Estimation of charge-ordering patterns inθ−ET2MM′(SCN)4(MM′=RbCo,RbZn,CsZn)by reflection spectroscopy

Temperature dependence of the polarized reflectance spectra have been measured on single crystals of $\theta -{\mathrm{ET}}_2{\mathrm{MM}}^{\prime }{\mathrm{}\left(\mathrm{SCN}\right)\mathrm{}}_4$ (abbreviated as $\theta {-MM}^{\prime };$ ${\mathrm{MM}}^{\prime }=\mathrm{RbCo},$ RbZn, and CsZn). The reflectance spectra of the θ-RbCo and θ-RbZn salts exhibit drastic change around 190 K for the samples cooled slowly $(dT/dt\sim -0.1\mathrm{K}/\mathrm{m}\mathrm{i}\mathrm{n}).\mathrm{}$ This spectral change disappears for the samples cooled rapidly $(dT/dt\sim -1\mathrm{K}/\mathrm{m}\mathrm{i}\mathrm{n}).\mathrm{}$ The reflectance spectra of the θ-CsZn salt exhibit Drude-like features down to 20 K. Charge-ordering patterns on these salts are deduced from the spectral analysis using the mean-field calculations.