Electron-phonon interaction for adiabatic anharmonic phonons

A model with Holstein-like electron-phonon coupling is studied in the limit of adiabatic phonons. The phonon distribution is anharmonic with two degenerate maxima. This model can be related to fermions in a correlated binary alloy and describes microscopic phase separation. We discuss the weak and strong electron-phonon coupling limit and present a qualitative phase diagram. In terms of the phononic displacements it consists of a homogeneous, an alternating, and a disordered phase. There is a first order phase transition between the homogeneous and the alternating phase, and second order phase transition between the alternating and the disordered phase. The opening of a gap inside the disordered phase is treated by a dynamical mean-field theory.Comment: 11 pages, 3 figures, revised and published versio

Computer simulator for a mobile telephone system

A software simulator was developed to help in the design of the LMSS. The simulator is used to study the characteristics and implementation requirements of the LMSS' configuration

Two-Dimensional Electrons in a Strong Magnetic Field with Disorder: Divergence of the Localization Length

Electrons on a square lattice with half a flux quantum per plaquette are considered. An effective description for the current loops is given by a two-dimensional Dirac theory with random mass. It is shown that the conductivity and the localization length can be calculated from a product of Dirac Green's functions with the {\it same} frequency. This implies that the delocalization of electrons in a magnetic field is due to a critical point in a phase with a spontaneously broken discrete symmetry. The estimation of the localization length is performed for a generalized model with $N$ fermion levels using a $1/N$--expansion and the Schwarz inequality. An argument for the existence of two Hall transition points is given in terms of percolation theory.Comment: 10 pages, RevTeX, no figure

Computer simulator for a mobile telephone system

A software simulator to help NASA in the design of the LMSS was developed. The simulator will be used to study the characteristics of implementation requirements of the LMSS's configuration with specifications as outlined by NASA

Two-component Bose gas in an optical lattice at single-particle filling

The Bose-Hubbard model of a two-fold degenerate Bose gas is studied in an optical lattice with one particle per site and virtual tunneling to empty and doubly-occupied sites. An effective Hamiltonian for this system is derived within a continued-fraction approach. The ground state of the effective model is studied in mean-field approximation for a modulated optical lattice. A dimerized mean-field state gives a Mott insulator whereas the lattice without modulations develops long-range correlated phase fluctuations due to a Goldstone mode. This result is discussed in comparison with the superfluid and the Mott-insulating state of a single-component hard-core Bose.Comment: 11 page

Interacting bosons in an optical lattice: Bose-Einstein condensates and Mott insulator

A dense Bose gas with hard-core interaction is considered in an optical lattice. We study the phase diagram in terms of a special mean-field theory that describes a Bose-Einstein condensate and a Mott insulator with a single particle per lattice site for zero as well as for non-zero temperatures. We calculate the densities, the excitation spectrum and the static structure factor for each of these phases.Comment: 17 pages, 5 figures; 1 figure added, typos remove