1:1 Alkali-TCNQ salts and the bond order wave (BOW) phase of half-filled linear Hubbard-type models

Abstract

The bond order wave (BOW) phase of half-filled linear Hubbard-type models is narrow and difficult to characterize aside from a few ground state properties. The BOW phase of a frustrated Heisenberg spin chain is wide and tractable. It has broken inversion symmetry C_i in a regular array and finite gap E_m to the lowest triplet state. The spin-BOW is exact in finite systems at a special point. Its elementary excitations are spin-½ solitons that connect BOWs with opposite phase. The same patterns of spin densities and bond orders appear in the BOW phase of Hubbard-type models. Infrared (IR) active lattice phonons or molecular vibrations are derivatives of P, the polarization along the stack. Molecular vibrations that are forbidden in regular arrays become IR active when C_i symmetry is broken. 1:1 Alkali-TCNQ salts contain half-filled regular TCNQ⁻ stacks at high temperature, down to 100 K in the Rb-TCNQ(II) polymorph whose magnetic susceptibility and polarized IR spectra indicate a BOW phase. More complete modeling will require explicit electronic coupling to phonons and molecular vibrations.