Fluctuations and order in a one-dimensional system. A spectroscopical study of the Peierls transition in ${\mathrm{K}}_2$Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$${\mathrm{Br}}_{0.3}$·3(${\mathrm{H}}_2$O)

Using optical spectroscopy in the energy range 1 meV-6 eV as a tool we have studied the Peierls transition in ${\mathrm{K}}_2$Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$ ${\mathrm{Br}}_{0.3}$·3(${\mathrm{H}}_2$O). We find a pseudogap at 0.2 eV at room temperature which transforms into a real gap as $\mathrm{T}\to 0$ without significantly changing the gap energy. From this we infer that although no long-range order exists at room temperature the rms order parameter has already reached its zero temperature value. At $T\approx 0$ we find a strong structure in $\sigma \left(\omega \right)$ at $\omega \sim 15$ ${\mathrm{cm}}^{-1\mathrm{}}$ which is assigned to an oscillation of the pinned charge-density wave (CDW). A simple phenomenological model allows us to estimate the contribution of the thermally unpinned CDW to the dc conductivity at $T>\mathrm{}0\mathrm{}$. We conclude that the contribution is 50% or less of the total conductivity.