M.O.J. Heikkinen, F. Massel, J. Kajala, M.J. Leskinen, G.-S. Paraoanu, P. Törmä

Aalto University, Helsinki, Finland

We predict a spin-asymmetric Josephson effect in which the two spin components of a Cooper pair are driven asymmetrically --- corresponding to driving a Josephson junction of two superconductors with different voltages for spin up and down electrons, respectively. We predict that the spin up and down components oscillate at the same frequency but with different amplitudes. Our results reveal that the standard description of the Josephson supercurrent in terms of coherent bosonic pair tunneling is insufficient. We provide an intuitive interpretation of the Josephson supercurrent as interference in Rabi oscillations of pairs and single particles, the latter causing the asymmetry. [1]

We also calculate the speed of sound for the FFLO state. We employ a single band Hubbard model for a population imbalanced two-component Fermi gas in an optical lattice. We derive the density response of the system using a generalised random phase approximation (GRPA). The speed of sound is found to be unisotropic. [2].

[1] M.O.J. Heikkinen, F. Massel, J. Kajala, M.J. Leskinen, G.-S. Paraoanu, and P. Törmä, Spin-asymmetric Josephson effect, submitted to Phys. Rev. Lett. arXiv:0911.4678.

[2] M.O.J. Heikkinen and P. Törmä, in preparation.