Spin texture of topological superconductor surface states

Noncentrosymmetric superconductors (NCS), where singlet and triplet pairing coexist, are examples of topologically non-trivial gapless systems, and hence display nondegenerate flat-band surface states at certain surfaces [1]. Using quasiclassical methods, we construct the edge-state wavefunctions and calculate the edge-state spectra [2]. We show that the edge states, in particular the flat bands, generically show strong spin polarization which is odd in the surface momentum [3]. The spin polarization is mostly fixed by the spin-orbit coupling, but also depends upon the relative strength of singlet to triplet pairing. Not only does the spin polarization suppress the effect of impurity scattering, but it also is responsible for strong edge currents at interfaces with ferromagnets [3]. [1] A. P. Schnyder and S. Ryu, Phys. Rev. B {\bf 84}, 060504(R) (2011). [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). [3] A. P. Schnyder, C. Timm, and P. M. R. Brydon, Phys. Rev. Lett. {\bf 111}, 077001 (2013).