Majorana flat bands, chiral Majorana edge states, and unidirectional Majorana edge states in noncentrosymmetric superconductors

We study two-dimensional noncentrosymmetric nodal superconductors under Zeeman field and clarify the field-angle dependence of topological properties. It has been shown that the nodal excitation acquires an excitation gap due to the Zeeman field perpendicular to antisymmetric spin-orbit coupling, and then gapful topological superconductivity is realized [A. Daido and Y. Yanase, Phys. Rev. B 94, 054519 (2016)]. We show that the system undergoes a gapful-gapless transition against tilting of the field. The gapless phase remains to show a finite band gap and unusual Majorana edge states in between the bulk bands. The Majorana edge states naturally propagate in a same direction between oppositely oriented edges. We elucidate relations of such unidirectional Majorana edge states with chiral edge states in the gapful topological superconducting phase and previously studied Majorana flat bands at zero Zeeman field. A compact formula of topological invariants characterizing the edge states is given. The gapful-gapless topological phase transition and associated evolution of Majorana states are demonstrated in a model for D + p-wave superconductivity. Experimental realization in recently fabricated cuprate heterostructures and heavy-fermion thin films is discussed.