Masafumi Udagawa

The Kitaev model has recently attracted considerable attention in broad areas of research owing to the topological nature and quantum spin-liquid (QSL) ground states. This is defined on a honeycomb lattice, and is exactly solvable due to the Ising conserved quantities on each hexagon. In this study, we investigate the thermodynamics of a three-dimensional extension of the Kitaev model defined on a hyperhoneycomb lattice. This model can be rewritten as a free Majorana fermion system coupled with Ising variables. Using this representation, we have performed the Monte Carlo simulation and analyzed the thermodynamic properties. We find that the model exhibits a finite-temperature phase transition between the QSLs and paramagnet in the whole parameter range. This result indicates that the QSL phases at low temperatures are always distinguished from the high-temperature paramagnet by a phase transition. We also find that the difference between QSL and paramagnet comes from the topological nature of the excitations.