Femtosecond optical response was investigated on a perovskite-type cobalt oxide, Pr0.5Ca0.5CoO3 during an insulator-metal (I-M) transition accompanied with the change in spin configuration. After photoirradiation at 30 K, the reflectivity showed a sudden and large increase with subsequent variation depending on the observed photon energy. An exact calculation of Maxwell's equations for the Pr0.5Ca0.5CoO3 after the photoirradiation enabled us to observe the ultrafast dynamics of I-M phase transition and the motion of the photonically created metallic domain at the velocity of ultrasonic wave.

We fabricated a one-dimensional magnetophotonic crystal (MPC), in which a magnetic layer of Co–ferrite (∼40 nm in thickness) is sandwiched by a couple of dielectric multilayer reflectors with (SiO2/TiO2)×7 structure. The Co–ferrite layer was synthesized by ferrite plating from aqueous solution at 90 °C. The MPC is so designed to enhance magneto-optical Faraday rotation θF at the Fabry–Pérot resonance wavelength λ of the multilayer structure. θF was observed to increase by factor of ∼5.4, but at λ=∼620 nm which is deviated from the target resonance (λ=740 nm) and at very weak transmissivity. This may be because the Co–ferrite layer is rough in surface and smaller in thickness than the target thickness at which Fabry–Pérot resonance is designed to occur.

A theory to describe the dielectric anomalies and the ferroelectric phase transition induced by oxygen isotope replacement in SrTiO 3 is developed.The proposed model gives consistent explanation between apparently contradictory experimental results on macroscopic dielectric measurements versus microscopic lattice dynamical measurements by neutron scattering studies. The essential feature is described by a 3-state quantum order-disorder system characterizing the degenerated excited states in addition to the ground state of TiO 6 cluster. The effect of isotope replacement is taken into account through the tunneling frequency between the excited states. The dielectric properties are analyzed by the mean field approximation (MFA), which gives qualitative agreements with experimental results throughout full range of the isotope concentration. The phase diagram in the temperature-tunneling frequency coordinate is studied by a QMC method to confirm the qualitative validity of the MFA analysis.

We study the ordered phases and the phase transitions in the stacked triangular antiferromagnetic Ising (STAFI) model with strong interplane coupling modeling CsCoCl 3 and CsCoBr 3 . We find that there exists an intermediate phase which consists of a single phase of so-called partial disordered (PD) type, and confirm the stability of this phase. The low temperature phase of this model is so-called two-sublattice ferri magnetic phase. The phase transition between the PD phase and two-sublattice ferri magnetic phase is of the first order. This sequence of the phases is homomorphic as that in the threedimensional generalized six-state clock model which have the same symmetry of the STAFI model. By studying distributions of domain walls in one dimensional chains connecting layered triangular lattices, we clarify the nature of the phase transition and give an interpretation of little anomaly of the specific heat.

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