Cooper Pairs with Broken Parity and Time-Reversal Symmetries in D-wave Superconductors

Paramagnetic effects are shown to result in the appearance of a triplet component of order parameter in a vortex phase of a d-wave superconductor in the absence of impurities. This component, which breaks both parity and time-reversal symmetries of Cooper pairs, is expected to be of the order of unity in a number of modern superconductors such as organic, high-Tc, and some others. A generic phase diagram of such type-IV superconductors, which are singlet ones at H=0 and characterized by singlet-triplet mixed Copper pairs with broken time-reversal symmetry in a vortex phase, is discussed.Comment: 10 pages, 1 figures, Phys. Rev. Lett., submitted (July 25 2005

Specific heat studies of pure Nb3Sn single crystals at low temperature

Specific heat measurements performed on high purity vapor-grown Nb$_3$Sn crystals show clear features related to both the martensitic and superconducting transitions. Our measurements indicate that the martensitic anomaly does not display hysteresis, meaning that the martensitic transition could be a weak first or a second order thermodynamic transition. Careful measurements of the two transition temperatures display an inverse correlation between both temperatures. At low temperature specific heat measurements show the existence of a single superconducting energy gap feature.Comment: Accepted in Journal of Physics: Condensed Matte

Evaluating free flux flow in low-pinning molybdenum-germanium superconducting films

Vortex dynamics in molybdenum-germanium superconducting films were found to well approximate the unpinned free limit even at low driving forces. This provided an opportunity to empirically establish the intrinsic character of free flux flow and to test in detail the validity of theories for this regime beyond the Bardeen-Stephen approximation. Our observations are in good agreement with the mean-field result of time dependent Ginzburg-Landau theory. PACS: 74.25.Sv,74.25.Wx,74.25.Uv,74.25.Op,74.25.F- Keywords: vortices, fluxon, Larkin, Ovchinnikov, upper critical magnetic fieldComment: This is the final revised version of a paper that is currently in press. It is expected to appear in Phys. Rev. B in 201

Two charges on plane in a magnetic field: special trajectories

A classical mechanics of two Coulomb charges on a plane $(e_1, m_1)$ and $(e_2, m_2)$ subject to a constant magnetic field perpendicular to a plane is considered. Special "superintegrable" trajectories (circular and linear) for which the distance between charges remains unchanged are indicated as well as their respectful constants of motion. The number of the independent constants of motion for special trajectories is larger for generic ones. A classification of pairs of charges for which special trajectories occur is given. The special trajectories for three particular cases of two electrons, (electron - positron), (electron - $\alpha$-particle) are described explicitly.Comment: 22 pages, 5 figure

Localization and superconducting proximity effect in sandwiched potassium films

Thin films of alkali metals when sandwiched at both surfaces by thin metal films loose their conductance. The superconducting proximity effect is used to investigate the change in the alkali film. On the length scale of the film thickness the electronic properties of the alkali film do not change noticeably although its conductance is dramatically reduced, corresponding to localized electrons.Comment: 13 pages, 5 figure

Localization and critical diffusion of quantum dipoles in two dimensions

We discuss quantum propagation of dipole excitations in two dimensions. This problem differs from the conventional Anderson localization due to existence of long range hops. We found that the critical wavefunctions of the dipoles always exist which manifest themselves by a scale independent diffusion constant. If the system is T-invariant the states are critical for all values of the parameters. Otherwise, there can be a "metal-insulator" transition between this "ordinary" diffusion and the Levy-flights (the diffusion constant logarithmically increasing with the scale). These results follow from the two-loop analysis of the modified non-linear supermatrix $\sigma$-model.Comment: 4.2 page

Diffractive paths for weak localization in quantum billiards

We study the weak localization effect in quantum transport through a clean ballistic cavity with regular classical dynamics. We address the question which paths account for the suppression of conductance through a system where disorder and chaos are absent. By exploiting both quantum and semiclassical methods, we unambiguously identify paths that are diffractively backscattered into the cavity (when approaching the lead mouths from the cavity interior) to play a key role. Diffractive scattering couples transmitted and reflected paths and is thus essential to reproduce the weak-localization peak in reflection and the corresponding anti-peak in transmission. A comparison of semiclassical calculations featuring these diffractive paths yields good agreement with full quantum calculations and experimental data. Our theory provides system-specific predictions for the quantum regime of few open lead modes and can be expected to be relevant also for mixed as well as chaotic systems.Comment: 5 pages, 3 figures, final version with extended discussion and added reference

Vortex instability in molybdenum-germanium superconducting films

We studied the high driving force regime of the current-voltage transport response in the mixed state of amorphous molybdenum-germanium superconducting films to the point where the flux flow becomes unstable. The observed nonlinear response conforms with the classic Larkin-Ovchinikov (LO) picture with a quasiparticle-energy-relaxation rate dominated by the quasiparticle recombination process. The measured energy relaxation rate was found to have a magnitude and temperature dependence in agreement with theory. PACS: 74.40.Gh, 74.25.Uv, 72.15.Lh, 73.50.Gr, 73.50.Fq Keywords: fluxon, vortices, TDGL, FFF, negative differential conductivity, NDC, non-monotonic IV curveComment: 5 pages, 3 figure

Conductance of Aharonov--Bohm Rings: From the Discrete to the Continuous Spectrum Limit

The dissipative conductance of an array of mesoscopic rings, subject to an a.c. magnetic flux is investigated. The magneto--conductance may change sign between canonical and grand-canonical statistical ensembles, as function of the inelastic level broadening and as function of the temperature. Differences between canonical and grand-canonical ensembles persist up to temperature of the order of the Thouless energy.Comment: 13 pages, 2 figures, REVTeX v2.1, WIS--93/121/Dec.--P

Optical conductivity of a granular metal at not very low temperatures

We study the finite-temperature optical conductivity, sigma(omega,T), of a granular metal using a simple model consisting of a array of spherical metallic grains. It is necessary to include quantum tunneling and Coulomb blockade effects to obtain the correct temperature dependence of sigma(omega, T), and to consider polarization oscillations to obtain the correct frequency dependence. We have therefore generalized the Ambegaokar-Eckern-Schoen (AES) model for granular metals to obtain an effective field theory incorporating the polarization fluctuations of the individual metallic grains. In contrast to the DC conductivity, which is determined by inter-grain charge transfer and obeys an Arrhenius law at low temperature, the AC conductivity is dominated by a resonance peak for intra-grain polarization oscillations, which has a power-law tail at low frequencies. More importantly, although the resonance frequency agrees with the classical prediction, the resonance width depends on intergrain quantum tunneling and Coulomb blockade parameters, in addition to the classical Drude relaxation within the grain. This additional damping is due to inelastic cotunneling of polarization fluctuations to neighbouring grains and it qualitatively differs from the DC conductivity in its temperature dependence quite unlike the expectation from Drude theory.Comment: Added figures, published version, 16 pages, REVTe