Bosonic Surface States and Acoustic Spectroscopy of Confined Superfluid ${}^3$He-B

Abstract

Using an effective field theory we study the low-lying bosonic excitations and their couplings to phonons at ultrasonic frequencies in superfluid ${}^3$He-B under strong confinement. We show that confinement induces a rich spectrum of low-lying bosons, including surface-bound bosonic states, as well as fine structure of long-lived massive bosons. Under sufficiently strong confinement we find a dynamical instability of the ${}^3$He-B film: the frequency of the surface-bound boson softens at finite wavevector, then develops a pole in the upper half of the complex frequency plane, signalling a dynamical instability of the translationally invariant superfluid vacuum towards pair-density-wave "crystallization". We discuss the signatures and observability of the low-lying bosonic spectrum based on analysis of the ultrasound attenuation from resonant excitation of the bosonic modes. We also note that surface-bound bosonic modes are not unique to ${}^3$He-B, but are expected to be common to unconventional superconductors with a multi-component order parameter.

5 pages, 5 figures

References in corpus (6)

• Symmetry Protected Topological Order and Spin Susceptibility in Superfluid 3He-B
• Crystalline order in superfluid 3He films
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Cited by in corpus (2)

• Realizing Quantum Materials with Helium: Helium films at ultralow temperatures, from strongly correlated atomically layered films to topological superfluidity
• Triangular pair density wave in confined superfluid ${}^3$He