The field-induced quantum spin-liquid phase of α−RuCl₃ is gapless

Throughout 2017, the material α−RuCl₃ has continued to inspire and fascinate those interested in correlated condensed matter. New experimental data now provide unique insight, and pose fresh challenges.

The motivation for much of the current work on α−RuCl₃ is that the material might be a realization of the Kitaev model, an exactly soluble spin model on the honeycomb lattice that contains topological quantum spin liquid (QSL) states of both gapped and gapless persuasions. The prospect of having a material in which the Kitaev model experimentally are intriguing, but the fingerprints available to date have been rather tenuous, leaving it unclear either how close α−RuCl₃ really comes to realizing the Kitaev model or what any more robust fingerprints should look like, especially when a magnetic field is applied.
 

An applied field drives the system from a magnetically ordered state into a disordered one that bears the hallmarks of a QSL. A whirlwind of controversy has blown up over this phase, with claims of varying reliability from many different experimental methods that it is gapped. The definitive technique for detecting magnetic excitations at very low energies is nuclear magnetic resonance (NMR), and now PSI theorist Bruce Normand, working with NMR colleagues at Renmin University of China in Beijing, has helped to decode the complex situation.


The team demonstrates that, beyond the quantum phase transition at a field of 7.5 T applied in the honeycomb plane, the high-field QSL has gapless spin excitations over a field range up to 16 T. These modes have cone-type dispersion branches, meaning that their density of states vanishes at the lowest energies, which indicates why they are hard to detect by less sensitive methods. Theoretically, these appear to be Dirac-type excitations rather than being of the Majorana type expected in the pure Kitaev model, and experimentally they become gapped when the field is applied out of the plane. This highly unconventional behaviour is not a feature of either Heisenberg or Kitaev magnets, and thus suggests that the physics of α−RuCl₃ is dominated by interactions beyond both models.