Thermal and magnetoelastic properties of a-RuCl3 in the field-induced low-temperature states

We discuss the implications that new magnetocaloric, thermal expansion, and magnetostriction data in a-RuCl3 single crystals have on its temperature-field phase diagram and uncover the magnetic-field dependence of an apparent energy gap structure ?(H) that evolves when the low-temperature antiferromagnetic order is suppressed. We show that, depending on how the thermal expansion data are modeled, ?(H) can show a cubic field dependence and remain finite at zero field, consistent with the pure Kitaev model hosting itinerant Majorana fermions and localized Z2 fluxes. Our magnetocaloric effect data provide, below 1K, unambiguous evidence for dissipative phenomena at Hc, a smoking gun for a first-order phase transition. Conversely, our results show little support for a phase transition from a QSL to a polarized paramagnetic state above Hc.