29,946 research outputs found
Many-Body Localization Transition in Random Quantum Spin Chains with Long-Range Interactions
While there are well established methods to study delocalization transitions
of single particles in random systems, it remains a challenging problem how to
characterize many body delocalization transitions. Here, we use a generalized
real-space renormalization group technique to study the anisotropic Heisenberg
model with long-range interactions, decaying with a power , which are
generated by placing spins at random positions along the chain. This method
permits a large-scale finite-size scaling analysis. We examine the full
distribution function of the excitation energy gap from the ground state and
observe a crossover with decreasing . At the full
distribution coincides with a critical function. Thereby, we find strong
evidence for the existence of a many body localization transition in disordered
antiferromagnetic spin chains with long range interactions.Comment: 6 pages, 4 figures, references adde
Search for Second and Third Generation Leptoquarks at CDF
We report the results of a search for second and third generation leptoquarks
using 88 of data recorded by the Collider Detector at Fermilab.
Color triplet technipions, which play the role of scalar leptoquarks, are
investigated due to their potential production in decays of strongly coupled
color octet technirhos. Events with a signature of two heavy flavor jets and
missing energy may indicate the decay of a second (third) generation leptoquark
to a charm (bottom) quark and a neutrino. As the data is found to be consistent
with Standard Model expectations, mass limits are determined.Comment: Talk given at DPF2000, Columbus (OH), 9-12 Aug 2000. 3 pages, 4
figures. Submitted to Int.J.Mod.Phys.
New nonlinear structures in a degenerate one-dimensional electron gas
The collective dynamics of nonlinear electron waves in an one-dimensional
degenerate electron gas is treated using the Lagrangian fluid approach. A new
class of solutions with a nontrivial space and time dependence is derived. Both
analytical and numerical results demonstrate the formation of stable,
breather-like modes, provided certain conditions are meet. For large amplitude
of the initial density perturbation, a catastrophic collapse of the plasma
density is predicted, even in the presence of the quantum statistical pressure
and quantum diffraction dispersive effects. The results are useful for the
understanding of the properties of general nonlinear structures in dense
plasmas
Dynamics of weakly coupled random antiferromagnetic quantum spin chains
We study the low-energy collective excitations and dynamical response
functions of weakly coupled random antiferromagnetic spin-1/2 chains. The
interchain coupling leads to Neel order at low temperatures. We use the
real-space renormalization group technique to tackle the intrachain couplings
and treat the interchain couplings within the Random Phase Approximation (RPA).
We show that the system supports collective spin wave excitations, and
calculate the spin wave velocity and spectra weight within RPA. Comparisons
will be made with inelastic neutron scattering experiments
quasi-one-dimensional disordered spin systems such as doped CuGeOComment: 4 page
Comment on "A note on the construction of the Ermakov-Lewis invariant"
We show that the basic results on the paper referred in the title [J. Phys.
A: Math. Gen. v. 35 (2002) 5333-5345], concerning the derivation of the Ermakov
invariant from Noether symmetry methods, are not new
Mixed normal conditional heteroskedasticity
Both unconditional mixed-normal distributions and GARCH models with fat-tailed conditional distributions have been employed for modeling financial return data. We consider a mixed-normal distribution coupled with a GARCH-type structure which allows for conditional variance in each of the components as well as dynamic feedback between the components. Special cases and relationships with previously proposed specifications are discussed and stationarity conditions are derived. An empirical application to NASDAQ-index data indicates the appropriateness of the model class and illustrates that the approach can generate a plausible disaggregation of the conditional variance process, in which the components' volatility dynamics have a clearly distinct behavior that is, for example, compatible with the well-known leverage effect. Klassifikation: C22, C51, G1
Asymmetric multivariate normal mixture GARCH
An asymmetric multivariate generalization of the recently proposed class of normal mixture GARCH models is developed. Issues of parametrization and estimation are discussed. Conditions for covariance stationarity and the existence of the fourth moment are derived, and expressions for the dynamic correlation structure of the process are provided. In an application to stock market returns, it is shown that the disaggregation of the conditional (co)variance process generated by the model provides substantial intuition. Moreover, the model exhibits a strong performance in calculating out–of–sample Value–at–Risk measures
Low Momentum Classical Mechanics with Effective Quantum Potentials
A recently introduced effective quantum potential theory is studied in a low
momentum region of phase space. This low momentum approximation is used to show
that the new effective quantum potential induces a space-dependent mass and a
smoothed potential both of them constructed from the classical potential. The
exact solution of the approximated theory in one spatial dimension is found.
The concept of effective transmission and reflection coefficients for effective
quantum potentials is proposed and discussed in comparison with an analogous
quantum statistical mixture problem. The results are applied to the case of a
square barrier.Comment: 4 figure
Testing conformal mapping with kitchen aluminum foil
We report an experimental verification of conformal mapping with kitchen
aluminum foil. This experiment can be reproduced in any laboratory by
undergraduate students and it is therefore an ideal experiment to introduce the
concept of conformal mapping. The original problem was the distribution of the
electric potential in a very long plate. The correct theoretical prediction was
recently derived by A. Czarnecki (Can. J. Phys. 92, 1297 (2014))
Modeling and predicting market risk with Laplace-Gaussian mixture distributions
While much of classical statistical analysis is based on Gaussian distributional assumptions, statistical modeling with the Laplace distribution has gained importance in many applied fields. This phenomenon is rooted in the fact that, like the Gaussian, the Laplace distribution has many attractive properties. This paper investigates two methods of combining them and their use in modeling and predicting financial risk. Based on 25 daily stock return series, the empirical results indicate that the new models offer a plausible description of the data. They are also shown to be competitive with, or superior to, use of the hyperbolic distribution, which has gained some popularity in asset-return modeling and, in fact, also nests the Gaussian and Laplace. Klassifikation: C16, C50 . March 2005
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