Interface currents in topological superconductor-ferromagnet junctions

Both fully gapped and nodal pairing states of noncentrosymmetric superconductors (NCS) display non-trivial topological properties, manifested by topologically protected dispersing and flat-band surface states [1,2]. Using a 2D model of an NCS, we show that the surface states typically have strong spin-polarization $s_{\mu=x,z}(k_y)$, which is odd in the surface-Brillouin-zone momentum $k_y$. Upon placing the NCS in proximity contact with a ferromagnet, the coupling to the exchange field gives a perturbative correction to the energy of these states $\propto s_\mu(k_y)$, thus generating an interface charge current $\propto \partial_{k_y}s_\mu(k_y)$ in the NCS. This is most clearly realized in a nodal NCS, where the weak dispersion acquired by the singly degenerate zero-energy flat bands leads to a strong enhancement of the interface current at low temperatures. We argue that this effect is a ``smoking-gun'' signature of the singly degenerate flat bands.\\[4pt] [1] A. P. Schnyder and S. Ryu, Phys. Rev. B {\bf 84}, 060504(R) (2011).\\[0pt] [2] P. M. R. Brydon, A. P. Schnyder, and C. Timm, Phys. Rev. B {\bf 84}, 020501(R) (2011); A. P. Schnyder, P. M. R. Brydon, and C. Timm, Phys. Rev. B {\bf 85}, 024522 (2012).