Basic aspects of the charge density wave instability of transition metal trichalcogenides NbSe₃and monoclinic-TaS₃

NbSe₃and monoclinic-TaS₃(m-TaS₃) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave Peierls transitions atTP1andTP2associated with type III and type I chains, respectively. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their Fermi surface (FS) and electron-hole response function. Because of the stronger inter-chain interactions, the FS and electron-hole response function are considerably more complex for NbSe₃thanm-TaS₃; however a common scenario can be put forward to rationalize the results. The intra-chain inter-band nesting processes dominate the strongest response for both type I and type III chains of the two compounds. Two well-defined maxima of the electron-hole response for NbSe₃are found with the (0a^*, 0c^*) and (1/2a^*, 1/2c^*) transverse components atTP1andTP2, respectively, whereas the second maximum is not observed form-TaS₃atTP2. Analysis of the different inter-chain coupling mechanisms leads to the conclusion that FS nesting effects are only relevant to set the transversea^*components in NbSe₃. The strongest inter-chain Coulomb coupling mechanism must be taken into account for the transverse coupling alongc^*in NbSe₃and along botha^*andc^*form-TaS₃. Phonon spectrum calculations reveal the formation of a giant 2k_FKohn anomaly form-TaS₃. All these results support a weak coupling scenario for the Peierls transition of transition metal trichalcogenides.