Characteristic energies, transition temperatures, and switching effects in clean SNS graphene nanostructures

We study proximity effects in clean nanoscale superconductor-normal metal-superconductor (S$\mid$N$\mid$S) graphene heterostructures using a self-consistent numerical solution to the continuum Dirac Bogoliubov-de Gennes (DBdG) equations. We obtain results for the pair amplitude and the local density of states (DOS), as a function of doping and of the geometrical parameters determining the width of the structures. The superconducting correlations are found to penetrate the normal graphene layers even when there is extreme mismatch in the normal and superconducting doping levels, where specular Andreev reflection dominates. The local DOS exhibits peculiar features, which we discuss, arising from the Dirac cone dispersion relation and from the interplay between the superconducting and Thouless energy scales. The corresponding characteristic energies emerge in the form of resonant peaks in the local DOS, that depend strongly on the doping level, as does the energy gap, which declines sharply as the relative difference in doping between the S and N regions is reduced. We also linearize the DBdG equations and develop an essentially analytical method that determines the critical temperature $T_c$ of an \sns nanostructure self-consistently. We find that for S regions that occupy a fraction of the coherence length, $T_c$ can undergo substantial variations as a function of the relative doping. At finite temperatures and by manipulating the doping levels, the self consistent pair amplitudes reveal dramatic transitions between a superconducting and resistive normal state of the structure. Such behavior suggests the possibility of using the proposed system as a carbon-based superconducting switch, turning superconductivity on or off by tuning the relative doping levels.Comment: 13 pages, figures include

Angular Dependence of the Superconducting Transition Temperature in Ferromagnet-Superconductor-Ferromagnet Trilayers

The superconducting transition temperature, $T_c$, of a ferromagnet (F) - superconductor (S) - ferromagnet trilayer depends on the mutual orientation of the magnetic moments of the F layers. This effect has been previously observed in F/S/F systems as a $T_c$ difference between parallel and antiparallel configurations of the F layers. Here we report measurements of $T_c$ in CuNi/Nb/CuNi trilayers as a function of the angle between the magnetic moments of the CuNi ferromagnets. The observed angular dependence of $T_c$ is in qualitative agreement with a F/S proximity theory that accounts for the odd triplet component of the condensate predicted to arise for non-collinear orientation of the magnetic moments of the F layers.Comment: 4 + \epsilon pages including 4 figures. To appear in Phys. Rev. Let

Micro Black Holes and the Democratic Transition

Unitarity implies that the evaporation of microscopic quasi-classical black holes cannot be universal in different particle species. This creates a puzzle, since it conflicts with the thermal nature of quasi-classical black holes, according to which all the species should see the same horizon and be produced with the same Hawking temperatures. We resolve this puzzle by showing that for the microscopic black holes, on top the usual quantum evaporation time, there is a new time-scale which characterizes a purely classical process during which the black hole looses the ability to differentiate among the species, and becomes democratic. We demonstrate this phenomenon in a well-understood framework of large extra dimensions, with a number of parallel branes. An initially non-democratic black hole is the one localized on one of the branes, with its high-dimensional Schwarzschild radius being much shorter than the interbrane distance. Such a black hole seemingly cannot evaporate into the species localized on the other branes, that are beyond its reach. We demonstrate that in reality the system evolves classically in time, in such a way that the black hole accretes the neighboring branes. The end result is a completely democratic static configuration, in which all the branes share the same black hole, and all the species are produced with the same Hawking temperature. Thus, just like their macroscopic counterparts, the microscopic black holes are universal bridges to the hidden sector physics.Comment: 35 pages, 5 figure

Superconducting proximity effects in metals with a repulsive pairing interaction

Studies of the superconducting proximity effect in normal conductor/superconductor $(N/S)$ junctions almost universally assume no effective electron-electron coupling in the $N$ region. While such an approximation leads to a simple description of the proximity effect, it is unclear how it could be rigorously justified. We reveal a much more complex picture of the proximity effect in $N/S$ bilayers, where $S$ is a clean s-wave BCS superconductor and $N$ is a simple metal with a repulsive effective electron coupling. We elucidate the proximity effect behavior using a highly accurate method to self-consistently solve the Bogoliubov-deGennes equations. We present our results for a wide range of values of the interface scattering, the Fermi wave vector mismatch, the temperature, and the ratio $g$ of the effective interaction strengths in the $N$ and $S$ region. We find that the repulsive interaction, represented by a negative $g$, strongly alters the signatures of the proximity effect as can be seen in the spatial dependence of the Cooper pair amplitude and the pair potential, as well as in the local density of states near the interface.Comment: 12 pages, including 10 figures. To appear in Phys. Rev.

Melting and structure of the vortex solid in strongly anisotropic layered superconductors with random columnar pins

We study the melting transition of the low-temperature vortex solid in strongly anisotropic layered superconductors with a concentration of random columnar pinning centers small enough so that the areal density of the pins is much less than that of the vortex lines. Both the external magnetic field and the columnar pins are assumed to be oriented perpendicular to the layers Our method, involving numerical minimization of a model free energy functional, yields not only the free energy values at the local minima of the functional but also the detailed density distribution of the system at each minimum: this allows us to study in detail the structure of the different phases. We find that at these pin concentrations and low temperatures, the thermodynamically stable state is a topologically ordered Bragg glass. This nearly crystalline state melts into an interstitial liquid (a liquid in which a small fraction of vortex lines remain localized at the pinning centers) in two steps, so that the Bragg glass and the liquid are separated by a narrow phase that we identify from analysis of its density structure as a polycrystalline Bose glass. Both the Bragg glass to Bose glass and the Bose glass to interstitial liquid transitions are first-order. We also find that a local melting temperature defined using a criterion based on the degree of localization of the vortex lines exhibits spatial variations similar to those observed in recent experiments.Comment: 17 page

Proximity effects and triplet correlations in Ferromagnet/Ferromagnet/Superconductor nanostructures

We report the results of a study of superconducting proximity effects in clean Ferromagnet/Ferromagnet/Superconductor (${\rm F_1F_2S}$) heterostructures, where the pairing state in S is a conventional singlet s-wave. We numerically find the self-consistent solutions of the Bogoliubov-de Gennes (BdG) equations and use these solutions to calculate the relevant physical quantities. By linearizing the BdG equations, we obtain the superconducting transition temperatures $T_c$ as a function of the angle $\alpha$ between the exchange fields in $\rm F_1$ and $\rm F_2$. We find that the results for $T_c(\alpha)$ in ${\rm F_1F_2S}$ systems are clearly different from those in ${\rm F_1 S F_2}$ systems, where $T_c$ monotonically increases with $\alpha$ and is highest for antiparallel magnetizations. Here, $T_c(\alpha)$ is in general a non-monotonic function, and often has a minimum near $\alpha \approx 80^{\circ}$. For certain values of the exchange field and layer thicknesses, the system exhibits reentrant superconductivity with $\alpha$: it transitions from superconducting to normal, and then returns to a superconducting state again with increasing $\alpha$. This phenomenon is substantiated by a calculation of the condensation energy. We compute, in addition to the ordinary singlet pair amplitude, the induced odd triplet pairing amplitudes. The results indicate a connection between equal-spin triplet pairing and the singlet pairing state that characterizes $T_c$. We find also that the induced triplet amplitudes can be very long-ranged in both the S and F sides and characterize their range. We discuss the average density of states for both the magnetic and the S regions, and its relation to the pairing amplitudes and $T_c$. The local magnetization vector, which exhibits reverse proximity effects, is also investigated.Comment: 14 pages including 11 figure

Adapting integrity checking techniques for concurrent operation executions

One challenge for achieving executable models is preserving the integrity of the data. That is, given a structural model describing the constraints that the data should satisfy, and a behavioral model describing the operations that might change the data, the integrity checking problem consists in ensuring that, after executing the modeled operations, none of the specified constraints is violated. A multitude of techniques have been presented so far to solve the integrity checking problem. However, to the best of our knowledge, all of them assume that operations are not executed concurrently. As we are going to see, concurrent operation executions might lead to violations not detected by these techniques. In this paper, we present a technique for detecting and serializing those operations that can cause a constraint violation when executed concurrently , so that, previous incremental techniques, exploiting our approach, can be safely applied in systems with concurrent operation executions guaranteeing the integrity of the data.Peer ReviewedPostprint (author's final draft

Distributions of Conductance and Shot Noise and Associated Phase Transitions

For a chaotic cavity with two indentical leads each supporting N channels, we compute analytically, for large N, the full distribution of the conductance and the shot noise power and show that in both cases there is a central Gaussian region flanked on both sides by non-Gaussian tails. The distribution is weakly singular at the junction of Gaussian and non-Gaussian regimes, a direct consequence of two phase transitions in an associated Coulomb gas problem.Comment: 5 pages, 3 figures include

Tunneling conductance in Superconductor/Ferromagnet junctions: a self consistent approach

We evaluate the tunneling conductance of clean Ferromomagnet/Superconductor junctions via a fully self-consistent numerical solution of the microscopic Bogoliubov-DeGennes equations. We present results for a relevant range of values of the Fermi wavevector mismatch (FWM), the spin polarization, and the interfacial scattering strength. For nonzero spin polarization, the conductance curves vary nonmonotonically with FWM. The FWM dependence of the self-consistent results is stronger than that previously found in non-self-consistent calculations, since, in the self-consistent case, the effective scattering potential near the interface depends on the FWM. The dependence on interfacial scattering is monotonic. These results confirm that it is impossible to characterize both the the FWM and the interfacial scattering by a single effective parameter and that analysis of experimental data via the use of such one-parameter models is unreliable.Comment: 12 pages, including 8 figure

Structure and kinematics of the Ayora-Cofrentes Diapir (eastern Betics). Role of basement faulting in the salt and suprasalt deformation of the Mesozoic cover

The Ayora-Cofrentes Diapir is a 34 km long N-trending salt wall cutting the Valencian Domain that is made by Middle to Upper Triassic salt. On both sides, it is flanked by parallel half grabens offsetting the subtabular strata of the Jurassic to Cretaceous suprasalt carbonate successions. Based on detailed geological mapping and cross-sections, one of them supported by a new magnetotelluric profile, this study analyses the structure and kinematics of this salt wall with the purpose of establishing the role played by the subsalt structure in its development; and, thus, help in the interpretation of diapirs currently incorporated in fold and thrust belts. In this regard, the study evidences the presence of a subsalt basement fault (the Ayora Fault), active as extensional during the Early Jurassic, and reactivated with a reverse throw sometime between the latest Cretaceous and middle Miocene. Both motions are older than the salt wall growth (middle-late Miocene). This strongly suggest that the Ayora-Cofrentes Diapir was not triggered by the vertical motion of the underlying Ayora basement fault but by thin-skinned processes for which this pre-existing basement fault appears to have played a crucial role as salt/suprasal strain localizer