Neutron Spectroscopy Evidence for a Possible Magnetic-Field-Induced Gapless Quantum-Spin-Liquid Phase in a Kitaev Material \alpha-RuCl_3

As one of the most promising Kitaev quantum-spin-liquid (QSL) candidates, \alpha-RuCl_3 has received a great deal of attention. However, its ground state exhibits a long-range zigzag magnetic order, which defies the QSL phase. Nevertheless, the magnetic order is fragile and can be completely suppressed by applying an external magnetic field. Here, we explore the evolution of magnetic excitations of \alpha-RuCl_3 under an in-plane magnetic field, by carrying out inelastic neutron scattering measurements on high-quality single crystals. Under zero field, there exist spin-wave excitations near the M point and a continuum near the \varGamma point, which are believed to be associated with the zigzag magnetic order and fractional excitations of the Kitaev QSL state, respectively. By increasing the magnetic field, the spin-wave excitations gradually give way to the continuous excitations. On the verge of the critical field \mu_0H_\rm c=7.5 T, the former ones vanish and only the latter ones are left, indicating the emergence of a pure QSL state. By further increasing the field strength, the excitations near the \varGamma point become more intense. By following the gap evolution of the excitations near the \varGamma point, we are able to establish a phase diagram composed of three interesting phases, including a gapped zigzag order phase at low fields, possibly gapless QSL phase near \mu_0H_\rm c, and gapped partially polarized phase at high fields. These results demonstrate that an in-plane magnetic field can drive \alpha-RuCl_3 into a long-sought QSL state near the critical field.