Spatially Modulated Superfluid State in Two-Dimensional 4He Films

The second layer of He-4 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 He-4 atomic density. For the first time, we show experimentally 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.

Automated summary

The study on the second layer of He-4 films reveals that the superfluid density is independent of frequency, providing unequivocal evidence of genuine superfluidity. The phase diagram established in this work shows the coexistence of superfluid phase, 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.