The seminal work of Anderson triggered a great deal of theoretical and experimental efforts to search for the novel quantum spin liquid (QSL) states in matters, and it has become one of central issues in contemporary condensed matter physics. The QSL state, a long-range quantum entangled state, is represented by a topological order and fractionalization of constituent magnetic moments. While the most QSL states have been described by deconfined spinons as an elementary excitation in frustrated magnets, Kitaev’s QSL state is exactly derived by fractionalizing the spin excitation into Majorana fermions in a two-dimensional honeycomb lattice, the so-called Kitaev lattice, with the ansatz of bond dependent Ising-like spin interaction. In the past decade, experimental realization of the fascinating Kitaev honeycomb QSL model has been eagerly pursued. In this talk, I will present the experimental evidences of fractionalized Majorana fermions in a high quality α-RuCl3 single crystal. Neutron and x-ray diffraction measurements reveal that the low-temperature crystal structure forms the perfect Ru-honeycomb lattice, which provides an ideal platform for the Kitaev honeycomb quantum spin lattice. Extensive thermodynamic and neutron spectroscopic measurements directly proved fractionalized Majorana fermion excitations as a result of thermal fractionalization of Jeff = ½ pseudospins, which is well reproduces by numerical predictions obtained from the Kitaev model.

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