A CRITERION FOR TWO KINDS OF PEIERLS TRANSITIONS

Two different kinds of Peierls transitions can exist depending on the anisotropic ratio y of intrachain and interchain coupling. γ has a critical value γ_o; if γ > γ_o, the Peierls transition will exhibit a CDW accompanied by a metal-semiconductor transition (kind I); if γγ_o, only CDW and superlattice occur at Peierls transition (kind II). Some experimental evidences are discussed.