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

In this work, we investigated polarization-resolved photoinduced carrier relaxation dynamics associated with charge ordering (CO) in geometrically frustrated organic conductors theta-(BEDT-TTF)(2)MZn(SCN)(4) (M = Rb, Cs) as a function of temperature. In theta-Rb (M = Rb), a polarization-dependent (anisotropic) and independent (isotropic) dynamics show similar temperature dependence. Moreover, discontinuous changes in transient reflec-tivity amplitude and an enhancement of a relaxation time are observed only upon heating near T-co = 195 K. This is associated with the first order phase transition due to long-range CO. However, in 0-Cs (M = Cs) where the frustration is stronger than that in 0-Rb, as temperature decreases below T-nco asymptotic to 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 asymptotic to 50 K, which is lower than the charge glass transition temperature of T-g similar to 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.