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

Colin Benjamin (1) , Thibaut Jonckheere (1) , Alex Zazunov (2) , Thierry Martin (1)

1 CPT - Centre de Physique Théorique - UMR 6207
2 NEEL - ThQC - Théorie Quantique des Circuits

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. We show that the mechanism leading to the $\pi$ 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.

Mots clés: molecular; spintronics; Josephson; supraconductivity; quantum dot; pi junction

Domaines: Supraconductivité [cond-mat.supr-con]; Systèmes mésoscopiques et effet Hall quantique [cond-mat.mes-hall]

Colin Benjamin, Thibaut Jonckheere, Alex Zazunov, Thierry Martin. Controllable pi junction in a Josephson quantum-dot device with molecular spin.. The European Physical Journal B: Condensed Matter and Complex Systems, 2007, 57, pp.279-289. ⟨10.1140/epjb/e2007-00167-6⟩. ⟨hal-00068625v4⟩