Imaging spinons in a 2D gapless quantum spin liquid

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 a spinon Fermi surface. This “ghost” Fermi surface (in an otherwise insulating material) is a key concept for understanding spin liquids and their relationship to other quantum phases. Directly imaging the spinon Fermi surface, however, is difficult due to the fractional and chargeless nature of spinons. I will discuss how we have used scanning tunneling microscopy (STM) to image density fluctuations arising from a spin liquid Fermi surface in single-layer 1T-TaSe2, a two-dimensional Mott insulator. Quantum spin liquid behavior was observed in isolated single layers of 1T-TaSe2 through long-wavelength modulations of the local density of states at Hubbard band energies. These modulations reflect a spinon Fermi surface instability in single-layer 1T-TaSe2 and allow direct experimental measurement of the spinon Fermi wavevector, in good agreement with theoretical predictions for a 2D quantum spin liquid. Our results establish single-layer 1T-TaSe2 as an ideal platform for studying novel two-dimensional quantum spin liquid phenomena.