Surface Andreev bound states of superfluid3He and Majorana fermions

What is it about?

This is a review of the surface bound states of superfluid 3He. Superfluid 3He provides a unique opportunity to investigate the topological surface states of the established spin-triplet p-wave superfluid. Superfluid 3He B-phase is categorized as a time-reversal invariant topological superfluid. Surface bound states should form due to the bulk-edge correspondence and be topologically protected. The bound states are predicted to be Majorana fermions satisfying the equivalence of the particle and anti-particle. Our transverse acoustic impedance (Z) measurements by changing the boundary condition of a wall from a diffusive scattering up to practically specular limit revealed the systematic dependence of the bound states on the surface boundary condition or the surface disorder. Usually, particles are localized by the disorder and gap appears at fermi energy. However, the topological surface states are not gappable in the presence of disorder. Our measurement of Z indicated no gap opening at fermi energy even under disorder, supporting the topological stability of the surface states. We also confirmed a peculiar sub-gap structure which is thought to be a reflection of anomalous scattering of Majorana fermions off the disorder. A growth of low-energy peak in Z was observed on a higher specularity wall, which is the clear indication of the surface linear dispersion called Majorana cone on a clean surface.

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