On the ‘Supersolid’ Response of the Second Layer of $$^4 \hbox {He}$$ 4 He on Graphite

Abstract

Recent torsional oscillator measurements on the second layer of 4 He adsorbed on graphite have identified an anomalous superfluid response over a coverage range near third-layer promotion, with four distinct coverage regimes. Here, we present details of the superfluid response in the coverage regime immediately below third-layer promotion. A scaling analysis of the inferred superfluid fraction shows the characteristic temperature governing the superfluid response to decrease, approaching zero near the coverage at which simulations predict the second layer to form a conventional incommensurate solid.

Atomically Layered Helium Films at Ultralow Temperatures: Model Systems for Realizing Quantum Materials

“…These results support the conclusion that the novel superfluid responses reported in [103,104] occur in a 2D solid phase. The following further work is desirable: to check the frequency independence of the supersolid response; to detect the response on a higher quality substrate; to understand the collective mode spectrum of the putative intertwined state; to seek an underlying microscopic theory which gives rise to this state.…”

“…Evidence for an emergent two-dimensional supersolid in the second layer of 4 He on graphite is reported in [103,104]. This work was motivated by the detection in earlier torsional oscillator experiments on this system, which found an anomalous mass decoupling over a narrow coverage range [24].…”

“…In that work, the destruction of superfluidity with increasing coverage was attributed to solidification of the film. The recent torsional oscillator study [103,104] was made over a fine grid of coverages down to temperatures approaching 1 mK. The results led to the proposal of a state of intertwined density wave and superfluid order.…”

Manifestations of Local Supersolidity of $^{4}$He around a Charged Molecular Impurity

“…Pioneering work on the superfuid response of 4 He around a C 20 fullerene has even hinted at something like nanoscale supersolidity, but only for specific helium numbers between 28 and 31 atoms, close to completion of the first solvation shell, whereas the phenomenon quickly vanishes when adding more 4 He atoms [38]. Similar effects have also been seen around the completion of the second layer of helium adsorbed on graphite [39][40][41].…”

Effect of zero-point motion on properties of quantum particles adsorbed on a substrate

“…There have been examinations e.g. in terms of neutron scattering [7][8][9][10], torsional oscillator [5,11,12], as well as heat capacity [13][14][15][16] measurements. There were adsorption studies [17] also on alkali substrates [18,19].…”