Phase transitions in models for coupled charge-density waves

Various phase transitions in models for coupled charge-density waves are investigated by means of the ε expansion, mean-field theory, and Monte Carlo simulations. At zero temperature the effective action for the system with appropriate commensurability effects is mapped onto the three- or four-dimensional XY model, depending on spatiotemporal fluctuations, under the corresponding symmetry-breaking fields. It is revealed that the three- and four-dimensional systems display a single transition between the clock order (with broken Z_M symmetry) and disorder. The nature of the phase transition depends crucially on the commensurability factor M: For M≥4, in particular, the transition belongs to the same university class as the XY model. On the other hand, in the presence of misfit causing frustration in the charge-density wave, the interchain coupling is observed to favor either the commensurate state or the incommensurate state depending on the initial configuration; this gives rise to hysteresis around the commensurate-incommensurate transition. Boundaries separating such phases with different symmetries are obtained in the parameter space consisting of the temperature, symmetry-breaking field, fluctuation strength, interchain coupling, and misfit.