Fermi surface of layered compounds and bulk charge density wave systems

Abstract

A review is given of recent angle-resolved photoemission (ARPES) experimentsand analyses on a series of layered charge density wave materials. Importantaspects of ARPES are recalled in view of its capability for bulk band, Fermisurface and spectral function mapping despite its surface sensitivity. Discussed areTaS2,TaSe2,NbTe2,TiSe2 andTiTe2 with structuresrelated to the so-called 1T polytype. Many of them undergo charge density wave transitions or exist with a distortedlattice structure. Attempts to explain the mechanism behind the structural reconstructionare given. Depending on the filling of the lowest occupied band a drastically differentbehaviour is observed. Whereas density functional calculations of the electronic energy andmomentum distribution reproduce well the experimental spectral weight distribution at theFermi energy, the ARPES energy distribution curves reveal that for some of the compoundsthe Fermi surface is pseudo-gapped. Two different explanations are given, the first based ondensity functional calculations accounting for the charge-density-wave-induced latticedistortion and the second relying on many-body physics and polaron formation.Qualitatively, both describe the observations well. However, in the future, in order to beselective, quantitative modelling will be necessary, including the photoemission matrixelements.