Regensburg 2010 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 7: SC: Heterostructures, Andreev Scattering, Proximity Effect, Coexistence

TT 7.11: Talk

Monday, March 22, 2010, 17:00–17:15, H20

Andreev Bound States and Transport in a Kondo Quantum Dot with Superconducting Leads — •Tabea Mandt and Johann Kroha — Physikalisches Institut, Universität Bonn, Germany

Despite intense theoretical efforts the nature of Andreev bound states and the resulting Josephson current in a Kondo dot coupled to two symmetric s-wave superconductors has remained controversial. We investigate the temperature- and coupling-dependent spectral and transport properties of this system using an extension of the infinite-U non-crossing approximation (NCA) to describe superconducting leads. Using a conserving approximation (generating functional), we extend previous selfconsistent calculations [1] to include multiple Andreev-reflections by summing up the infinite series of ladder diagrams in the relevant anomalous channel (Cooper pair exchange) of the quantum dot Green’s function. The infinite number of Andreev reflections is necessary to describe true bound states and, hence, to answer the open problems about the number of bound states and, possibly, their spectral width. For the evaluation, the summation is efficiently formulated in terms of coupled Bethe-Salpeter equations for the renormalized pseudoparticle vertices, which can be decoupled due to a separation of energy scales. The phase diagram of the junction and the Josephson current–phase relation are established and related to the phase-dependent position of the subgap bound states.

[1] G. Sellier, Th. Kopp, J. Kroha, Y. S. Barash, Phys. Rev. B 72, 174502 (2005).