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SFS Josephson pi-junctions

Superconductor-Ferromagent-Superconductor Josephson junctions may have a phase shift of pi in the ground state due to the oscillations of the order parameter in the ferromagnetic layer.

In collaboration with Valery Ryazanov's group of ISSP, Russia, we perform phase-sensitive experiments that allow direct observation of a pi-phase shift in an rf-SQUID containing an SFS Josephson junction as a function of beta_L and temperature.

We fabricate Nb-CuNi-Nb junctions, with CuNi as a weak ferromagnet. Such structures exhibit a transition into pi state with temperature. We are interested in 0- to pi-crossover region, where coexistence of the two phases and secondary Josephson harmonics were predicted.

We are investigating the potential that SFS pi-junctions have as the components of the superconducting qubit. In the superconducting qubit, a phase shift of pi is necessary to produce a degenerate double-well potential. We are fabricating pi junctions with higher than currently available Ic and R. Low resistance junctions introduce dissipation into the quantum circuit causing decoherence.

Papers:

S. M. Frolov, D. J. Van Harlingen, V. A. Oboznov, V. V. Bolginov, and V. V. Ryazanov Measurement of the current-phase relation of superconductor/ferromagnet/superconductor pi Josephson junctions Phys. Rev. B 70, 144505 (2004)