithms, lattice types, and
updating schemes and find in each case the same universal behavior at
macroscopic times, despite of surprising differences during the early
non-universal stages.Comment: 9 pages, 3 figures, RevTeX, submitted to Phys. Rev. Let
Non Markovian persistence in the diluted Ising model at criticality
We investigate global persistence properties for the non-equilibrium critical
dynamics of the randomly diluted Ising model. The disorder averaged persistence
probability of the global magnetization is found to decay
algebraically with an exponent that we compute analytically in a
dimensional expansion in . Corrections to Markov process are
found to occur already at one loop order and is thus a novel
exponent characterizing this disordered critical point. Our result is
thoroughly compared with Monte Carlo simulations in , which also include a
measurement of the initial slip exponent. Taking carefully into account
corrections to scaling, is found to be a universal exponent,
independent of the dilution factor along the critical line at , and
in good agreement with our one loop calculation.Comment: 7 pages, 4 figure
Surface and interface study of pulsed-laser-deposited off-stoichiometric NiMnSb thin films on Si(100) substrate
We report a detailed study of surface and interface properties of
pulsed-laser deposited NiMnSb films on Si (100) substrate as a function of film
thickness. As the thickness of films is reduced below 35 nm formation of a
porous layer is observed. Porosity in this layer increases with decrease in
NiMnSb film thickness. These morphological changes of the ultra thin films are
reflected in the interesting transport and magnetic properties of these films.
On the other hand, there are no influences of compositional in-homogeneity and
surface/interface roughness on the magnetic and transport properties of the
films.Comment: 13 pages, 7 figures, Submitted to Phys. Rev.
Interparticle Potential up to Next-to-leading Order for Gravitational, Electrical, and Dilatonic Forces
Long-range forces up to next-to-leading order are computed in the framework
of the Einstein-Maxwell-dilaton system by means of a semiclassical approach to
gravity. As has been recently shown, this approach is effective if one of the
masses under consideration is significantly greater than all the energies
involved in the system. Further, we obtain the condition for the equilibrium of
charged masses in the system.Comment: 19 pages, 19 figures, RevTeX4.1. Revised version, Title change
Entangled random pure states with orthogonal symmetry: exact results
We compute analytically the density of Schmidt
eigenvalues, distributed according to a fixed-trace Wishart-Laguerre measure,
and the average R\'enyi entropy for reduced
density matrices of entangled random pure states with orthogonal symmetry
. The results are valid for arbitrary dimensions of the
corresponding Hilbert space partitions, and are in excellent agreement with
numerical simulations.Comment: 15 pages, 5 figure
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