Possible coexistence of Superfluidity and Spatial Order in Monolayers of Helium-4

The superfluidity coexisting with the spatial order is predicted in the proposed quantum liquid crystal (QLC) phase of the 2nd monolayer of ⁴He on graphite from the heat capacity (HC) [1] and the torsional oscillator (TO) studies [2]. So far, it is not clear if the QLC phase, which exists within a small density region (9%) between the liquid and solid phases, shows the superfluidity due to large uncertainties in determination of the sample density (r). Recently, we succeeded in eliminating this problem by carrying out simultaneous HC and TO measurement on the same samples. The results show unambiguously finite frequency (f) shifts due to superfluidity in the QLC phase at T < 0.3 K. Also found is that the coefficient of the background T-linear dependence of f (T) at T > 0.3 K starts to increase once the thermodynamic characteristic of the QLC phase appears in the HC measurement with increasing r. This can be attributed to stiffening of the ⁴He and graphite composite, being consistent with the partial spatial order in the QLC phase.

References
[1] S. Nakamura et al., PRB 94, 180501(R) (2016).
[2] P.A. Crowell and J.D. Reppy, PRL 70, 3291 (1993); PRB 53, 2701 (1996). Y. Shibayama et al., J. Phys.: Conf. Ser. 150, 032096 (2009). J. Nyéki et al., Nat. Phys. 13, 455 (2017).