Spatially Modulated Superfluid State in Two-Dimensional $^4$He Films

TL;DR

This study investigates the second layer of $^4$He films on graphite, revealing a superfluid phase that coexists with structural orders, indicating potential for exotic quantum states like spatially modulated superfluid and supersolid phases.

Contribution

It provides experimental evidence of genuine superfluidity in $^4$He films and maps the phase diagram showing coexistence with structural orders, highlighting the second layer as a candidate for exotic quantum states.

Findings

Superfluid density is frequency-independent, confirming genuine superfluidity.

Coexistence of superfluidity with hexatic density-wave order.

Identification of a phase with both superfluid and solid characteristics.

Abstract

The second layer of $^4$He films adsorbed on a graphite substrate is an excellent experimental platform to study the interplay between superfluid and structural orders. Here, we report a rigid two-frequency torsional oscillator study on the second layer as a function of temperature and $^4$He atomic density. We find that the superfluid density is independent of frequency, which can be interpreted as unequivocal evidence of genuine superfluidity. The phase diagram established in this work reveals that a superfluid phase coexists with hexatic density-wave correlation and a registered solid phase. This suggests the second layer as a candidate for hosting two exotic quantum ground states: the spatially modulated superfluid and supersolid phases, resulting from the interplay between superfluid and structural orders.