Et2Me2Sb[Pd(dmit)2]2 (dmit=1,3-dithiol-2-thione-4,5-dithiolate,see Figure 1 (a)) has a two-dimensional triangular lattice of Pd(dmit)2 dimers and shows a unique insulator to insulator phase transition. The schematic view of the Pd(dmit)2 layer of the high- and low-temperature phases are shown in Figures 1 (b) and (c), respectively. The system has a half-filling character due to the very strong dimerization. The strong charge correlation turns the high-temperature phase into a dimer Mott insulating phase (DM phase, hereafter), which only has monovalent dimers. This material exhibits a real space ordering of charges in the low-temperature phase, resulting in another type of insulating phase. The valence of the dimers is not uniform, and the two types of dimers, which have different intermolecular spacing, are ordered as shown schematically in Figure 1(c). The microscopic mechanism involved in this low-temperature phase is based on the unique nature of the multi-level electronic structure of the dimer of Pd(dmit)2 instead of the inter-site Coulombic repulsion (Figure 1(d)) . Thus, this low temperature phase is referred to as the ‘charge-separated’ (CS) phase to distinguish it from the conventional charge-ordering phase.
Figure 1. (a) Molecular structure of Pd(dmit)2. (b) Schematic crystal structure of the high-temperature dimer Mott insulating phase of Et2Me2Sb[Pd(dmit)2]2 in the Pd(dmit)2 plane. (c) Schematic crystal structure of the low-temperature charge-separated phase in the Pd(dmit)2 plane. (d) Schematic energy diagram of the Pd(dmit)2 monomer and dimers. Closed circles represent electrons.
We found the photoinduced insulator-to-insulator phase transition in this system. This unique CS phase is clearly identified by the reflectivity spectrum . The reflectivity shows characteristic peak structures in the near-IR range (solid lines in Figure 2 (a)) that are due to the intra-dimer photoexcitation between the bonding and anti-bonding states of the molecular orbitals (indicated by arrows in Figure 1(d)). These peak energies directly reflect the degree of dimerization. The transient reflectivity spectra at 50 K and 0.5 ps after photoexcitation are measured by Pump-Probe type time-resolved spectroscopy (Dt~150 fs) and plotted in Figure 2(a) as closed and opened circles. We reproduced the obtained spectra quantitatively by assuming a homogeneous state of the surface, and the dielectric function (e) can be described by a linear combination of in the DM (eHT) and initial (eLT) states. The detailed procedure of the analysis was described in ref.  and the references therein. The photoinduced state is the DM phase judging from this correspondence of the calculated curve with the experimental data as shown in Figure 2(b). Based on the value of fitting parameter, one excitation photon converts approximately five dimers to the photoinduced state at 0.5 ps after the photoexcitation. The conversion rate exceeds 1, suggesting the cooperative nature of the photoinduced phenomena.
Figure 2. (a) Closed and open circles represent the photoinduced reflectivity spectra (E||a-axis) with excitation photon energies of 1.55 eV and 1.18 eV, respectively, at 0.5 ps after photo-irradiation. The solid lines show the spectra without photoexcitation at 50 K (blue, CS phase) and 200 K (red, DM phase). The dashed line shows the simulated reflectivity spectrum using the multi-layer model . (b) Schematic view of the photoinduced phase just after photoexcitation .
The obtained results strongly suggest the occurrence of insulator-to-insulator PIPT, which is a new class of PIPT from the CS to DM phase and is different from the conventional insulator-to-metal PIPT.