Gapless spin liquids: Stability and possible experimental relevance

For certain crystalline systems, most notably the organic compound EtMe3Sb[Pd(dmit)(2)](2), experimental evidence has accumulated of an insulating state with a high density of gapless neutral excitations that produce Fermi-liquid-like power laws in thermodynamic quantities and thermal transport. This has been taken as evidence of a fractionalized spin-liquid state. In this Rapid Communication, we argue that if the experiments are taken at face value, the most promising spin-liquid candidates are a Z(4) spin liquid with a spinon-Fermi surface and no broken symmetries, or a Z(2) spin liquid with a spinon-Fermi surface and at least one of the following spontaneously broken: (a) time-reversal and inversion, (b) translation, or (c) certain point-group symmetries. We present a solvable model on the triangular lattice with an (a) type Z(2) spin-liquid ground state. DOI: 10.1103/PhysRevB.87.140402