Fragility of surface states in topological superfluid 3He

P J Heikkinen,J M Parpia,P Sharma,A Vorontsov,A Casey,N Zhelev,J Saunders,L V Levitin,X Rojas

doi:10.1038/s41467-021-21831-y

Abstract

Superfluid 3He, with unconventional spin-triplet p-wave pairing, provides a model system for topological superconductors, which have attracted significant interest through potential applications in topologically protected quantum computing. In topological insulators and quantum Hall systems, the surface/edge states, arising from bulk-surface correspondence and the momentum space topology of the band structure, are robust. Here we demonstrate that in topological superfluids and superconductors the surface Andreev bound states, which depend on the momentum space topology of the emergent order parameter, are fragile with respect to the details of surface scattering. We confine superfluid 3He within a cavity of height D comparable to the Cooper pair diameter ξ0. We precisely determine the superfluid transition temperature Tc and the suppression of the superfluid energy gap, for different scattering conditions tuned in situ, and compare to the predictions of quasiclassical theory. We discover that surface magnetic scattering leads to unexpectedly large suppression of Tc, corresponding to an increased density of low energy bound states.

Highlights

Superfluid 3He, with unconventional spin-triplet p-wave pairing, provides a model system for topological superconductors, which have attracted significant interest through potential applications in topologically protected quantum computing
Majorana zero modes (MZMs) may be corrupted by non-topological low energy Andreev bound states (ABS)[22], which can be present as a result of details of interfaces and materials properties in such systems[23]
We have shown that it is possible to cool 3He confined within precisely engineered nanoscale cavities into the superfluid phases[24], and detect the nuclear magnetic resonance (NMR) response of the small 3He sample using an ultra-sensitive spectrometer[25]

Summary

Introduction

Superfluid 3He, with unconventional spin-triplet p-wave pairing, provides a model system for topological superconductors, which have attracted significant interest through potential applications in topologically protected quantum computing. In early work it was shown that both the NMR frequency shift of superfluid 3He confined in a stack of mylar sheets[36] and the superfluid fraction within the pores of packed powders[37] depend on the surface 4He coverage They both increased with increased coverage, indicating the expected reduced pair breaking with higher specularity. Transverse acoustic impedance measurements, and quasiclassical analysis, revealed in superfluid 3He-B the energy density of states of midgap surface-bound excitations depends on specularity[38,39,40,41]. Their spectrum shows a Majorana-like cone as the conditions for fully specular scattering are approached

Methods

Results

Conclusion