Band structures of the layer compounds 1T-TaS2and 2H-TaSe2in the presence of commensurate charge-density waves

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Abstract
Angle-resolved photoemission spectroscopy (ARPES) measurements of the E(k/sub ///) band structure are reported for 1T-TaS2 and 2H-TaSe2 at temperatures where commensurate CDWS are well developed. Empirical tight-binding calculations of the band structure in the presence of the experimentally known CDW superlattice are also reported. In the case of the square root 13* square root 13 CDW in 1T-TaS2, experiment and theory both reveal a gross distortion of the band structure, in which the Ta-derived d band collapses into three sub-band manifolds separated by gaps. The thirteenth electron is predicted to reside in a conduction band only 50 meV wide, where it is susceptible to a Mott-Anderson transition. In the case of 2H-TaSe2, additional peaks are seen in the ARPES data in the presence of the CDW, but the effects are too weak to distinguish between a band-structure distortion and weak Umklapp associated with the 3*3 superlattice. Calculated densities of states, decomposed according to the inequivalent atomic sites, are presented.