Resolving the origin of planar thermal Hall effect in the Kitaev magnet α-RuCl₃ by field-angle-resolved specific heat measurements

Recent experiments have shown that the layered honeycomb material α-RuCl₃ exhibits several anomalous features that are consistent with expectations of the Kitaev quantum spin liquid (KQSL) when a magnetic field is applied in the honeycomb plane. Most remarkably, finite planar thermal Hall conductivity has been observed, whose magnitude is close to the half-integer quantization value expected for the chiral edge currents of Majorana fermions. However, there are attempts to offer a different explanation by the bosonic edge excitations due to topological magnons. A key to distinguishing between fermionic and bosonic origins of unusual features in the high-field state of α-RuCl₃ is the difference in the field angle dependence of the excitation gap. The Majorana gap in the KQSL is expected to become zero for the field along the Ru-Ru bond direction (b-axis). This is in stark contrast to the bosonic case; the magnon gap remains finite even for the field along the b-axis where the topological chiral edge current changes sign. Here we study low-energy excitations in the high-field phase of α-RuCl₃. Using a two-axis rotator, we measured field-angle-resolved specific heat C at very low temperatures down to ∼ 250 mK. When the field is aligned to the b-axis, we resolve finite residual C/T² in the zero-temperature limit. This indicates the gapless linear energy dispersion (Dirac cone) of Majorana fermions in the field-induced KQSL state.