Fractionalized excitations in the partially magnetized spin liquid candidate YbMgGaO₄

Quantum spin liquids (QSLs) are exotic states of matter characterized by emergent gauge structures and fractionalized elementary excitations. In the recently discovered triangular lattice QSL candidate YbMgGaO₄, dispersive spinon excitations with a clear upper excitation edge has been observed, but the nature of its ground state is still under debate. In this talk, we present neutron scattering measurement on YbMgGaO₄ under various magnetic fields. Our data reveal a dispersive spin excitation continuum with clear upper and lower excitation edges under a weak magnetic field (H = 2.5 T). Moreover, a spectral crossing emerges at the Gamma point at the Zeeman-split energy. The corresponding redistribution of the spectral weight and its field-dependent evolution is inconsistent with a magnon behavior, but instead resembles the theoretical prediction based on the inter-band and intra-band spinon particle-hole excitations associated with the Zeeman-split spinon bands. These results provide a strong evidence for fractionalized excitations and spinon Fermi surfaces in the partially magnetized QSL state in YbMgGaO₄.