Surface-dominated finite size effects in nanoconfined superfluid helium

TL;DR

This study investigates how nanoconfinement in superfluid helium affects its superfluid density, revealing surface-bound excitations as a key factor in finite-size effects and scaling behavior.

Contribution

It demonstrates that surface-bound excitations cause suppression of superfluid density in nanoconfined helium, explaining previously unexplained finite-size scaling phenomena.

Findings

Superfluid density is suppressed in 25 and 50 nm channels.

Suppression is due to roton-like thermal excitations with a 5 K energy gap.

Surface-bound excitations explain the lack of finite-size scaling in suppression.

Abstract

Superfluid $^4$He (He-II) is a widely studied model system for exploring finite-size effects in strongly confined geometries. Here, we study He-II confined in mm-scale channels of 25 and 50 nm height at high pressures using a nanofluidic Helmholtz resonator. We find that the superfluid density is measurably suppressed in the confined geometry from the transition temperature down to 0.6 K. Importantly, this suppression can be accounted for by roton-like thermal excitation with an energy gap of 5 K. We show that the surface-bound excitations lead to the previously unexplained lack of finite-size scaling of suppression of the superfluid density.