Competitive spin-flipped normal reflection and equal-spin Andreev reflection in heterojunctions of nodal-line superconductors

The superconducting phase of topological semimetals has become a promising route for the implementation of topological superconductivity and non-Abelian Majorana fermions. Here, we investigate quantum transport in the junctions composed of a nodal-line superconductor, i.e., a topological nodal-line semimetal with superconducting pairing. It is shown that two topologically nontrivial regions exist in the surface Brillouin zone of the nodal-line superconductor labeled by the transverse momentum. Specifically, topological regions with topological invariants $\mathcal{N}=1$ and $\mathcal{N}=2$ host a single and a pair of Majorana zero modes at the surface, respectively. We show that the single Majorana zero mode in the $\mathcal{N}=1$ region can induce resonant equal-spin Andreev reflection, while the Majorana pair in the $\mathcal{N}=2$ region can lead to resonant spin-flipped normal reflection. Both reflection processes can give rise to spin currents but with different polarization directions, whose proportions can be adjusted by a Zeeman field. We also study the transport properties of the device with a sandwich structure and predict that strong crossed Andreev reflection can be implemented with a finite bias. Detection schemes for all these novel transport properties, which can be revealed by spin-resolved transport signatures, are proposed. Our work paves the way for the implementation and detection of Majorana fermions in nodal-line superconductors.