Evidence for a spinon Fermi surface in a triangular lattice quantum spin liquid candidate

TL;DR

This study provides experimental evidence for a quantum spin liquid with a spinon Fermi surface in a triangular lattice material, using neutron scattering to observe characteristic broad spin excitations.

Contribution

First direct experimental evidence of a spinon Fermi surface in a triangular lattice quantum spin liquid candidate, confirming theoretical predictions.

Findings

Broad spin excitations observed across the Brillouin zone

Persistent diffusive spin excitation at low energies

Clear upper excitation edge consistent with spinon particle-hole continuum

Abstract

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.