  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-1/2 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.
