Non-interacting electrons and the metal-insulator transition in 2D with correlated impurities

While standard scaling arguments show that a system of non-interacting electrons in two dimensions and in the presence of uncorrelated disorder is insulating, in this work we discuss the case where inter-impurity correlations are included. We find that for point-like impurities and an infinite inter-impurity correlation length a mobility edge exists in 2D even if the individual impurity potentials are random. In the uncorrelated system we recover the scaling results, while in the intermediate regime for length scales comparable to the correlation length, the system behaves like a metal but with increasing fluctuations, before strong localization eventually takes over for length scales much larger than the correlation length. In the intermediate regime, the relevant length scale is not given by the elastic scattering length but by the inter-impurity correlation length, with important consequences for high mobility systems.Comment: 4 page

Seeing Anderson Localization

Anderson localization was discovered 50 years ago to describe the propagation of electrons in the presence of disorder. The main prediction back then, was the existence of disorder induced localized states, which do not conduct electricity. Many years later it turns out, that the concept of Anderson localization is much more general and applies to almost any type of propagation in time or space, when more than one parameter is relevant (like phase and amplitude). Here we propose a new optical scheme to literally see Anderson localization by varying the optical wavelength or angle of incidence to tune between localized and delocalized states. The occurrence of Anderson localization in the propagation of light, in particular, has become the focus of tremendous interest due to the emergence of new optical technologies and media, such as low dimensional and disordered optical lattices. While several experiments have reported the measurement of Anderson localization of light, many of the observations remain controversial because the effects of absorption and localization have a similar signature, i.e., exponential decrease of the transmission with the system size. In this work, we discuss a system, where we can clearly differentiate between absorption and localization effects because this system is equivalent to a perfect filter, only in the absence of any absorption. Indeed, only one wavelength is perfectly transmitted and all others are fully localized. These results were obtained by developing a new theoretical framework for the average optical transmission through disordered media.Comment: 5 pages, 4 figure

Comment on ``Periodic wave functions and number of extended states in random dimer systems'

There are no periodic wave-functions in the RDM but close to the critical energies there exist periodic envelopes. These envelopes are given by the non-disordered properties of the system.Comment: RevTex file, 1 page, Comment X. Huang, X. Wu and C. Gong, Phys. Rev. B 55, 11018 (1997

The strengthening of reentrant pinning by collective interactions in the peak effect

Since it was first observed about 40 years ago [1], the peak effect has been the subject of numerous research mainly impelled by the desire to determine its exact mechanisms. Despite these efforts, a consensus on this question has yet to be reached. Experimentally, the peak effect indicates a transition from a depinned vortex phase to a reentrant pinning phase at high magnetic field. To study the effects of intrinsic pinning on the peak effect, we consider Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ superconducting metallic glasses in which the vortex pinning force varies depending on the Fe content and in which a huge peak effect is seen as a function of magnetic field. The results are mapped out as a phase diagram in which it is readily seen that the peak effect becomes broader with decreasing pinning force. Typically, pinning can be understood by increased pinning centers, but here, we show that reentrant pinning is due to the strengthening of interactions (while decreasing pinning strength). Our results demonstrate the strengthening of the peak effect by collective effects.Comment: 4 pages, 4 figure