The Kitaev quantum spin liquid (QSL) model has fascinated condensed matter physicists for a decade because they are exactly solvable, provide a variety of QSL phases, and are relevant for transition metal compounds such as irridates and ruthenates. This model implements the exchange frustration by bond-dependent Ising-like spin interaction on the ideal two-dimensional honeycomb lattice resulting in an exactly solvable topological QSL ground state and fractional spin excitations represented by noble Majorana fermions. In the past decade, experimental realization of the Kitaev QSL model has been eagerly pursued and now it becomes a challenging to explore exotic quantum phases appearing in real materials due to additional perturbative spin interactions.

In this talk, I will present the experimental evidences of Majorana fermions in a layered van der Waals compound α-RuCl3. 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 demonstrated Majorana fermionic excitations as a result of thermal fractionalization of Jeff = ½ pseudospins, which is well reproduced by numerical simulations obtained from the Kitaev model. Furthermore, I will present recent progress in pursuit of exotic quantum phases and critical behaviors invoked by an external magnetic field or pressure on this material.