Finite field regime for a quantum spin liquid in α−RuCl3

An external magnetic field can induce a transition in α−RuCl3 from an ordered zigzag state to a disordered state that is possibly related to the Kitaev quantum spin liquid. Here, we present field-dependent inelastic neutron scattering and magnetocaloric effect measurements implying the existence of an additional transition out of the quantum spin-liquid phase at an upper field limit Bu. The neutron scattering shows three distinct regimes of magnetic response. In the low-field ordered state the response shows magnon peaks; the intermediate-field regime shows only continuum scattering, and above Bu the response shows sharp magnon peaks at the lower bound of a strong continuum. Measurable dispersion of magnon modes along the (0,0,L) direction implies non-negligible interplane interactions. Combining the magnetocaloric effect measurements with other data, a T−B phase diagram is constructed. The results constrain the range where one might expect to observe quantum spin-liquid behavior in α−RuCl3.