Field-angle-resolved specific heat measurements in the Kitaev material α-RuCl₃

Recently, 4d⁵ or 5d⁵ honeycomb materials with strong spin-orbit coupling have attracted great attention as a candidate for realizing a Kitaev quantum spin liquid (QSL). However, most of the candidate materials show magnetic orders at low temperatures. Indeed, α-RuCl₃, one of the most promising candidates, also exhibits a zigzag antiferromagnetic order below T_N (~ 7 K). However, the magnetic order of this material can be suppressed by applying a critical magnetic field H* of 7.5 T parallel to the honeycomb plane. Recent thermal transport measurements, as well as spectroscopic studies such as inelastic neutron scattering, suggest the emergence of spin fractionalization into Majorana fermions in the field-induced QSL phase. Here, we report specific heat measurements under in-plane magnetic fields up to 7 T on electron-irradiated α-RuCl₃ single crystals, in which a systematic suppression of T_N and H* was observed. We find a remarkable six-fold oscillation in the field-angle dependence of the specific heat at low temperatures above H*. This result is consistent with the field-angle dependence of the Majorana gap expected in the Kitaev QSL.