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

Reconstruction of time-consistent species trees

Background The history of gene families—which are equivalent to event-labeled gene trees—can to some extent be reconstructed from empirically estimated evolutionary event-relations containing pairs of orthologous, paralogous or xenologous genes. The question then arises as whether inferred event-labeled gene trees are “biologically feasible” which is the case if one can find a species tree with which the gene tree can be reconciled in a time-consistent way. Results In this contribution, we consider event-labeled gene trees that contain speciations, duplications as well as horizontal gene transfer (HGT) and we assume that the species tree is unknown. Although many problems become NP-hard as soon as HGT and time-consistency are involved, we show, in contrast, that the problem of finding a time-consistent species tree for a given event-labeled gene can be solved in polynomial-time. We provide a cubic-time algorithm to decide whether a “time-consistent” species tree for a given event-labeled gene tree exists and, in the affirmative case, to construct the species tree within the same time-complexity

Tracing Gold Nanoparticle Charge by Electrolyte Insulator Semiconductor Devices

A capacitive field-effect electrolyte-insulator-semiconductor (EIS) device was applied for the first time to trace the charge of supported gold nanoparticles (Au-NPs) induced by oxygen plasma treatment or due to storing in aqueous oxidation and reduction solutions. In addition, X-ray photoelectron spectroscopy (XPS) has been used as a reference method to establish the various charge states of the Au-NPs resulting from the different treatment steps. After the oxygen-plasma treatment, a shift of the capacitance-voltage (C-V) curve (and flatband potential) of the Au-NP-covered p-Si-SiO2 EIS structure by about -300 mV was found. The exposure of the EIS sensor surface to an oxidative and a reductive solution resulted in a shift of the C-V curve for -85 and +81 mV, respectively. These observations correlate well with corresponding binding energy shifts in Au 4f core spectra in XPS experiments. The obtained results may open new opportunities for biosensing and biochips based on nanoparticle-charge-gated field-effect devices. © 2011 American Chemical Society.status: publishe