An alternative route to charge density wave formation in multi-band systems
The common view of charge density wave (CDW) formation was originally posed by Kohn: the tendency towards ordering is particularly strong in low dimensions, because the Fermi surface has parallel parts, referred to as nesting. This leads to a divergence in the Lindhard susceptibility, determining the magnitude and direction of the ordering vector Q. However, several publications raise the question as to whether nesting alone is sufficient to explain the observed ordering direction Q, particularly in dimensions higher than one. A central question is whether the selection of the CDW ordering vector is always driven by an electronic instability, or if the ordering vector could instead be determined by a lattice distortion driven by some other mechanism exploiting the role of the electron-phonon coupling.
For exploring a possible relation between anisotropic electron-phonon coupling and CDW ordering selection, it is desirable to map out the coupling strength in momentum space. To this goal, we analyze data from Raman experiments (Fig. 3.17) and the related selection rules for the CDW ErTe3 material (TCDW1 = 265 K, followed by a second one at TCDW2 = 155 K) and demonstrate that the lifting of band degeneracies enhances the light-scattering sensitivity (known as focusing effect) and, concomitantly, the electron-phonon coupling at ordering vectors that do not coincide with those predicted by nesting alone. Hence, while electron-phonon coupling is known to be important in CDW systems, on a microscopic basis the focusing effect seems to be a more generic paradigm for multiband materials.