Critical Velocity in the Presence of Surface Bound States in Superfluid $^3$He-B

TL;DR

This study investigates the nonlinear damping behavior of a MEMS oscillator in superfluid $^3$He-B, revealing a critical velocity lower than the Landau velocity, likely due to quasiparticle escape from surface bound states.

Contribution

It demonstrates the influence of surface bound states on critical velocity and nonlinear damping in superfluid $^3$He-B, providing new insights into quasiparticle dynamics.

Findings

Damping increases rapidly above a threshold velocity.

Critical velocity is much lower than the Landau critical velocity.

Nonlinear behavior attributed to quasiparticle escape from surface states.

Abstract

A microelectromechanical oscillator with a gap of 1.25 $\mu$m was immersed in superfluid $^3$He-B and cooled below 250 $\mu$K at various pressures. Mechanical resonances of its shear motion were measured at various levels of driving force. The oscillator enters into a nonlinear regime above a certain threshold velocity. The damping increases rapidly in the nonlinear region and eventually prevents the velocity of the oscillator from increasing beyond the critical velocity which is much lower than the Landau critical velocity. We propose that this peculiar nonlinear behavior stems from the escape of quasiparticles from the surface bound states into the bulk fluid.