New Mechanism for a Charge-Density-Wave Instability

It is shown that a two-dimensional energy band with saddle points at the Fermi energy is unstable against charge density wave formation. The distorted phase is metallic with only a relatively small area of Fermi surface truncated at the saddle points. It is suggested that this model applies to the layer compounds $2H$-Ta${\mathrm{Se}}_2$, $2H$-Ta${\mathrm{S}}_2$, $2H$-Nb${\mathrm{Se}}_2$, and 4Hb-Ta${\mathrm{S}}_2$.