Single and two-particle energy gaps across the disorder-driven superconductor-insulator transition

The competition between superconductivity and localization raises profound questions in condensed matter physics. In spite of decades of research, the mechanism of the superconductor-insulator transition (SIT) and the nature of the insulator are not understood. We use quantum Monte Carlo simulations that treat, on an equal footing, inhomogeneous amplitude variations and phase fluctuations, a major advance over previous theories. We gain new microscopic insights and make testable predictions for local spectroscopic probes. The energy gap in the density of states survives across the transition, but coherence peaks exist only in the superconductor. A characteristic pseudogap persists above the critical disorder and critical temperature, in contrast to conventional theories. Surprisingly, the insulator has a two-particle gap scale that vanishes at the SIT, despite a robust single-particle gap.Comment: 7 pages, 5 figures (plus supplement with 4 pages, 5 figures

Localization of preformed Cooper pairs in disordered superconductors

International audienceThe most profound effect of disorder on electronic systems is the localization of the electrons transforming an otherwise metallic system into an insulator. If the metal is also a superconductor then, at low temperatures, disorder can induce a pronounced transition from a superconducting into an insulating state. An outstanding question is whether the route to insulating behaviour proceeds through the direct localization of Cooper pairs or, alternatively, by a two-step process in which the Cooper pairing is first destroyed followed by the standard localization of single electrons. Here we address this question by studying the local superconducting gap of a highly disordered amorphous superconductor by means of scanning tunnelling spectroscopy. Our measurements reveal that, in the vicinity of the superconductor-insulator transition, the coherence peaks in the one-particle density of states disappear whereas the superconducting gap remains intact, indicating the presence of localized Cooper pairs. Our results provide the first direct evidence that the superconductor-insulator transition in some homogeneously disordered materials is driven by Cooper-pair localization

Carrier scattering in metals and semiconductors

The transport properties of solids, as well as the many optical phenomena in them are determined by the scattering of current carriers. ``Carrier Scattering in Metals and Semiconductors'' elucidates the state of the art in the research on the scattering mechanisms for current carriers in metals and semiconductors and describes experiments in which these mechanisms are most dramatically manifested.The selection and organization of the material is in a form to prepare the reader to reason independently and to deal just as independently with available theoretical results and experimenta

Electrons and Disorder in Solids

This book has been written for those who study or professionally deal with solid state physics. It contains modern concepts about the physics of electrons in solids. It is written using a minimum of mathematics. The emphasis is laid on various physical models aimed at stimulating creative thinking. The book helps the reader choose the most efficient scheme of an experiment or the optimal algorithm of a calculation. Boltzmann and hopping types of conductivity are compared. Thequalitative theory of weak localization is presented and its links with the true localization and metal-insulator trans

Phase separation in oxygen deficient HoBa₂Cu₃O_7-δ single crystals: effect of high pressure and twin boundaries

We investigate the influence of high hydrostatic pressure on the electrical resistance in the ab-plane in HoBa2Cu3O7-δ single crystals with oxygen deficiency. It is determined that the high-pressure-induced redistribution of labile oxygen enhances phase separation, which is accompanied by structural relaxation and ascending diffusion within the volume of the sample. This results in a significant displacement of the temperature intervals that correspond to metal-to-dielectric-type transitions. It is determined that the formation of the low-temperature phase can occur at the twin boundaries