Low energy theory of disordered graphene

At low values of external doping graphene displays a wealth of unconventional transport properties. Perhaps most strikingly, it supports a robust 'metallic' regime, with universal conductance of the order of the conductance quantum. We here apply a combination of mean field and bosonization methods to explore the large scale transport properties of the system. We find that, irrespective of the doping level, disordered graphene is subject to common mechanisms of Anderson localization. However, at low doping a number of renormalization mechanisms conspire to protect the conductivity of the system, to an extend that strong localization may not be seen even at temperatures much smaller than those underlying present experimental work.Comment: 4 page

Quasi-Particle density of states and Thouless conductance of disordered d-wave superconductors

We present a numerical study of the quasi-particle density of states (DoS) of two-dimensional d-wave superconductors in the presence of disorder, focusing on the influence of the range of the disorder. We find qualitatively different behavior for smooth and short-ranged disorder. In the former case, we find power law scaling of the DoS with an exponent depending on the strength of the disorder and the superconducting order parameter in quantitative agreement with the theory of Nersesyan {\em et al.}. For strong disorder, a qualitative change to an energy independent DoS occurs. In contrast, for short-ranged disorder of sufficient strength, we find localization by analyzing the system size dependence of the Thouless numbers. Near zero energy we find a micro gap in the DoS. The width of this micro gap is given by the mean level spacing of a localization volume. From the system size and disorder dependence of the width of the micro gap we derive the dependence of the localization length on the disorder strength.Comment: 2 pages, to appear in J. Phys. Soc. Jpn., proceedings Localisation 2002 (Tokyo, Japan

Class D spectral peak in Majorana quantum wires

Proximity coupled spin-orbit quantum wires have recently been shown to support midgap Majorana states at critical points. We show that in the presence of disorder these systems are prone to the buildup of a second bandcenter anomaly, which is of different physical origin but shares key characteristics with the Majorana state: it is narrow in width, insensitive to magnetic fields, carries unit spectral weight, and is rigidly tied to the band center. Depending on the parity of the number of subgap quasiparticle states, a Majorana mode does or does not coexist with the impurity generated peak. The strong 'entanglement' between the two phenomena may hinder an unambiguous detection of the Majorana by spectroscopic techniques.Comment: 4+ pages, 3 figures, qualitative discussion is adde

Effective field theory of the disordered Weyl semimetal

In disordered Weyl semimetals, mechanisms of topological origin lead to the protection against Anderson localization, and at the same time to different types of transverse electromagnetic response -- the anomalous Hall, and chiral magnetic effect. We here apply field theory methods to discuss the manifestation of these phenomena at length scales which are beyond the scope of diagrammatic perturbation theory. Specifically we show how an interplay of symmetry breaking and the chiral anomaly leads to a field theory containing two types of topological terms. Generating the unconventional response coefficients of the system, these terms remain largely unaffected by disorder, i.e. information on the chirality of the system remains visible even at large length scales.Comment: 4 pages, 1 figur

Theory of the strongly disordered Weyl semimetal

In disordered Weyl semimetals, mechanisms of topological origin lead to novel mechanisms of transport, which manifest themselves in unconventional types of electromagnetic response. Prominent examples of transport phenomena particular to the Weyl context include the anomalous Hall effect, the chiral magnetic effect, and the formation of totally field dominated regimes of transport in which the longitudinal conductance is proportional to an external magnetic field. In this paper, we discuss the manifestations of these phenomena at large length scales including the cases of strong disorder and/or magnetic field which are beyond the scope of diagrammatic perturbation theory. Our perhaps most striking finding is the identification of a novel regime of drift/diffusion transport where diffusion at short scales gives way to effectively ballistic dynamics at large scales, before a re-entrance to diffusion takes place at yet larger scales. We will show that this regime plays a key role in understanding the interplay of the various types of magnetoresponse of the system. Our results are obtained by describing the strongly disordered system in terms of an effective field theory of Chern-Simons type. The paper contains a self-contained derivation of this theory, and a discussion of both equilibrium and non-equilibrium (noise) transport phenomena following from it.Comment: 26 pages, 7 figure

Spectral and Transport Properties of d-Wave Superconductors With Strong Impurities

One of the remarkable features of disordered d-wave superconductors is strong sensitivity of long range properties to the microscopic realization of the disorder potential. Particularly rich phenomenology is observed for the -- experimentally relevant -- case of dilute distributions of isolated impurity centers. Building on earlier diagrammatic analyses, the present paper derives and analyses a low energy effective field theory of this system. Specifically, the results of previous diagrammatic T-matrix approaches are extended into the perturbatively inaccessible low energy regimes, and the long range (thermal) transport behaviour of the system is discussed. It turns out that in the extreme case of a half-filled tight binding band and infinitely strong impurities (impurities at the unitary limit), the system is in a delocalized phase.Comment: 14 pages, two figures include