Elastic Anomaly of Helium Films at a Quantum Phase Transition

TL;DR

This paper investigates elastic anomalies in helium films near a quantum phase transition, revealing how their elastic properties change with coverage and temperature, and providing insights into their quantum ground states.

Contribution

It is the first to determine the energy band structure of helium films from elastic measurements and links elastic anomalies to quantum phase transitions.

Findings

Helium films stiffen under AC strain at low temperatures.

The onset temperature of stiffening approaches 0 K near critical coverage.

Ground states are gapped and possibly Mott insulators or Mott glasses.

Abstract

Helium films show various quantum phases that undergo quantum phase transitions by changing coverage n. We found anomalous elastic phenomena in bosonic 4He and fermionic 3He films adsorbed on a glass substrate. The films stiffen under AC strain at low temperature with an excess dissipation. The onset temperature of the stiffening decreases to 0 K as n approaches a critical coverage nc. The elastic anomaly is explained by thermal activation of helium atoms from the localized to extended states with a distributed energy gap. We determine for the first time the energy band structure of helium films from elasticity. The ground states of 4He and 3He at n < n_c are identically gapped and compressible, which are possibly a sort of Mott insulator or Mott glass.