In this work, polarized pump-probe spectroscopy was carried out to investigate the effects of a structural ordering of molecules on photoinduced phase separation (PIPS) in the organic conductors $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_{2}X$ [$X=\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br} (\kappa -Br)$ and $\mathrm{Cu}{\left(\mathrm{NCS}\right)}_2$ ($\kappa$-NCS)]. We found that the anisotropic response for the probe polarization appeared at around $T_{\mathrm{g}}$, where the glasslike structural transition occurs. The anisotropy can be a result of a transient destruction of the local ordering of molecules, indicating a connection between the glasslike transition and PIPS. Moreover, we found that the PIPS response gradually develops with decreasing temperature in $\kappa$-Br, whereas it steeply increases in $\kappa$-NCS. This qualitative difference suggests that the structural ordering caused by a PIPS is more crucial in $\kappa$-NCS than in $\kappa$-Br.

• Received 23 January 2017
• Revised 15 September 2017

DOI:https://doi.org/10.1103/PhysRevB.96.134311