The phase diagram and the structure of CDW state in high magnetic field in quasi-1D materials: mean-field approach

We develop the mean-field theory of a charge-density wave (CDW) state in magnetic field and study the properties of this state below the transition temperature. We show that the CDW state with shifted wave vector in high magnetic field (CDW$_x$ phase) has at least double harmonic modulation on the most part of the phase diagram. In the perfect nesting case the single harmonic CDW state with shifted wave vector exists only in a very narrow region near the tricritical point where the fluctuations are very strong. We show that the transition from CDW$_0$ to CDW$_x$ state below the critical temperature is accompanied by a jump of the CDW order parameter and of the wave vector rather than by their continuous increase. This implies a first order transition between these CDW states and explains the strong hysteresis accompanying this transition in many experiments. We examine how the phase diagram changes in the case of imperfect nesting.