Commensurate-Incommensurate Transition in Submonolayer $^3$He on Graphite

TL;DR

This study investigates the phase transition of submonolayer $^3$He on graphite, revealing striped domain-wall phases and a quantum nematic state through high-precision heat capacity measurements.

Contribution

It provides new experimental evidence of striped domain-wall phases and the quantum nematic state in $^3$He monolayers on graphite, elucidating the nature of the commensurate-incommensurate transition.

Findings

Identification of two striped domain-wall phases, $oldsymbol{ ext{α}_1}$ and $oldsymbol{ ext{α}_2}$.

Observation of a second-order transition from $oldsymbol{ ext{α}_2}$ to $oldsymbol{ ext{α}_1}$.

Detection of T-linear heat capacity indicating one-dimensional phonons in $oldsymbol{ ext{α}_1}$.

Abstract

We report high-precision heat-capacity measurements of submonolayer $^3$He adsorbed on highly crystalline graphite, revealing new aspects of the commensurate$-$incommensurate transition. Below 1\~K, two possible striped domain-wall phases emerge: $\alpha_1$ with variable wall spacing and $\alpha_2$ with fixed spacing. The $T$-linear heat capacity in $\alpha_1$ arises from one-dimensional phonons along the walls. $\alpha_2$ melts into $\alpha_1$ at a critical density via a second-order transition, consistent with a quantum nematic (quantum liquid-crystal) state in $\alpha_1$, and reconciling thermodynamic and prior nuclear-magnetic data.