Eur. Phys. J. B 57, 279-289 (2007)
https://doi.org/10.1140/epjb/e2007-00167-6

Controllable π junction in a Josephson quantum-dot device with molecular spin

¹ Centre de Physique Théorique (Centre de Physique Théorique is UMR 6207 du CNRS, associated with Université de la Méditérannée, Université de Provence, and Université de Toulon), Case 907, Luminy, 13288 Marseille Cedex 9, France
² LEPES, 25 Avenue des Martyrs, 38000 Grenoble, France
³ Université de la Méditérannée, 13288 Marseille Cedex 9, France

Corresponding authors: ^a cbiop@yahoo.com - ^b jonckhee@cpt.univ-mrs.fr

Received: 20 February 2007
Published online: 13 June 2007

Abstract

We consider a model for a single molecule with a large frozen spin sandwiched in between two BCS superconductors at equilibrium, and show that this system has a π junction behavior at low temperature. The π shift can be reversed by varying the other parameters of the system, e.g., temperature or the position of the quantum dot level, implying a controllable π junction with novel application as a Josephson current switch. We show that the mechanism leading to the π shift can be explained simply in terms of the contributions of the Andreev bound states and of the continuum of states above the superconducting gap. The free energy for certain configuration of parameters shows a bistable nature, which is a necessary pre-condition for achievement of a qubit.