Andreev-Majorana bound states in superfluids

We consider Andreev-Majorana (AM) bound states with zero energy on surfaces, interfaces and vortices in different phases of the $p$-wave superfluids. We discuss the chiral superfluid $^3$He-A, and time reversal invariant phases: superfluid $^3$He-B, planar and polar phases. The AM zero modes are determined by topology in bulk, and they disappear at the quantum phase transition from the topological to non-topological state of the superfluid. The topology demonstrates the interplay of dimensions. In particular, the zero-dimensional Weyl points in chiral superfluids (the Berry phase monopoles in momentum space) give rise to the one-dimensional Fermi arc of AM bound states on the surface and to the one-dimensional flat band of AM modes in the vortex core. The one-dimensional nodal line in the polar phase produces the two-dimensional flat band of AM modes on the surface. The interplay of dimensions also connects the AM states in superfluids with different dimensions. For example, the topological properties of the spectrum of bound states in the three-dimensional $^3$He-B is connected to the properties of the spectrum in the two-dimensional planar phase (thin film).

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