Phase diagram for charge-density waves in a magnetic field

The influence of an external magnetic field on a quasi-one-dimensional system with a charge-density wave (CDW) instability is treated within the random-phase approximation which includes both CDW and spin-density wave correlations. We show that the CDW is sensitive to both orbital and Pauli effects of the field. In the case of perfect nesting, the critical temperature decreases monotonically with the field, and the wave vector of the instability starts to shift above some critical value of magnetic field. Depending on the ratio between the spin and charge coupling constants and on the direction of the applied magnetic field, the wave-vector shift is either parallel (CDWₓ order) or perpendicular (CDWy order) to the most conducting direction. The CDWₓ order is a field-dependent linear combination of the charge- and spin-density waves and is sensible only to the Pauli effect. The wave-vector shift in CDWy depends on the interchain coupling, but the critical temperature does not. This order is affected by the confinement of the electronic orbits. By increasing the relative strength of the orbital effect with respect to the Pauli effect, one can destroy the CDWy, establishing either a CDWₓ or a CDW₀ (corresponding to the perfect nesting wave vector). By increasing the imperfect nesting parameter, one passes from the regime where the critical temperature decreases with the field to the regime where it is initially enhanced by the orbital effect and eventually suppressed by the Pauli effect. For a bad nesting, the quantized phases of the field-induced CDW appear. {} 1996 The American Physical Society.