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.

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 frequency response of the multilayered acoustic cloak made of isotropic materials is analyzed by the acoustic scattering theory. When concealing a rigid cylinder, the acoustic total scattering cross section (TSCS) monotonically increases with frequency. If concealing penetrable materials, the cloak-induced reverse-phase monopole and dipole resonances can increase the TSCS around the resonant frequencies, while the in-phase monopole resonance decreases the TSCS. The results are confirmed by numerical simulations of the finite element method. © 2008 American Institute of Physics.

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.