Quest for the ground state in two-dimensional antiferromagnetic solid 3He

A low-density solid 3He film adsorbed on graphite surface forms an ideal two-dimensional (2D) quantum spin system with nuclear spin S=1/2 on a triangular lattice. In the antiferromagnetic region called the 4/7 phase, the existence of the competing higher order multiple spin exchange (MSE) interactions in addition to a geometrical frustration, makes the system strongly frustrated. The ground state has been investigated both theoretically and experimentally, but it is still an open question. The NMR measurements for the 4/7 phase adsorbed on both one layer of 4He and two layers of HD preplated graphite have been made down to the temperatures of 10 μK which is by a factor of 10 lower than the previous. The observed susceptibility shows a gradual increase with no abrupt change. This fact strongly suggests that the triangular antiferromagnet with the higher order multiple exchanges has a gapless spin liquid ground state.