3He Films as Model Strongly Correlated Fermion Systems

Abstract

Helium films on graphite are atomically layered. This allows a wide variety of studies of strong correlations in two dimensions with density as a continuously tunable parameter. Studies of a monolayer of 3He adsorbed on graphite plated by a bi‐layer of HD find a divergence of effective mass with increasing density, corresponding to a Mott‐Hubbard transition between a 2D Fermi liquid and a quantum spin liquid phase. While the Fermi liquid survives in 2D, non‐Fermi liquid features remain at finite T, recent theories find that this correction arises from the spin component of the backscattering amplitude. In another experiment a 3He film is grown on graphite plated by a bi‐layer of 3He. The first 3He layer only solidifies in the presence of an overlayer. However in the regime in which the system comprises a 3He fluid bilayer, we observe a striking maximum in the temperature dependence of both heat capacity and magnetization. This feature is driven towards T = 0 with increasing film coverage, suggestive of a quantum critical point. Well below the maximum a linear temperature dependence of the heat capacity is recovered; the coverage dependence of the effective mass identifies a (bandwidth driven) Mott‐Hubbard transition at 9.8 nm−2.