Wannier-Orbital theory and ARPES for the quasi-1D conductor LiMo$_{6}$O$_{17}$
In this set of three papers, we present the results of a combined study by density-functional (LDA) band theory (NMTO) and angle-resolved photoemission spectroscopy (ARPES) of lithium purple bronze, 2(Li$_{1x}$Mo$_{6}$O$_{17}$). 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 2D and formed by three Mo $t_{2g}$-like extended Wannier orbitals (WOs) per cell, each one giving rise to a 1D band running at a 120$^{\circ }$ angle to the two others. A structural "dimerization" from $\mathbf{c}/2$ to $\mathbf{c}$ 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 band structure, expressed as a six-band, analytical tight-binding (TB) Hamiltonian, is verified in detail by the recognition and application of an ARPES selection rule that enables, for the first time, 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 TB 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.