Condensed Matter Theory

Abstract:

Dirty quantum Hall ferromagnets (QHFMs) provide a setting both for new problems in the theory of magnetism with quenched disorder, and for new realisations of old problems. In the first category, the fact that spin textures in Heisenberg QHFMs carry charge leads to a coupling between charged impurities and magnetic order. This coupling drives a zero-temperature transition between a ferromagnet at low disorder and a spin glass at strong disorder, and controls screening and the nature of excitations in the disorder-dominated ground state. In the second category, random fields coupling linearly to the order parameter appear in some Ising QHFMs, and transport measurements appear to indicate field-induced domain states at low temperature.

Abstract:

We present compelling experimental evidence for a successful electrostatic trapping of two-dimensional dipolar excitons that results in stable formation of a well-confined, high-density and spatially uniform dipolar exciton fluid. We show that, for at least half a microsecond, the exciton fluid sustains a density higher than the critical density for degeneracy if the exciton fluid temperature reaches the lattice temperature within that time. This method should allow for the study of strongly interacting bosons in two dimensions at low temperatures, and possibly lead towards the observation of quantum phase transitions of two-dimensional interacting excitons, such as superfluidity and crystallization. © 2006 The American Physical Society.