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Register a new userEffects of the phase coherence on the local density of states in superconducting proximity structures
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We theoretically study the local density of states in superconducting proximity structure where two superconducting terminals are attached to a side surface of a normal-metal wire. Using the quasiclassical Greens function method, the energy spectrum is obtained for both of spin-singlet $s$-wave and spin-triplet $p$-wave junctions. In both of the cases, the decay length of the proximity effect at the zero temperature is limited by a depairing effect due to inelastic scatterings. In addition to the depairing effect, in $p$-wave junctions, the decay length depends sensitively on the transparency at the junction interfaces, which is a unique property to odd-parity superconductors where the anomalous proximity effect occurs.
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Attaching a superconductor in good contact with a normal metal makes rise to a proximity effect where the superconducting correlations leak into the normal metal. An additional contact close to the first one makes it possible to carry a supercurrent through the metal. Forcing this supercurrent flow along with an additional quasiparticle current from one or many normal-metal reservoirs makes rise to many interesting effects. The supercurrent can be used to tune the local energy distribution function of the electrons. This mechanism also leads to finite thermoelectric effects even in the presence of electron-hole symmetry. Here we review these effects and discuss to which extent the existing observations of thermoelectric effects in metallic samples can be explained through the use of the dirty-limit quasiclassical theory.
Oxygen NMR is used to probe the local influence of nonmagnetic Zn and magnetic Ni impurities in the superconducting state of optimally doped high Tc YBa2Cu3O7. Zn and Ni induce a staggered paramagnetic polarization, similar to that evidenced above Tc, with a typical extension xi=3 cell units for Zn and xi>=3 for Ni. In addition, Zn is observed to induce a local density of states near the Fermi Energy in its neighbourhood, which also decays over about 3 cell units. Its magnitude decreases sharply with increasing temperature. This allows direct comparison with the STM observations done in BiSCO.
We investigate the electron-phonon cooling power in disordered electronic systems with a special focus on mesoscopic superconducting proximity structures. Employing the quasiclassical Keldysh Greens function method, we obtain a general expression for the cooling power perturbative in the electron-phonon coupling, but valid for arbitrary electronic systems out of equilibrium. We apply our theory to several disordered electronic systems valid for an arbitrary relation between the thermal phonon wavelength and the electronic mean free path due to impurity scattering. Besides recovering the known results for bulk normal metals and BCS superconductors, we consider two experimentally relevant geometries of superconductor-normal metal proximity contacts. Both structures feature a significantly suppressed cooling power at low temperatures related to the existence of a minigap in the quasiparticle spectrum. This improved isolation low cooling feature in combination with the high tunability makes such structures highly promising candidates for quantum calorimetry
High temperature superconductors with a Tc above 40 K have been found to be strongly correlated electron systems and to have a layered structure. Guided by these rules, Kamihara et al. discovered a Tc up to 26 K in the layered La(O1-xFx)FeAs. By replacing La with tri-valence rare-earth elements RE of smaller ionic radii, Tc has subsequently been raised to 41-52 K. Many theoretical models have been proposed emphasizing the important magnetic origin of superconductivity in this compound system and a possible further Tc-enhancement in RE(O1-xFx)FeAs by compression. This later prediction appears to be supported by the pressure-induced Tc-increase in La(O0.89F0.11)FeAs observed. Here we show that, in contrast to previous expectations, pressure can either suppress or enhance Tc, depending on the doping level, suggesting that a Tc exceeding 50s K may be found only in the yet-to-be discovered compound systems related to but different from R(O1-xFx)FeAs and that the Tc of La(O1-xFx)FeAs and Sm(O1-xFx)FeAs may be further raised to 50s K.
I investigate superconducting states in a quasi-2D Holstein model using the dynamical cluster approximation (DCA). The effects of spatial fluctuations (non-local corrections) are examined and approximations neglecting and incorporating lowest-order vertex corrections are computed. The approximation is expected to be valid for electron-phonon couplings of less than the bandwidth. The phase diagram and superconducting order parameter are calculated. Effects which can only be attributed to theories beyond Migdal--Eliashberg theory are present. In particular, the order parameter shows momentum dependence on the Fermi-surface with a modulated form and s-wave order is suppressed at half-filling. The results are discussed in relation to Hohenbergs theorem and the BCS approximation.
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