Spin-Liquid--to--Spin-Liquid Transition in Kitaev Magnets Driven by   Fractionalization
  While phase transitions between magnetic analogs of three states of matter --- a long-range ordered state, paramagnet, and spin liquid --- have been extensively studied, the possibility of "liquid-liquid" transitions, namely, between different spin liquids, remains elusive. By introducing the additional Ising coupling into the honeycomb Kitaev model with bond asymmetry, we discover that the Kitaev spin liquid turns into a spin-nematic quantum paramagnet before a magnetic order is established by the Ising coupling. The quantum phase transition between the two liquid states accompanies a topological change driven by fractionalized excitations, the $Z_2$ gauge fluxes, and is of first order. At finite temperatures, this yields a persisting first-order transition line that terminates at a critical point located deep inside the regime where quantum spins are fractionalized. It is suggested that similar transitions may occur in other perturbed Kitaev magnets with bond asymmetry. 
Joji Nasu, Yasuyuki Kato, Junki Yoshitake, Yoshitomo Kamiya, Yukitoshi   Motome
cond-mat.str-el cond-mat.mtrl-sci cond-mat.stat-mech