Collective excitation of a short-range charge ordering in θ -(BEDT-TTF) 2 CsZn(SCN) 4

We find a characteristic low-energy peak structure located in the range 100-300 cm-1 in the optical conductivity spectra of a quasi-two-dimensional organic compound with a triangular lattice, θ-(BEDT-TTF)2CsZn(SCN)4, in which two different types of short-range charge orderings (COs) coexist. Upon lowering the temperature, the low-energy peak becomes significant and shifts to much lower frequencies only for the polarization of Ea, in contrast to the other broad electronic bands in the midinfrared region. On introducing disorder, the low-energy peak is strongly suppressed in comparison with the broad electronic bands. This result indicates that the low-energy peak is attributed to a collective excitation that originates from the short-range CO with a relatively long-period 3×3 pattern. The present results shed light on the understanding of the low-energy excitation in the glassy electronic state, where the charge degrees of freedom remain at low temperatures.