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

Abstract

We discuss the implications that new magnetocaloric, thermal expansion and magnetostriction data in $\alpha$-RuCl${}_3$ single crystals have on its temperature-field phase diagram and uncover the magnetic-field dependence of an apparent energy gap structure $\Delta (H)$ that evolves when the low temperature antiferromagnetic order is suppressed. We show that, depending on how the thermal expansion data is modeled, $\Delta (H)$ can show a cubic field dependence and remain finite at zero field, consistent with the pure Kitaev model hosting itinerant Majorana fermions and localized ${\mathbb{Z}}_2$ fluxes. Our magnetocaloric effect data provides, below $1\mathrm{K}$, unambiguous evidence for dissipative phenomena at $H_{\mathrm{c}}$, smoking gun for a first order phase transition. Our results, on the other hand, show little support for a phase transition from a QSL to a polarized paramagnetic state above $H_{\mathrm{c}}$.