$1T\text{−}{\mathrm{Ta}\mathrm{S}}_2$ is a cluster Mott insulator on the triangular lattice with 13 Ta atoms forming a star of David cluster as the unit cell. We derive a two-dimensional $XXZ$ spin-$1/2$ model with a four-spin ring exchange term to describe the effective low energy physics of a monolayer $1T\text{−}{\mathrm{Ta}\mathrm{S}}_2$, where the effective spin-$1/2$ degrees of freedom arises from the Kramers degenerate spin-orbital states on each star of David. A large scale density matrix renormalization group simulation is further performed on this effective model and we find a gapless spin liquid phase with a spinon Fermi surface at a moderate to large strength region of the four-spin ring exchange term. All peaks in the static spin structure factor are found to be located on the “$2k_F$” surface of a half-filled spinon on the triangular lattice. Experiments to detect the spinon Fermi surface phase in $1T\text{−}{\mathrm{Ta}\mathrm{S}}_2$ are discussed.

• Received 12 March 2018
• Revised 12 May 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.046401