STRUCTURAL AND ELECTRONIC ORIGIN OF THE HIDDEN NESTING AND CHARGE-DENSITY WAVES IN TRANSITION-METAL OXIDES AND BRONZES

By employing the concept of hidden nesting, we examined why charge density wave (CDW) instabilities occur in the two-dimensional (2D) metals, purple bronzes AMo6O17 (A = K, Na, Tl), Magneli phases Mo4O11 and monophosphate tungsten bronzes (MPTB's). These oxide metals consist of metal-oxygen (M-O) layers made up of MO6 (M = Mo, W) octahedra, and the t2g-block bands of these M-O layers are partially filled. The M-O layers are divided into three sets of parallel chains containing t2g-orbitals, and the t2g-orbitals of each MO6 octahedron act as delta-orbitals toward the M-O axes perpendicular to the chains. Consequently, the t2g-block bands Of AMo6O17, Mo4O11 and MPTB's can be described in terms of three hidden one-dimensional (1D) bands, which explains why these 2D metals possess three hidden 1D Fermi surfaces and give rise to the associated CDW instabilities. The hidden nesting concept is applicable to the diphosphate tungsten bronzes, which are apparently 2D metallic because their W-O layers have 1D bands dispersive in two orthogonal directions. In general, the electronic instabilities of any systems with several partially filled bands are expected to arise from hidden nesting.