Wannier orbital theory and angle-resolved photoemission spectroscopy for the quasi-one-dimensional conductor LiMo6 O17. I. Six-band t2g Hamiltonian

In this and the two following papers, we present the results of a combined study by density-functional band theory and angle-resolved photoemission spectroscopy (ARPES) of lithium purple bronze, Li1xMo6O17. This material is particularly notable for its unusually robust quasi-one-dimensional (quasi-1D) behavior. The band structure, in a large energy window around the Fermi energy, is basically two-dimensional and formed by three Mo t2g-like extended Wannier orbitals (WOs), each one giving rise to a 1D band running at a 120 angle to the two others. A structural "dimerization"from c/2 to c gaps the xz and yz bands while leaving the xy bands metallic in the gap but resonantly coupled to the gap edges and, hence, to the two other directions. The resulting complex shape of the quasi-1D Fermi surface (FS), verified by our ARPES, thus depends strongly on the Fermi energy position in the gap, implying a great sensitivity to Li stoichiometry of properties dependent on the FS, such as FS nesting or superconductivity. The theory is verified in detail by the recognition and application of an ARPES selection rule that enables the separation in ARPES spectra of the two barely split xy bands and the observation of their complex split FS. The strong resonances prevent either a two-band tight-binding model or a related real-space ladder picture from giving a valid description of the low-energy electronic structure. Down to a temperature of 6 K we find no evidence for a theoretically expected downward renormalization of perpendicular single-particle hopping due to LL fluctuations in the quasi-1D chains. This paper I introduces the material, motivates our study, summarizes the Nth-order muffin-tin orbital (NMTO) method that we use, analyzes the crystal structure and the basic electronic structure, and presents our NMTO calculation of the t2g low-energy WOs and the resulting tight-binding Hamiltonian for the six lowest energy bands, only the four lowest being occupied. Thus this paper sets the theoretical framework and nomenclature for the following two papers.