Two-dimensional antiferromagnetism of solid 3He adsorbed on plated graphite

Bilayer 3 He: A Simple Two-Dimensional Heavy-Fermion System with Quantum Criticality

Two-dimensional helium-3 (3He) provides a simple model for the experimental investigation of the emergence of quantum complexity in a strongly correlated Fermi system. We have observed two-dimensional, two-band heavy-fermion behavior in bilayer films of 3He atoms when adsorbed on the surface of graphite preplated by a solid bilayer of 4He. Thermodynamic measurements on this system showed that the relevant control parameter is the total density of the 3He film. The 3He bilayer system can be driven toward a quantum critical point at which the effective mass appears to diverge, interband coupling vanishes, and a local-moment state appears. It opens a new testing ground for theories of quantum criticality in heavy-fermion materials.

Gapless spin liquid behavior in two-dimensional solid 3He

Direct demagnetization has been made for two-dimensional solid 3He in both the paramagnetic and the antiferromagnetic phases. The lowest temperature is about 10 microK, judging from the observed magnetization for the paramagnetic solid 3He. The magnetization of the antiferromagnetic solid 3He shows a gradual increase to about 10 microK for the 4/7 phase adsorbed on both one layer of 4He and two layers of HD preplated graphite. This strongly suggests that the triangular antiferromagnet with the higher order multiple exchange has a quantum spin liquid ground state with nearly zero or extremely small spin gap less than 10 microK.

Evidence for a Mott-Hubbard transition in a two-dimensional 3He fluid monolayer

The heat capacity and magnetization of a fluid 3He monolayer adsorbed on graphite plated with a bilayer of HD have been measured in the temperature range 1-60 mK. Approaching the density at which the monolayer solidifies into a sqrt[7]xsqrt[7] commensurate solid, we observe an apparent divergence of the effective mass and magnetization corresponding to a T=0 Mott-Hubbard transition between a 2D Fermi liquid and a magnetically disordered solid. The observations are consistent with the Brinkman-Rice-Anderson-Vollhardt scenario for a metal-insulator transition. We observe a leading order T2 correction to the linear term in heat capacity.

Low-temperature magnetization of submonolayer 3He adsorbed on HD preplated graphite

NMR studies of submonolayer 3He adsorbed on a bilayer of HD preplated graphite have been made down to 100 &mgr;K, which is more than 1 order of magnitude smaller than the exchange energy ( J). In the highly frustrated antiferromagnetic solid region, the magnetization obeys a Curie-Weiss law even at temperatures around J and then increases gradually down to 100 &mgr;K. Nevertheless, it does not show any anomalous behavior corresponding to a spin gap. The normalized magnetization versus the reduced temperature ( T/J) is independent of the density just after solidification. This is consistent with the result in the high-temperature region, that the main multiple-spin exchanges have a similar density dependence.