Probing uniform and nonuniform charge ordering with polarized femtosecond optical pulses in geometrically frustrated θ-(BEDT-TTF)_{2}MZn(SCN)_{4} (M=Rb, Cs)

Abstract

In this work, we investigated polarization-resolved photoinduced carrier relaxation dynamics associated with charge ordering (CO) in geometrically frustrated organic conductors θ-(BEDT-TTF)_{2}MZn(SCN)_{4} (M = Rb, Cs) as a function of temperature. In θ-Rb (M = Rb), a polarization-dependent (anisotropic) and independent (isotropic) dynamics show similar temperature dependence. Moreover, discontinuous changes in transient reflectivity amplitude and an enhancement of a relaxation time are observed only upon heating near T_{co}=195K. This is associated with the first order phase transition due to long-range CO. However, in θ-Cs (M = Cs) where the frustration is stronger than that in θ-Rb, as temperature decreases below T_{nco}≈ 150 K, the anisotropic component only slightly decreases while the isotropic one increases gradually without hysteresis. The behaviors indicate that CO is formed nonuniformly, that is, multiple short-range CO domains appear and are distributed with different orientations. Moreover, we found that the anisotropic dynamics with long-relaxation times suppress below T_{s}≈ 50 K, which is lower than the charge glass transition temperature of T_{g}∼ 100 K, whereas the isotropic dynamics grows monotonically. The distinct difference between them suggests the occurrence of a structural transition in which the lattice deforms randomly to match a glassy charge distribution due to the charge-lattice coupling.