Weak Mott insulators on the triangular lattice: Possibility of a gapless nematic quantum spin liquid

Abstract

We study the energetics of Gutzwiller projected BCS states of various symmetries for the triangular lattice antiferromagnet with a four-particle ring exchange using variational Monte Carlo methods. In a range of parameters the energetically favored state is found to be a projected ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ paired state which breaks lattice rotational symmetry. We show that the properties of this nematic or orientationally ordered paired spin-liquid state as a function of temperature and pressure can account for many of the experiments on organic materials. We also study the ring-exchange model with ferromagnetic Heisenberg exchange and find that among the studied ans\"atze, a projected $f$-wave state is the most favorable.