  A quantum spin liquid is an exotic quantum state of matter in which spins are
highly entangled and remain disordered down to zero temperature. Such a state
of matter is potentially relevant to high-temperature superconductivity and
quantum-information applications, and experimental identification of a quantum
spin liquid state is of fundamental importance for our understanding of quantum
matter. Theoretical studies have proposed various quantum-spin-liquid ground
states, most of which are characterized by exotic spin excitations with
fractional quantum numbers (termed `spinon'). Here, we report neutron
scattering measurements that reveal broad spin excitations covering a wide
region of the Brillouin zone in a triangular antiferromagnet YbMgGaO4. The
observed diffusive spin excitation persists at the lowest measured energy and
shows a clear upper excitation edge, which is consistent with the particle-hole
excitation of a spinon Fermi surface. Our results therefore point to a QSL
state with a spinon Fermi surface in YbMgGaO4 that has a perfect spin-1/2
triangular lattice as in the original proposal of quantum spin liquids.
