Neutron-scattering study of the "one-dimensional" conductor K2Pt(CN)4Br0.3.3.2D2O (KCP)

Abstract

The triple-axis neutron technique has been used to make a detailed temperature-dependent study of the scattering associated with the $2k_F$ instability discovered by Comes et al. and subsequently studied by Renker et al. in the "one-dimensional" (1-D) conductor ${\mathrm{K}}_2$Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$ ${\mathrm{Br}}_{0.3}$.3.2${\mathrm{D}}_2$ O(KCP). At low temperatures, they found that the elastic scattering on the planes perpendicular to the conducting $c$ axis at $Q_z=2\mathrm{}{k}_F$ was confined to relatively narrow peaks, and that with increasing temperature these peaks became diffuse perpendicular to $\overrightarrow{\mathrm{c}}$. We find that the elastic scattering can be quantitatively described by assuming there are static (or quasistatic) charge-density waves (CDW) in the parallel Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$ chains, with a temperature-dependent interchain correlation length. The observed asymmetry of the scattering perpendicular to $\overrightarrow{\mathrm{c}}$ is due to the variation of the structure factor of the Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$ complexes, which respond as a unit to the CDW. At elevated temperatures, the elastic scattering approaches the 1-D form expected for uncorrelated CDW's in "chains" of Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$. The temperature variation of the amplitude of the CDW in each chain is obtained from the data, and is found to decrease slowly with increasing temperature above 80 K. The width of the scattering along the chain direction remains very narrow at all temperatures, and for $T\lesssim 160$ K a lower limit to the intrachain correlation length of 300 Å has been set. The inelastic scattering in the vicinity of the $2k_F$ instability extends to lower energies than previously observed and in fact appears to fall to $E\approx 0$ for all temperatures in the range 80-300 K. We interpret these results to indicate that at room temperature the CDW distortion has already manifested itself in the lattice, and that the development of interchain correlations at low temperatures is not directly related to a Peierls-type transition.