Excitations in the field-induced quantum spin liquid state of α-RuCl3

The celebrated Kitaev quantum spin liquid (QSL) is the paradigmatic example of a topological magnet with emergent excitations in the form of Majorana Fermions and gauge fluxes. Upon breaking of time-reversal symmetry, for example in an external magnetic field, these fractionalized quasiparticles acquire non-Abelian exchange statistics, an important ingredient for topologically protected quantum computing. Consequently, there has been enormous interest in exploring possible material realizations of Kitaev physics and several candidate materials have been put forward, recently including α-RuCl 3 . In the absence of a magnetic field this material orders at a finite temperature and exhibits low-energy spin wave excitations. However, at moderate energies, the spectrum is unconventional and the response shows evidence for fractional excitations. Here we use time-of-flight inelastic neutron scattering to show that the application of a sufficiently large magnetic field in the honeycomb plane suppresses the magnetic order and the spin waves, leaving a gapped continuum spectrum of magnetic excitations. Our comparisons of the scattering to the available calculations for a Kitaev QSL show that they are consistent with the magnetic field induced QSL phase. Condensed Matter Physics: magnetic field drives spins to a liquid A sufficiently large magnetic field suppresses long-range magnetic order in α-RuCl 3 , leaving a disordered state with a gapped continuum spectrum of magnetic excitations, similar to that expected for the famous Kitaev quantum spin liquid. An international team led by Stephen E. Nagler from Oak Ridge National Laboratory in the USA performed time-of-flight neutron scattering to study low energy magnetic excitations of α-RuCl 3 . They observed that the application of a sufficiently large magnetic field to this material suppressed spin waves associated with the long-range order, and drove it to an unusual excited state. By comparison with calculations, these results are consistent with the Kitaev quantum spin liquid state in a magnetic field. The results provide important information of a possible route to producing gapped excitations related to magnetic Majorana Fermions towards topologically protected quantum computation.