  Quantum spin liquid is an exotic quantum state of matter in magnets. This
state is a spin analogue of the liquid helium which does not solidify down to
the lowest temperature due to strong quantum fluctuations. In conventional
fluids, liquid and gas possess the same symmetry and adiabatically connect to
each other by bypassing the critical end point. We find that the situation is
qualitatively different in quantum spin liquids realizing in a
three-dimensional Kitaev model; both gapless and gapped quantum spin liquid
phases at low temperatures are always distinguished from the high-temperature
paramagnet (spin gas) by a phase transition. The results challenge common
belief that the absence of thermodynamic singularity down to the lowest
temperature is a symptom of a quantum spin liquid.
