Phase Diagram for Charge Density Waves in a Magnetic Field

TL;DR

This paper investigates how magnetic fields influence charge density waves in quasi-one-dimensional systems, revealing the roles of orbital and Pauli effects, and identifying conditions for different CDW orders and phase transitions.

Contribution

It provides a detailed theoretical analysis of magnetic field effects on CDWs, including wave vector shifts, critical temperature behavior, and the emergence of quantized phases, considering both orbital and spin interactions.

Findings

Critical temperature decreases monotonously with magnetic field in perfect nesting.

Wave vector shifts depend on the ratio of spin and charge couplings and field direction.

Quantized phases of field-induced CDWs appear in systems with poor nesting.

Abstract

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 monotonously 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 ($CDW_x$ order) or perpendicular ($CDW_y$ order) to the most conducting direction. The $CDW_x$ 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 $CDW_y$ 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 $CDW_y$, establishing either a $CDW_x$, or a $CDW_0$ (corresponding to perfect nesting wave vector). We also show that 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.