4,911 research outputs found
Supercurrents on Asymmetric Orbifolds
We study -heterotic string on asymmetric orbifolds associated
with semi-simple simply-laced Lie algebras. Using the fact that -model
allows different twists, we present a new N=1 space-time supersymmetric model
whose supercurrent appears from twisted sectors but not untwisted sector.Comment: 7 pages, Latex, KOBE-TH-93-0
Quantum Hall effect and the topological number in graphene
Recently unusual integer quantum Hall effect was observed in graphene in
which the Hall conductivity is quantized as , where is the electron charge and is the
Planck constant. %\cite{Novoselov2005,Zheng2005}, %although it can be explained
in the argument of massless Dirac fermions, To explain this we consider the
energy structure as a function of magnetic field (the Hofstadter butterfly
diagram) on the honeycomb lattice and the Streda formula for Hall conductivity.
The quantized Hall conductivity is obtained to be odd integer, times two (spin degrees of freedom) when a uniform magnetic field is
as high as 30T for example. When the system is anisotropic and described by the
generalized honeycomb lattice, Hall conductivity can be quantized to be any
integer number. We also compare the results with those for the square lattice
under extremely strong magnetic field.Comment: 4 pages, 10 figure
On nonparametric and semiparametric testing for multivariate linear time series
We formulate nonparametric and semiparametric hypothesis testing of
multivariate stationary linear time series in a unified fashion and propose new
test statistics based on estimators of the spectral density matrix. The
limiting distributions of these test statistics under null hypotheses are
always normal distributions, and they can be implemented easily for practical
use. If null hypotheses are false, as the sample size goes to infinity, they
diverge to infinity and consequently are consistent tests for any alternative.
The approach can be applied to various null hypotheses such as the independence
between the component series, the equality of the autocovariance functions or
the autocorrelation functions of the component series, the separability of the
covariance matrix function and the time reversibility. Furthermore, a null
hypothesis with a nonlinear constraint like the conditional independence
between the two series can be tested in the same way.Comment: Published in at http://dx.doi.org/10.1214/08-AOS610 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Zero modes and the edge states of the honeycomb lattice
The honeycomb lattice in the cylinder geometry with zigzag edges, bearded
edges, zigzag and bearded edges (zigzag-bearded), and armchair edges are
studied. The tight-binding model with nearest-neighbor hoppings is used. Edge
states are obtained analytically for these edges except the armchair edges. It
is shown, however, that edge states for the armchair edges exist when the the
system is anisotropic. These states have not been known previously. We also
find strictly localized states, uniformly extended states and states with
macroscopic degeneracy.Comment: 6 pages 8 figure
An Oracle Approach for Interaction Neighborhood Estimation in Random Fields
We consider the problem of interaction neighborhood estimation from the
partial observation of a finite number of realizations of a random field. We
introduce a model selection rule to choose estimators of conditional
probabilities among natural candidates. Our main result is an oracle inequality
satisfied by the resulting estimator. We use then this selection rule in a
two-step procedure to evaluate the interacting neighborhoods. The selection
rule selects a small prior set of possible interacting points and a cutting
step remove from this prior set the irrelevant points. We also prove that the
Ising models satisfy the assumptions of the main theorems, without restrictions
on the temperature, on the structure of the interacting graph or on the range
of the interactions. It provides therefore a large class of applications for
our results. We give a computationally efficient procedure in these models. We
finally show the practical efficiency of our approach in a simulation study.Comment: 36 pages, 10 figure
- …