Hunting Majorana Fermions in Kitaev Magnets

A Majorana fermion is a fermionic particle that is its own antiparticle. Since the theoretical discovery by Ettore Majorana in 1937, the exotic particle has long been searched in particle physics. In the last few decades, however, it has attracted renewed interest in condensed matter physics, where it can be realized as an elementary excitation (quasiparticle) in quantum states of matter, such as the fractional quantum Hall states and topological superconductors. In this review, we discuss another platform for Majorana fermions, the quantum spin liquid. The quantum spin liquid is a bizarre quantum phase of insulating magnets, firstly proposed by Philip Anderson in 1973, in which interacting magnetic moments remain disordered down to the lowest temperature under strong quantum fluctuations. They are characterized by topological entanglement and fractional excitations, whose possible application to topological quantum computation is recently discussed intensively. As a prime candidate for such exotic states, we here focus on the Kitaev magnets, a subgroup of the spin-orbit Mott insulators, which have been a subject of intense research initiated by the seminal works by Alexei Kitaev in 2006 and by G. Jackeli and G. Khaliullin in 2009. After a brief overview of the Kitaev model and the fractionalization of spins in the exact ground state, we review recent explosive development in this rapidly growing field, with a focus on numerical solutions of the Kitaev model at finite temperatures and the comparison with experiments. The key concept is thermal fractionalization-two types of fractional excitations manifest themselves at largely different temperatures. This leads to distinct thermodynamics and spin dynamics in a variety of experimentally measurable quantities. We discuss such peculiar behaviors as the signatures of fractional quasiparticles, in careful comparison with the available experimental data for the candidate materials of the Kitaev magnets. Our review gives an overview of the current status of the identification of Majorana fermions in the Kitaev magnets, which would serve as a basis for further experimental and theoretical studies toward the manipulation of the exotic particles for topological quantum computation.