Heat capacities of the anomalous fluid phase in two-dimensional ³He

We measured heat capacities of two dimensional ³He adsorbed on graphite preplated with a ⁴He monolayer (³He/ ⁴He/gr) in a wide temperature (0.3 ≤ T ≤ 80 mK) and density range (1.3 ≤ ρ ≤ 7.3 nm^-2). We found that the system behaves as a normal Fermi fluid at low densities between 1.3 and 5.6 nm^-2 where the quasiparticle effective mass seems to diverge at a density for the 4/7 phase (ρ_4/7 = 6.8 nm^-2). At higher densities but below ρ_4/7, we observed anomalous temperature dependencies of heat capacity with two round maxima near 1 and 30 mk. With increasing density in this region, a high temperature weight of heat capacity decreases selectively leaving the 30 mK peak, while the 1 mK peak develops. This unexpected behavior can not be explained by the conventional two-phase co-existence model. Instead, we propose here that the 2D normal Fermi fluid is continuously transformed to the 4/7 Mott localized phase through a new quantum phase where a hopping of the zero-point vacancy plays an important role.