Evidence for quantum spin liquid behaviour in single-layer 1T-TaSe2 from scanning tunnelling microscopy

Two-dimensional triangular-lattice antiferromagnets are predicted under some conditions to exhibit a quantum spin liquid ground state whose low-energy behavior is described by spinon Fermi surface. Directly imaging the resulting spinons, however, difficult due their fractional, chargeless nature. Here we use scanning tunneling spectroscopy image density modulations arising from surface instability in single-layer 1T-TaSe$_2$, two-dimensional Mott insulator. We first demonstrate existence of localized spins arranged on triangular lattice 1T-TaSe$_2$ contacting it metallic 1H-TaSe$_2$ layer and measuring Kondo effect. Subsequent spectroscopic isolated, reveals long-wavelength at Hubbard band energies that reflect modulations. This allows direct experimental measurement wavevector, good agreement with theoretical predictions for 2D liquid. These results establish as new platform studying novel quantum-spin-liquid phenomena.