Observation of Superfluidity in 2^nd Layer Helium-4 Films adsorbed on Grafoil Investigated by a Double-resonant Frequency Torsional Oscillator

Helium film adsorbed on graphite substrates provides rich phase diagram due to the interplay of interactions between helium atoms and substrate. A torsional oscillator experiment of Crowell and Reppy [1] reported first reentrant superfluidity in the second layer of ⁴He films adsorbed on Grafoam., which was reproduced recently [2, 3]. Both experiments suggest the existence of a possible 2D supersolid phase, although an ab initio Monte Carlo simulation disagree with the interpretation [4]. Here, we investigated superfluidity and elasticity dependent response in the second layer of ⁴He film adsorbed on Grafoil with a TO with two different resonant frequency. The measurements were performed in the atomic coverages ranging from 17 to 20 atoms/nm². We found no frequency dependent TO responses that can be attributed to the rigidity change. Instead, frequency independent response appeared with a dissipation peak, demonstrating the evidence of superfludity in the 2^nd layer helium films on Grafoil.

References
[1] P. A. Crowell and J. D. Reppy, Phys. Rev. B 53, 2701 (1996).
[2] Y. Shibayama, et al., J. Phys. Conf. Ser. 150, 032096 (2009).
[3] J. Nyeki et al., Nature Physics .35, 40 (2017).
[4] P. Corboz et al., Rev. B 78, 245414 (2008).