Electronic structures of halogen bridged mixed-valence (MV) complexes Cs2Au2X6 (X=I, Br, and Cl)have been investigated by means of reflectivity measurements. We have observed five polarization-dependent opti-cal transitions in the energy range of 0.6-5.5eV. The lower-lying three bands below～3eV are due to the intermolecular transitions from AuX2 to Aux4 molecules, while the others are assigned to the intramolecular transitions. The optical ap Egap increases from 1.31 eV for X=I to 2.04 eV for X=Cl, due to enhancement of the nearest-neighboring Coulomb repulsion V and the Jahn-Teller distortion of AuX4 (S). We further investi-gated extemal pressure effects on the electronic state, and found pressure-induced disappearance of the Raman-active Au-Br stretching modes, which indicates an electronic phase transition from the MV state to a single-valence (SV) state.

We have found a photoinduced phase transition (PIPT) in the mixed-valence (MV) gold complex Cs2Au2Br6 from the MV state (ambient pressure phase) to the single-valence (SV) phase (high-pressure phase) in the presure region from 6.4to 6.8 Gpa. We have observed an accumulation effect of each pulse shot, and have ascribed it to stabilization of the photogenerated SV clusters due to the cooperative Jahn-Teller instability of Au2+ ions. The PIPT shows a distinct threshold behavior against the photon density as well as the shots number, which has been interpreted in terms of te critical nucleation of the photoinjected SV clusters. We further found that the phototransformed SV phase is not stable below 5.5 GPa. and changes into the former MV phase after a long dead time of～104s.

Dynamics of the spin system has been invesdtigated in a ferromagnetic La0.7 Ca0.3 MnO3 film by means of a time-resolved pump-probe method. In a weak excitation condition, the relaxation time T increases critically on approaching the critical temperature Tc (=262K) from the low-temperature side as Tx|T/Tc-1|-1.39 for T>200k. Surprisingly, the same scaling relation holds even after strong excitation in a much wider temperature range down to～10K. We will discuss these behaviors in terms of photoinduced growth of the charge-ordering fluctuations.

Photoexcitation effects on the spin states were investigated for a prototypical spin-crossover complex [Fe(ptz)6] (BF4)2 (ptz=1-propyltetrazole) by the pump and probe technique at 77K. We have found (I) superlinear increase of density nHs of the photoexcited high-spin molecules when the excitation power density P exceeded a critical value (=120mW/cm2) and (ii) optical hysteresis between the P-increasing and the P-decreasing runs. These characteristic features are well reproduced by the Monte Carlo simulation based on the Ising model, in which the spin state of the respective Fe2+ion is represented by the pseudospins.

The authors demonstrate that a class of ultrasonic metamaterial, which is composed of subwavelength resonant units built up by parallel-coupledelmholtz resonators with identical resonant frequency, possesses broadocally resonant forbidden bands. The bandwidths are strongly dependent onhe number of resonators in each unit. The broadening of bands is ascribedo the change of effective acoustic impendence. The coupling effects onhe wave vector and negative dynamic modulus are discussed. Numericalimulations by finite element method further confirm the theoreticalesults. © 2008 American Institute of Physics.DOI: 10.1063/1.2839401