Origin of oscillatory structures in the magnetothermal conductivity of the putative Kitaev magnet α-RuCl3

The layered honeycomb magnet α-RuCl3 has been suggested to exhibit a field-induced quantum spin liquid state, in which the reported large thermal Hall effect close to the half-quantized value still remains a subject of debate. Recently, oscillatory structures of the magnetothermal conductivity were reported and interpreted as quantum oscillations of charge-neutral particles. To investigate the origin of these oscillatory structures, we performed a comprehensive measurement of the in-plane magnetothermal conductivity κ(H) down to low temperature (100 mK), as well as magnetization M, for single crystals grown by two different techniques: Bridgman and chemical vapor transport. The results show a series of dips in κ(H) and peaks in the field derivative of M located at the same fields independent of the growth method. We argue that these structures originate from the field-induced phase transitions rather than from quantum oscillations. The positions of several of these features are temperature-dependent and connected to the magnetic phase transitions in zero field: the main transition at 7 K and weaker additional transitions, which likely arise from secondary phases at 10 K and 13 K. In contrast to what is expected for quantum oscillations, the magnitude of the structure in κ(H) is smaller for the higher conductivity crystal and decreases rapidly upon cooling below 1 K.