Phase-breaking effects in superconducting heterostructures

Abstract

We present a theoretical analysis of a zero-temperature charge transport in a double-barrier structure formed by normal and superconducting electrodes with a partially dephasing mesoscopic region between two insulating layers. A scattering theory approach permits us to investigate a crossover from phase-coherent to sequential carrier transmission caused by inelastic phase-randomizing events. For a weakly transmitting junction, we derive a simple expression describing their effect on the superconducting tunneling density of states. For moderate-strength barriers, numerically simulated conductance-versus-voltage spectra exhibit a double-peaked structure in the case of s-wave superconductors and a dramatic reduction of a zero-bias maximum for d-wave pairing.Because of the gapped energy spectrum of a superconductor S, current I versus voltage V characteristics of mesoscopic devices formed by normal N, and S regions are strongly nonlinear and their measurement is one of the most high-resolution probes for analyzing quasiparticle spectra in S electrodes. 1 At the same time, the method is known to be an extremely interface-sensitive technique, with curves strongly governed by the nature of a transition region between N and S electrodes. Most of the theoretical results in this field have been obtained under the assumption of quantum-coherent transport. 2 What is less established is the effect of incoherent scattering events. After the work of Dynes et al., 3 it is usually considered by introducing a damping parameter ⌫ into the normalized quasiparticle density of states N T (). This paper is motivated by recent findings that cannot be described by the Dynes formula obtained from an entirely ad hoc procedure and valid only for an s-wave superconductor very close to its gap value ⌬ s . The experiments were carried out for contacts with doped copper and manganese perovskites, where a weak Cu-O or Mn-O bond oxygen easily outdiffuses from the surface reducing the oxygen stoichiometry near the intrinsic metal oxide surface. 4 As it was argued in Ref. 5, it should result in an enhancement of antiferromagnetic spin fluctuations. Strong inelastic scatterings of transferring carriers from excitations located in and/or near the insulating layer in a tunnel device not only modify the background characteristic and smear N T (), but also produce gradual changes of the gap features in conductance spectra. It follows, in particular, from our experiment for a cuprate LaBa 2 Cu 3 O 7Ϫx ͑Ref. 6͒ that was designed to directly address the issue of environment-induced decoherence. Another nonconventional finding is a double-peaked structure in the lead gap region in conductance spectra for contacts between a manganite and a superconducting Pb ͓the inset ͑a͒ in Fig. 5 of Ref. 7͔. As it will be clarified below, such anomalies can arise as an effect of a near-interface decohering mechanism on the carrier transmission across a superconductng heterojunction. The aim of this work is to present a theoretical analysis of the impact of inelastic scatt...

Resolving the order parameter of high-Tcsuperconductors through quantum pumping spectroscopy

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