Coherent State Wave Functions on the Torus
  In the study of the quantum Hall effect there are still many unresolved problems. One of these is how to generate representative wave functions for ground states on other geometries than the planar and spherical. We study one such geometry, the toroidal one, where the periodic boundary conditions must be properly taken into account.   As a tool to study the torus we investigate the properties of various types of localized states, similar to the coherent states of the harmonic oscillator, which are maximally localized in phase space. We consider two alternative definitions of localized states in the lowest Landau level (LLL) on a torus. One is the projection of the coordinate delta function onto the LLL. Another definition, proposed by Haldane & Rezayi, is to consider the set of functions which have all their zeros at a single point. Since all LLL wave functions on a torus, are uniquely defined by the position of their zeros, this defines a set of functions that are expected to be localized around the point maximally far away from the zeros.   These two families of localized states have many properties in common with the coherent states on the plane and on the sphere, e.g. a simple resolution of unity and a simple self-reproducing kernel. However, we show that only the projected delta function is maximally localized.   We find that because of modular covariance, there are severe restrictions on which wave functions that are acceptable on the torus. As a result, we can write down a trial wave function for the $\nu=\frac{2}{5}$ state, that respects the modular covariance, and has good numerical overlap with the exact coulomb ground state.   Finally we present preliminary calculations of the antisymmetric component of the viscosity tensor for the proposed, modular covariant, $\nu=\frac{2}{5}$ state, and find that it is in agreement with theoretical predictions. 
Mikael Fremling
cond-mat.str-el