In this work, we investigated polarization-resolved photoinduced carrier relaxation dynamics associated with charge ordering (CO) in geometrically frustrated organic conductors $\theta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_{2}M\mathrm{Zn}{\left(\mathrm{SCN}\right)}_4$ (M = Rb, Cs) as a function of temperature. In $\theta \text{−}\mathrm{Rb}$ (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 $T_{\mathrm{co}}=195\mathrm{K}$. This is associated with the first order phase transition due to long-range CO. However, in $\theta \text{−}\mathrm{Cs}$ (M = Cs) where the frustration is stronger than that in $\theta \text{−}\mathrm{Rb}$, as temperature decreases below $T_{\mathrm{nco}}\approx$ 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 $T_{\mathrm{s}}\approx$ 50 K, which is lower than the charge glass transition temperature of $T_{\mathrm{g}}\sim$ 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.

• Received 4 August 2022
• Accepted 8 December 2022

DOI:https://doi.org/10.1103/PhysRevResearch.5.013024

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society