Density wave v.s. magnetic breakdown: Mechanism of high-field phase transition in α-(BEDT-TTF)2XHg(SCN)4

We study the density wave instability under magnetic fields for the two-band two-dimensional electron system with the interband coupling due to the magnetic breakdown. We consider a model system having a pair of open Fermi surfaces and a closed Fermi surface, whose low-temperature state is a density wave phase caused by the nesting of open Fermi surfaces. Under the influence of magnetic fields, the density wave transition is suppressed by the magnetic breakdown coupling with the closed Fermi surface. We apply this model to the high-magnetic-field phase transition observed in the quasi-two-dimensional organic conductors (BEDT-TTF)2XHg(SCN)4.