Surface Majorana fermions and bulk collective modes in superfluid 3He-B

TL;DR

This paper investigates how bulk quantum fluctuations in superfluid 3He-B influence topologically protected Majorana surface states, revealing a coupling to spin-orbit Goldstone modes that may induce a phase transition.

Contribution

It demonstrates the coupling between bulk Goldstone modes and surface Majorana fermions, leading to an effective interaction and potential quantum phase transition in superfluid 3He-B.

Findings

Bulk Goldstone mode couples to Majorana surface states

Induces an effective short-range interaction among Majorana fermions

Possible quantum phase transition to a gapped phase near the surface

Abstract

The theoretical study of topological superfluids and superconductors has so far been carried out largely as a translation of the theory of noninteracting topological insulators into the superfluid language, whereby one replaces electrons by Bogoliubov quasiparticles and single-particle band Hamiltonians by Bogoliubov-de Gennes Hamiltonians. Band insulators and superfluids are, however, fundamentally different: while the former exist in the absence of inter-particle interactions, the latter are broken symmetry states that owe their very existence to such interactions. In particular, unlike the static energy gap of a band insulator, the gap in a superfluid is due to a dynamical order parameter that is subject to both thermal and quantum fluctuations. In this work, we explore the consequences of bulk quantum fluctuations of the order parameter in the $B$ phase of superfluid $^3$He on the topologically protected Majorana surface states. Neglecting the high-energy amplitude modes, we find that one of the three spin-orbit Goldstone modes in $^3$He-$B$ couples to the surface Majorana fermions. This coupling in turn induces an effective short-range two-body interaction between the Majorana fermions, with coupling constant inversely proportional to the strength of the nuclear dipole-dipole interaction in bulk $^3$He. A mean-field theory estimate of the value of this coupling suggests that the surface Majorana fermions in $^3$He-$B$ are in the vicinity of a quantum phase transition to a gapped time-reversal symmetry breaking phase.