To elucidate the underlying nature of the hidden order (HO) state in heavy-fermion compound URu(2)Si(2), we measure electrical transport properties of ultraclean crystals in a high field, low temperature regime. Unlike previous studies, the present system with much less impurity scattering resolves a distinct anomaly of the Hall resistivity at H;{*} = 22.5 T, well below the destruction field of the HO phase = or approximately 36 T. In addition, a novel quantum oscillation appears above a magnetic field slightly below H;{*}. These results indicate an abrupt reconstruction of the Fermi surface, which implies a possible phase transition well within the HO phase caused by a band-dependent destruction of the HO parameter.

Since the discovery of the Verwey transition in magnetite, transition metal compounds with pyrochlore structures have been intensively studied as a platform for realizing remarkable electronic phase transitions. We report on a phase transition that preserves the cubic symmetry of the β-pyrochlore oxide CsW2O6, where each of W 5d electrons are confined in regular-triangle W3 trimers. This trimer formation represents the self-organization of 5d electrons, which can be resolved into a charge order satisfying the Anderson condition in a nontrivial way, orbital order caused by the distortion of WO6 octahedra, and the formation of a spin-singlet pair in a regular-triangle trimer. An electronic instability due to the unusual three-dimensional nesting of Fermi surfaces and the strong correlations of the 5d electrons characteristic of the pyrochlore oxides are both likely to play important roles in this charge-orbital-spin coupled phenomenon.

Polarized far-infrared (FIR) spectroscopic measurements and FIR magneto-optical studies were performed on the inorganic spin-Peierls compound CuGeO3. An absorption line, which was found at 98 cm −1 in the dimerized phase (D phase), was assigned to a folded phonon mode of B3u symmetry. The splitting of the folded mode into two components in the incommensurate phase (IC phase) has been observed for the first time. A new broad absorption centered at 63 cm −1 was observed only in the E b axis polarization, which was assigned to a magnetic excitation from singlet ground state to a continuum state. 78.66.-w,78.30.-j,63.22.+m

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