Possible quantum liquid crystal phases of helium monolayers

TL;DR

This paper investigates helium monolayers on graphite, proposing the existence of quantum liquid crystal phases like quantum hexatic, and discusses potential supersolid or superhexatic transitions based on specific heat anomalies.

Contribution

It introduces the possibility of quantum liquid crystal phases in helium monolayers, a novel state of matter, supported by specific heat anomaly analysis and phase diagram investigation.

Findings

Observation of specific heat anomalies at 1.4 K and 0.9 K.

Identification of anomalies as 2D melting transitions.

Proposal of quantum hexatic or commensurate phases as low-temperature states.

Abstract

The second-layer phase diagrams of $^4$He and $^3$He adsorbed on graphite are investigated. Intrinsically rounded specific-heat anomalies are observed at 1.4 and 0.9 K, respectively, over extended density regions in between the liquid and incommensurate solid phases. They are identified to anomalies associated with the Kosterlitz-Thouless-Halperin-Nelson-Young type two-dimensional melting. The prospected low temperature phase (C2 phase) is a commensurate phase or a $\textit{quantum hexatic}$ phase with quasi-bond-orientational order, both containing $\textit{zero}$-$\textit{point}$ defectons. In either case, this would be the first atomic realization of the $\textit{quantum liquid crystal}$, a new state of matter. From the large enhancement of the melting temperature over $^3$He, we propose to assign the observed anomaly of $^4$He-C2 phase at 1.4 K to the hypothetical supersolid or superhexatic transition.