A bird’s-eye view of charge and spin density wave from first principles calculations

Résumé

This thesis deals with the electronic structure of low dimensional metals in the form of either single layers or bulk. Low dimensional metals often exhibit electronic instabilities like charge or spin density waves. A central role in most of the theoretical approaches to understand these instabilities is played by the notion of Fermi surface nesting. Very often this is considered to be the driving force of these instabilities. Yet a careful examination of the experimental information casts strong doubts about the appropriateness of such notion for a considerable number of these conductors. The object of the present thesis is an appraisal of this situation based on the calculation of the Lidhard response function as well as the phonon band structure for a series of low dimensional conductors based on accurate first-principles DFT calculations. Altogether we have studied from the first principles calculations standpoint the following classes of materials: transition metal dichalcogenides (TiSe2, TiTe2 and NbSe2), oxides (blue bronze, monophosphate tungsten bronzes, layered perovskite-related AnBnO3n+2 niobates and titanates, Magnèli phases Mo4O11 and Mo8O23), transition metal trichalcogenides (NbS3, NbSe3, TaS3, ZrTe3) and tetrachalcogenides (TaTe4, (TaSe2)4I), Bechgaards salts ( (TMTSF)2X with X = ClO4, NO3, PF6) and rare earth intermetallics (LaAgSb2) . In this document we report the results for six of these materials: TiSe2, TiTe2, NbSe2, the potassium blue bronze, the Bechgaard salts and TaTe4. As a result of this work, we have been able to show that: 1) The CDW mechanism in TiSe2, TiTe2, NbSe2 and TaTe4 is a phonon mediated mechanism and completely unrelated to the Fermi surface nesting mechanism; 2) 6 different structure can coexist in the CDW state of NbSe2; 3) in the blue bronze the Peierls transition can be well accounted for by the weak electron-phonon coupling theory in the adiabatic approximation. 4) that in the Bechgaard salts the Lindhard response is found to change considerably with temperature and these changes are clearly associated with dimensional crossovers.