Spin-Dependent Acoustic Response in the Nonunitary A 1 and A 2 Phases of Superfluid He 3 under High Magnetic Fields

What is it about?

Under high magnetic field, superfluid helium-three A1 phase and A2 phase which are unconventional and non-unitary superfluid appear and those phases show almost same behavior without the transition temperature. However, we observed a spin dependence transverse acoustic impedance between A1 phase and A2 phase. The transverse acoustic impedance is a highly sensitive tool of the surface states of superfluid. The surface states of unconventional superconductor and superfluid attract many attention such as Andreev bound states, topological superfluid, odd-frequency pairing superfluid and so on. In this paper, an anomalously difference of the temperature dependence of the impedance was observed between the A1-phase and the A2-phase. This non-symmetric response implies that the exchange of transverse momentum between the surface states and a wall is dependent on the spin states. Such a spin-dependent process has never been observed and is possibly a unique feature in non-unitary superfluids.

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Why is it important?

Depending on the characters of condensates and the surface boundary conditions, various kinds of surface states form near the surface; some of them are topological and some of them are non-topological. Since the surface states draw increasing attention recently, there exists a strong demand to classify them and unveil their physical properties. Surface states of A1- and A2-phases on diffusive surfaces, however, has never been studied due to the difficulties in achieving the multi-extreme conditions. While non-unitary phases have been proposed in several heavy fermion superconductors, their surface states have neither been investigated in detail suffered from uncontrollable surface disorders and impurities. Superfluid A1- and A2-phases are rare examples of unambiguously established non-unitary states and good test grounds for studying the surface states of non-unitary states.