Field-induced quantum criticality in the Kitaev system α − RuCl 3

$\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ has attracted enormous attention since it has been proposed as a prime candidate to study fractionalized magnetic excitations akin to Kitaev's honeycomb-lattice spin liquid. We have performed a detailed specific-heat investigation at temperatures down to $0.4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in applied magnetic fields up to $9\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ for fields parallel to the $ab$ plane. We find a suppression of the zero-field antiferromagnetic order, together with an increase of the low-temperature specific heat, with increasing field up to ${\ensuremath{\mu}}_{0}{H}_{\text{c}}\ensuremath{\approx}6.9$ T. Above ${H}_{\text{c}}$, the magnetic contribution to the low-temperature specific heat is strongly suppressed, implying the opening of a spin-excitation gap. Our data point toward a field-induced quantum critical point at ${H}_{\text{c}}$; this is supported by universal scaling behavior near ${H}_{\text{c}}$. Remarkably, the data also reveal the existence of a small characteristic energy scale well below 1 meV, above which the excitation spectrum changes qualitatively. We relate the data to theoretical calculations based on a ${J}_{1}\ensuremath{-}{K}_{1}\ensuremath{-}{\mathrm{\ensuremath{\Gamma}}}_{1}\ensuremath{-}{J}_{3}$ honeycomb model.