Jeudi 14 mars 2013 à 10h15
Salle 234, Ecole de Physique

Transport properties through a quantum dot coupled to normal and superconducting leads

Akihisa Koga, Tokyo Institute of Technology, Japan

Recently electron transport through nanofabrications has attracted much interest. One of the simplest systems is a quantum dot with discrete energy levels, which gives us a stage to study fundamental quantum physics. The quantum dot system coupled to the normal and superconducting leads is one of the interesting systems to study how the Kondo screening and the Andreev reflections affect transport properties. In fact, the Kondo-enhanced Andreev transport has been discussed in the InAs quantum dot contacted with the normal and superconducting electrodes [1]. These nontrivial aspects related to the interplay between the Coulomb interactions and proximity-induced on-dot pairing have been discussed by various theoretical approaches. However, some analytical approaches introduce a sort of the nontrivial mean-field to discuss the nonequilibrium phenomena beyond the lower-order calculations and/or to satisfy the current conservation through the quantum dot, which may not be controllable in the strong coupling region. Therefore, the unbiased and robust method for nonequilibrium properties is desired to discuss the transient phenomena and steady state of quantum many-body systems. To this end, we make use of the continuous-time quantum Monte Carlo (CTQMC) method based on the Keldysh formalism [2]. Here we extend the CTQMC method in the continuous-time auxiliary field formulation [3] to treat the superconducting state in the Nambu formalism. By applying the method to the Anderson impurity model, we calculate the particle number, double occupancy, pairing correlations and currents. We then discuss the transient phenomena and steady state in the quantum dot system. [1] R. S. Deacon, et al., Phys. Rev. Lett. 104, 076805 (2010); Phys. Rev. B 81, 121308 (2010). [2] P. Werner, T. Oka, and A. J. Millis, Phys. Rev. B 79, 035320 (2009). [3] E. Gull, P. Werner, O. Parcollet, and M. Troyer, Europhys. Lett. 82, 57003 (2008).