Photoinduced phase separation with local structural ordering in organic molecular conductors

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 $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}₂X$ [$X=\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}₂]\mathrm{Br} (\ensuremath{\kappa}\ensuremath{-}Br)$ and $\mathrm{Cu}{(\mathrm{NCS})}₂$ ($\ensuremath{\kappa}$-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 $\ensuremath{\kappa}$-Br, whereas it steeply increases in $\ensuremath{\kappa}$-NCS. This qualitative difference suggests that the structural ordering caused by a PIPS is more crucial in $\ensuremath{\kappa}$-NCS than in $\ensuremath{\kappa}$-Br.