Abstract
We propose the existence of a substantial charge current parallel to the interface between a noncentrosymmetric superconductor and a metallic ferromagnet. Our analysis focuses upon two complementary orbital-angular-momentum pairing states of the superconductor, exemplifying topologically nontrivial states which are gapped and gapless in the bulk, respectively. Utilizing a quasiclassical scattering theory, we derive an expression for the interface current in terms of Andreev reflection coefficients. Performing a systematic study of the current, we find stark qualitative differences between the gapped and gapless superconductors, which reflect the very different underlying topological properties. For the fully gapped superconductor, there is a sharp drop in the zero-temperature current as the system is tuned from a topologically nontrivial to a trivial phase. We explain this in terms of the sudden disappearance of the contribution to the current from the subgap edge states at the topological transition. The current in the gapless superconductor is characterized by a dramatic enhancement at low temperatures, and exhibits a singular dependence on the exchange-field strength in the ferromagnetic metal at zero temperature. This is caused by the energy shift of the strongly spin-polarized nondegenerate zero-energy flat bands due to their coupling to the exchange field. We argue that the interface current provides a novel test of the topology of the superconductor, and discuss prospects for the experimental verification of our predictions.
Highlights
The discovery that gapped single-particle Hamiltonians can have a nontrivial topology, depending on their dimensionality and the presence of time-reversal and particlehole symmetries [1, 2, 3, 4, 5], has sparked a massive search for topological materials
Increasing the strength of the singlet pairing leads to a sign change of the negative-helicity gap, which marks the transition into a state with trivial topology [27]
We find that an interface current appears in the noncentrosymmetric superconductors (NCS) for a FM with magnetization components along the x- or z-axes, and reverses direction with the magnetization
Summary
The discovery that gapped single-particle Hamiltonians can have a nontrivial topology, depending on their dimensionality and the presence of time-reversal and particlehole symmetries [1, 2, 3, 4, 5], has sparked a massive search for topological materials. In two dimensions, gapped DIII systems may be topologically nontrivial and possess a nonzero Z2 topological number An example of such a state is given by the NCS with Rashba spin-orbit coupling, s-wave form factor of the gap, and majority-triplet pairing [25, 26, 27, 28]. The absence of the flat bands for an NCS with s-wave form factor leads to a very weak edge current due to the interplay between the spin-orbit coupling and the spin polarization induced by the exchange field. Note that applying an exchange field to the entire NCS leads to an energy shift of the edge states [13, 31, 38] but distorts the Fermi surfaces in a way which is inconsistent with a zero-momentum pairing state [39, 40].
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