Diffusion in quantum crystals

Abstract

Astrophysical black hole candidates are thought to be the Kerr black hole predicted by General Relativity. However, in order to confirm the Kerr-nature of these objects, we need to probe the geometry of the space-time around them and check that observations are consistent with the predictions of the Kerr metric. That can be achieved, for instance, by studying the properties of the electromagnetic radiation emitted by the gas in the accretion disk. The high-frequency quasi-periodic oscillations observed in the X-ray flux of some stellar-mass black hole candidates might do the job. As the frequencies of these oscillations depend only very weakly on the observed X-ray flux, it is thought they are mainly determined by the metric of the space-time. In this paper, I consider the resonance models proposed by Abramowicz and Kluzniak and I extend previous results to the case of non-Kerr space-times. The emerging picture is more complicated than the one around a Kerr black hole and there is a larger number of possible combinations between different modes. I then compare the bounds inferred from the twin peak high-frequency quasi-periodic oscillations observed in three micro-quasars (GRO J1655-40, XTE J1550-564, and GRS 1915+105) with the measurements from the continuum-fitting method of the same objects. For Kerr black holes, the two approaches do not provide consistent results. In a non-Kerr geometry, this conflict may be solved if the observed quasi-periodic oscillations are produced by the resonance ν θ : ν r = 3 : 1, where ν θ and ν r are the two epicyclic frequencies. It is at least worth mentioning that the deformation from the Kerr solution required by observations would be consistent with the one suggested in another recent work discussing the possibility that steady jets are powered by the spin of these compact objects.

Asymmetric tunneling, Andreev reflection and dynamic conductance spectra in strongly correlated metals

“…Therefore, we can conclude that the asymmetric part of the conductances described by Eqs. (12) and (18) are in good qualitative agreement with the experimental facts displayed in Figures 2, 3, 5, 6 and 7, while the asymmetry starts from FCQPT yielding the FC state with particle-hole symmetry violation.…”

“…It follows from Eq. (12), that when V ≃ 2T and FC occupies a noticeable part of Fermi volume, (p f − p i )/p F ≃ 1, so that the asymmetric part becomes comparable with differential conductivity,…”

“…This behavior permits to study the asymmetric conductance as a function of temperature and is in good agreement with the behavior described by Eqs. (12) and (18). Therefore, we can conclude that the asymmetric part of the conductances described by Eqs.…”

“…It is highly desirable to probe the other properties of the heavy electron liquid like quasiparticle occupation numbers, which are not directly linked to the density of states or to the behavior of the effective mass M * . Both scanning tunneling microscopy (STM) and point contact spectroscopy (PCS) based on Andreev reflection (AR) [12] being sensitive to both the density of states and quasiparticle occupation numbers are ideal techniques to study the effects of particle-hole symmetry violation, making the differential tunneling conductivity and dynamic conductance to be asymmetric function of applied voltage. This asymmetry can be observed when HF metals and HTSC are either normal or superconducting.…”

Premature switching in graphene Josephson transistors

“…At V<2Δ, there are a series of smaller conductance peaks; such sub-gap features, which are commonly observed in superconductor/normal metal/superconductor (SNS) junctions, arise from the phenomenon of multiple Andreev reflection in conductance. During an Andreev reflection, an electron in the normal metal incident on the N/S interface is reflected as a hole, while transferring 2e to the superconducting Cooper pair condenstate [25]. In SNS junctions, an electron in graphene can be reflected back and forth several times between the two N/S interfaces, gaining energy eV each time it transverses the junction, until it accumulates sufficient energy to exit graphene into superconducting electrode as a quasiparticle.…”