Hall resistance in the hopping regime, a "Hall Insulator"?

The Hall conductivity and resistivity of strongly localized electrons at low temperatures and at small magnetic fields are obtained. It is found that the results depend on whether the conductivity or the resistivity tensors are averaged to obtain the macroscopic Hall resistivity. In the second case the Hall resistivity always {\it diverges} exponentially as the temperature tends to zero. But when the Hall resistivity is derived from the averaged conductivity, the resulting temperature dependence is sensitive to the disorder configuration. Then the Hall resistivity may approach a constant value as $T\to 0$. This is the Hall insulating behavior. It is argued that for strictly dc conditions, the transport quantity that should be averaged is the resistivity.Comment: Late

Nonequilibrium orbital magnetization of strongly localized electrons

The magnetic response of strongly localized electrons to a time-dependent vector potential is considered. The orbital magnetic moment of the system, away from steady-state conditions, is obtained. The expression involves the tunneling and phonon-assisted hopping currents between localized states. The frequency and temperature dependence of the orbital magnetization is analyzed as function of the admittances connecting localized levels. It is shown that quantum interference of the localized wave functions contributes to the moment a term which follows adiabatically the time-dependent perturbation.Comment: RevTeX 3.

Effective Actions, Boundaries and Precision Calculations of Casimir Energies

We perform the matching required to compute the leading effective boundary contribution to the QED lagrangian in the presence of a conducting surface, once the electron is integrated out. Our result resolves a confusion in the literature concerning the interpretation of the leading such correction to the Casimir energy. It also provides a useful theoretical laboratory for brane-world calculations in which kinetic terms are generated on the brane, since a lot is known about QED near boundaries.Comment: 5 pages. revtex; Added paragraphs describing finite-conductivity effects and effects due to curvatur

The ground state of a general electron-phonon Hamiltonian is a spin singlet

The many-body ground state of a very general class of electron-phonon Hamiltonians is proven to contain a spin singlet (for an even number of electrons on a finite lattice). The phonons interact with the electronic system in two different ways---there is an interaction with the local electronic charge and there is a functional dependence of the electronic hopping Hamiltonian on the phonon coordinates. The phonon potential energy may include anharmonic terms, and the electron-phonon couplings and the hopping matrix elements may be nonlinear functions of the phonon coordinates. If the hopping Hamiltonian is assumed to have no phonon coordinate dependence, then the ground state is also shown to be unique, implying that there are no ground-state level crossings, and that the ground-state energy is an analytic function of the parameters in the Hamiltonian. In particular, in a finite system any self-trapping transition is a smooth crossover not accompanied by a nonanalytical change in the ground state. The spin-singlet theorem applies to the Su-Schrieffer-Heeger model and both the spin-singlet and uniqueness theorems apply to the Holstein and attractive Hubbard models as special cases. These results hold in all dimensions --- even on a general graph without periodic lattice structure.Comment: 25 pages, no figures, plainte

Chiral Lagrangians

An overview of the field of Chiral Lagrangians is given. This includes Chiral Perturbation Theory and resummations to extend it to higher energies, applications to the muon anomalous magnetic moment, $\epsilon^\prime/\epsilon$ and others.Comment: Invited talk at the XX International Symposium on Lepton and Photon Interactions at High Energies 23rd-28th July 2001, Rome Italy, 15 pages, uses ws-p10x7.cls Changes: 2 references added, numbers in g-2 hadronic changed slightl

The Self-Trapping Line of the Holstein Molecular Crystal Model in One Dimension

The ground state of the Holstein molecular crystal model in one dimension is studied using the Global-Local variational method, analyzing in particular the total energy, kinetic energy, phonon energy, and interaction energy over a broad region of the polaron parameter space. Through the application of objective criteria, a unique curve is identified that simply, accurately, and robustly locates the self-trapping transition separating small polaron and large polaron behavior

Surface Phase Transitions Induced by Electron Mediated Adatom-Adatom Interaction

We propose that the indirect adatom-adatom interaction mediated by the conduction electrons of a metallic surface is responsible for the $\sqrt{3}\times \sqrt{3}\Leftrightarrow 3\times 3$ structural phase transitions observed in Sn/Ge (111) and Pb/Ge (111). When the indirect interaction overwhelms the local stress field imposed by the substrate registry, the system suffers a phonon instability, resulting in a structural phase transition in the adlayer. Our theory is capable of explaining all the salient features of the $\sqrt{3}\times \sqrt{3}\Leftrightarrow 3\times 3$ transitions observed in Sn/Ge (111) and Pb/Ge (111), and is in principle applicable to a wide class of systems whose surfaces are metallic before the transition.Comment: 4 pages, 5 figure

Pion and Sigma Polarizabilities and Radiative Transitions

Fermilab E781 plans measurements of gamma-Sigma and $\gamma$-pion interactions using a 600 GeV beam of Sigmas and pions, and a virtual photon target. Pion polarizabilities and radiative transitions will be measured in this experiment. The former can test a precise prediction of chiral symmetry; the latter for a_1(1260) ----> pi + gamma is important for understanding the polarizability. The experiment also measures polarizabilities and radiative transitions for Sigma hyperons. The polarizabilities can test predictions of baryon chiral perturbation theory. The radiative transitions to the Sigma*(1385) provide a measure of the magnetic moment of the s-quark. Previous experimental and theoretical results for gamma-pi and gamma-Sigma interactions are given. The E781 experiment is described.Comment: 13 pages text (tex), Tel Aviv U. Preprint TAUP 2204-94, uses Springer-Verlag TEX macro package lecproc.cmm (appended at end of tex file, following \byebye), which requires extracting lecproc.cmm and putting this file in your directory in addition to the tex file (mmcd.tex) before tex processing. lecproc.cmm should be used following instructions and guidelines available from Springer-Verlag. Submitted to the Proceedings of Workshop on Chiral Dynamics, Massachusetts Institute of Technology, July 1994, Eds. A. Bernstein, B. Holstein. Replaced Oct. 4 to add TAUP preprint number. Replaced Oct. 12 to correct Pb target thickness from 1.3% interaction to 0.3

Ground State and Excitations of Disordered Boson Systems

After an introduction to the dirty bosons problem, we present a gaussian theory for the ground state and excitations. This approach is physically equivalent to the Bogoliubov approximation. We find that ODLRO can be destroyed with sufficient disorder. The density of states and localization of the elementary excitations are discussed. (To appear in JLTP Proceedings of the Conference on Condensed Bose Systems at the University of Minnesota, 1993.)Comment: 13 pages. (postscript file because of the figures inserted in the text.