Subgap features due to quasiparticle tunneling in quantum dots coupled to superconducting leads

We present a microscopic theory of transport through quantum dot set-ups coupled to superconducting leads. We derive a master equation for the reduced density matrix to lowest order in the tunneling Hamiltonian and focus on quasiparticle tunneling. For high enough temperatures transport occurs in the subgap region due to thermally excited quasiparticles, which can be used to observe excited states of the system for low bias voltages. On the example of a double quantum dot we show how subgap transport spectroscopy can be done. Moreover, we use the single level quantum dot coupled to a normal and a superconducting lead to give a possible explanation for the subgap features observed in the experiments published in Appl. Phys. Lett. 95, 192103 (2009).Comment: 18 pages, 20 figures, revised according to published versio

The Role of Vortices in the Mutual Coupling of Superconducting and Normal-Metal Films

I propose a possible explanation to a recently observed ``cross-talk'' effect in metal-insulator-metal trilayers, indicating a sharp peak near a superconducting transition in one of the metal films. Coulomb interactions are excluded as a dominant coupling mechanism, and an alternative is suggested, based on the local fluctuating electric field induced by mobile vortices in the superconducting layer. This scenario is compatible with the magnitude of the peak signal and its shape; most importantly, it addresses the {\it non-reciprocity} of the effect in exchanging the roles of the films.Comment: 13 pages, RevTe

Spin-twist driven persistent current in a strongly correlated two-dimensional electron system: a manifestation of the gauge field

A persistent current, coupled with the spin state, of purely many-body origin is shown to exist in Nagaoka's ferromagnetic state in two dimensions (2D). This we regard as a manifestation of a gauge field, which comes from the surrounding spin configuration and acts on the hole motion, being coupled to the Aharonov-Bohm flux. This provides an example where the electron-electron interaction exerts a profound effect involving the spins in clean two-dimensional lattice systems in sharp contrast to continuum or spinless fermion systems.Comment: 11 pages, typeset using Revtex 3.0, Phys. Rev. B in press, 2 figures available upon request at [email protected]

Charge Transport in the Dense Two-Dimensional Coulomb Gas

The dynamics of a globally neutral system of diffusing Coulomb charges in two dimensions, driven by an applied electric field, is studied in a wide temperature range around the Berezinskii-Kosterlitz-Thouless transition. I argue that the commonly accepted ``free particle drift'' mechanism of charge transport in this system is limited to relatively low particle densities. For higher densities, I propose a modified picture involving collective ``partner transfer'' between bound pairs. The new picture provides a natural explanation for recent experimental and numerical findings which deviate from standard theory. It also clarifies the origin of dynamical scaling in this context.Comment: 4 pages, RevTeX, 2 eps figures included; some typos corrected, final version to be published in Phys. Rev. Let

Pairing Fluctuation Theory of Superconducting Properties in Underdoped to Overdoped Cuprates

We propose a theoretical description of the superconducting state of under- to overdoped cuprates, based on the short coherence length of these materials and the associated strong pairing fluctuations. The calculated $T_c$ and the zero temperature excitation gap $\Delta(0)$, as a function of hole concentration $x$, are in semi-quantitative agreement with experiment. Although the ratio $T_c/\Delta(0)$ has a strong $x$ dependence, different from the universal BCS value, and $\Delta(T)$ deviates significantly from the BCS prediction, we obtain, quite remarkably, quasi-universal behavior, for the normalized superfluid density $\rho_s(T)/\rho_s(0)$ and the Josephson critical current $I_c(T)/I_c(0)$, as a function of $T/T_c$. While experiments on $\rho_s(T)$ are consistent with these results, future measurements on $I_c(T)$ are needed to test this prediction.Comment: 4 pages, 3 figures, REVTeX, submitted to Phys. Rev. Let

Pinhole calculations of the Josephson effect in 3He-B

We study theoretically the dc Josephson effect between two volumes of superfluid 3He-B. We first discuss how the calculation of the current-phase relationships is divided into a mesoscopic and a macroscopic problem. We then analyze mass and spin currents and the symmetry of weak links. In quantitative calculations the weak link is assumed to be a pinhole, whose size is small in comparison to the coherence length. We derive a quasiclassical expression for the coupling energy of a pinhole, allowing also for scattering in the hole. Using a selfconsistent order parameter near a wall, we calculate the current-phase relationships in several cases. In the isotextural case, the current-phase relations are plotted assuming a constant spin-orbit texture. In the opposite anisotextural case the texture changes as a function of the phase difference. For that we have to consider the stiffness of the macroscopic texture, and we also calculate some surface interaction parameters. We analyze the experiments by Marchenkov et al. We find that the observed pi states and bistability hardly can be explained with the isotextural pinhole model, but a good quantitative agreement is achieved with the anisotextural model.Comment: 20 pages, 21 figures, revtex

Microscopic Theory of Josephson Mesoscopic Constrictions

We present a microscopic theory for the d.c. Josephson effect in model mesoscopic constrictions. Our method is based on a non-equilibrium Green function formalism which allows for a self-consistent determination of the order parameter profile along the constriction. The various regimes defined by the different length scales (Fermi wavelength $\lambda_F$, coherence length $\xi_0$ and constriction length $L_C$) can be analyzed, including the case where all these lengths are comparable. For the case $\lambda_F \tilde{<} (L_C,\xi_0)$ phase oscillations with spatial period $\lambda_F/2$ can be observed. In the case of $L_C>\xi_0$ solutions with a phase-slip center inside the constriction can be found, in agreement with previous phenomenological theories.Comment: 4 pages (RevTex 3.0), 3 postscript figures available upon request, 312456-C

Resistively-shunted superconducting quantum point contacts

We have studied the Josephson dynamics of resistively-shunted ballistic superconducting quantum point contacts at finite temperatures and arbitrary number of conducting modes. Compared to the classical Josephson dynamics of tunnel junctions, dynamics of quantum point contacts exhibits several new features associated with temporal fluctuations of the Josephson potential caused by fluctuations in the occupation of the current-carrying Andreev levels.Comment: 5 pages, RevTex, 3 postscript figures include

Hopping Conduction in Uniaxially Stressed Si:B near the Insulator-Metal Transition

Using uniaxial stress to tune the critical density near that of the sample, we have studied in detail the low-temperature conductivity of p-type Si:B in the insulating phase very near the metal-insulator transition. For all values of temperature and stress, the conductivity collapses onto a single universal scaling curve. For large values of the argument, the scaling function is well fit by the exponentially activated form associated with variable range hopping when electron-electron interactions cause a soft Coulomb gap in the density of states at the Fermi energy. The temperature dependence of the prefactor, corresponding to the T-dependence of the critical curve, has been determined reliably for this system, and is proportional to the square-root of T. We show explicitly that nevlecting the prefactor leads to substantial errors in the determination of the scaling parameters and the critical exponents derived from them. The conductivity is not consistent with Mott variable-range hopping in the critical region nor does it obey this form for any range of the parameters. Instead, for smaller argument of the scaling function, the conductivity of Si:B is well fit by an exponential form with exponent 0.31 related to the critical exponents of the system at the metal- insulator transition.Comment: 13 pages, 6 figure

Superconductive properties of thin dirty SN bilayers

The theory of superconductivity in thin SN sandwiches (bilayers) in the diffusive limit is developed within the standard Usadel equation method, with particular emphasis on the case of very thin superconductive layers, d_S << d_N. The proximity effect in the system is governed by the interlayer interface resistance (per channel) \rho_{int}. The case of relatively low resistance (which can still have large absolute values) can be completely studied analytically. The theory describing the bilayer in this limit is of BCS type but with the minigap (in the single-particle density of states) E_g << \Delta substituting the order parameter \Delta in the standard BCS relations; the original relations are thus severely violated. In the opposite limit of an opaque interface, the behavior of the system is in many respects close to the BCS predictions. Over the entire range of \rho_{int}, the properties of the bilayer are found numerically. Finally, it is shown that the results obtained for the bilayer also apply to more complicated structures such as SNS and NSN trilayers, SNINS and NSISN systems, and SN superlattices.Comment: 15 pages (including 10 EPS figures), REVTeX. Version 2: minor changes; added references, a note is added concerning applicability of our results to SNINS and NSISN systems. To appear in Phys. Rev. B on March 1, 200