The intersection of topology and strong electron correlations has given rise to a plethora of exotic phenomena. One such phenomenon is the fractional quantum Hall effect, where quasiparticles carry fractions of electron charge. More recently, the Kitaev quantum spin liquid (QSL) has garnered attention due to the emergence of Majorana fermions and non-Abelian anyons resulting from the fractionalization of electron spins. The Kitaev model, which represents a spin-1/2 on a honeycomb lattice interacting through bond-dependent Ising ferromagnetic couplings, has been observed in the spin-orbit Mott insulator α-RuCl3. Several measurements, including specific heat, Raman scattering, and inelastic neutron scattering, provide evidence for the spin fractionalization in α-RuCl3. Moreover, the half-integer quantized thermal Hall conductance provides evidence for the formation of a topologically nontrivial state consistent with the Kitaev model .
The half-integer quantized thermal Hall conductance, which appears even for a magnetic field with no out-of-plane components (planar thermal Hall effect), supports the formation of a topologically nontrivial state consistent with the Kitaev model  . Recently, we performed low-temperature measurements of high-resolution specific heat  and planar thermal Hall conductivity with rotating in-plane fields. We find that a distinct closure of the low-energy bulk gap is observed concomitantly with the sign reversal of the planar thermal Hall effect. The general discussion of topological bands shows that this is the hallmark of an angle-rotation-induced topological phase transition of fermions, providing conclusive evidence for the Majorana-fermion origin of the thermal Hall .
These results provide direct signatures of topologically protected chiral currents of charge neutral Majorana fermions at the edge and non-Abelian anyons in the bulk of the crystal. The recent scanning tunneling microscopy measurements of monolayer α- and b- RuCl3 will also be discussed if time allows.
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