Laboratoire de Physique Théorique
Toulouse - UMR 5152

Quantum spin liquid (QSL) is an exotic quantum state of matter in insulating magnets, where long-range ordering is suppressed down to the lowest temperature. Several experimental candidates of QSL have been recently nominated thus far. In their characterization, the absence of thermodynamic anomalies is experimentally regarded as a hallmark of QSL. On the other hand, the finite-temperature properties theoretically have not been well known due to the difficulty in the analysis for models with a QSL ground state such as the negative sign problem in quantum Monte Carlo (QMC) simulations. To clarify this problem, we address three kinds of Kitaev models on a honeycomb, a decorated honeycomb, and a three-dimensional hyperhonycomb lattices. The Kitaev model is one of the solvable quantum spin models, where the ground state is given by QSLs. In addition, the Kitaev-type interaction is relevant to the recently found Ir oxides Li2IrO3. Here, we investigate the finite-temperature properties in these Kitaev models by using the QMC simulation in the Majorana representation that we developed. Our method is free from any biased approximation as well as the negative sign problem. We find that these models commonly exhibit a two-stage entropy release, indicating thermal fractionalization of the quantum spins into two kinds of Majorana fermions. Moreover, there is no phase transition at nonzero temperature in the Kitaev model on the honeycomb lattice, but we find that the models on the decorated honeycomb and the hyperhoneycomb lattices exhibit the finite-temperature phase transitions. In the former case, the transition is associated with the time reversal symmetry breaking in the chiral QSL. On the other hand, in the latter case, the transition cannot be characterized within the Ginzburg-Landau-Wilson framework. We find the difference between QSL and paramagnet comes from the topological nature of the excitations.