Nature 540, 559-562 (2016).
Neutron scattering measurements of the triangular-lattice antiferromagnet YbMgGaO4 reveal broad spin excitations, covering a wide region of the Brillouin zone. The observed diffusive spin excitation persists at the lowest measured energy and shows a clear upper excitation edge, consistent with the particle–hole excitation of a spinon Fermi surface. The results therefore point to the existence of a quantum spin liquid state with a spinon Fermi surface in YbMgGaO4, which has a perfect spin-1/2 triangular lattice as in the original proposal of quantum spin liquids by P.W. Anderson (Mater. Res. Bull., 1973)