Controllable pi junction in a Josephson quantum-dot device with molecular spin.
Résumé
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 $\pi$ junction behavior at low temperature. The $\pi$ 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 $\pi$ junction with novel application as a Josephson current switch. In contrast to previous works the importance of the contribution from the continuum of states above the superconducting gap is brought out. This fact along with the novel result that for a large spin (ferromagnetic coupling regime) one has complete absence of Andreev bound state current could lead to an important application in controlling heat transport or thermoelectric refrigeration. The free energy for certain configuration of parameters shows a bistable nature, which is a necessary pre-condition for achievement of a qubit.