Estimation of charge-ordering patterns in θ-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-ET2M M' (SCN)(4) (abbreviated as theta-M M' ; M M' = RbCo, RbZn, and CsZn). The reflectance spectra of the theta-RbCo and theta-RhZn salts exhibit drastic change around 190 K for the samples cooled slowly (dT/dt similar to -0.1 K/min). This spectral change disappears for the samples cooled rapidly (dT/dt similar to -1 K/min). The reflectance spectra of the theta-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.