title: Localization and critical diffusion of quantum dipoles in two dimensions.
abstract: We discuss quantum propagation of dipole excitations in two dimensions. This problem differs from the conventional Anderson localization due to the existence of long-range hops. We find that the critical wave functions of the dipoles always exist which manifest themselves by a scale independent diffusion constant. If the system is T invariant the states are critical for all values of the parameters. Otherwise, there can be a "metal-insulator" transition between this "ordinary" diffusion and the Levy flights (the diffusion constant logarithmically increasing with the scale). These results follow from the two-loop analysis of the modified nonlinear supermatrix σ model.
authors: Physics Department, Columbia University, New York, New York 10027, USA.
authors: I L
authors: IL
authors: Aleiner
authors: B L
authors: BL
authors: Altshuler
authors: K B
authors: KB
authors: Efetov