Mott transition and heavy fermion state in the pyrochlore Hubbard model

We investigate the interplay between geometrical frustration and strong electron correlation based upon the pyrochlore Hubbard model. In the half-filling case, using the perturbative expansion in terms of electron correlation, we show that the self-energy shows a divergent behavior leading the system into the Mott insulating state, in which quantum disordered spin liquid without magnetic long-range order realizes. In the hole-doped case, we obtain heavy-fermion-like Fermi liquid state. We also calculate the neutron cross section which is well consistent with recent neutron scattering experiments for itinerant pyrochlore systems.Comment: 5 pages, 5 figures, revised version accepted for the publication in Phys. Rev.

When does "Time" become "Observable" ?

We investigate some spectral properties of time operators which are obtained through Canonical Commutation Relation (CCR) and Positive Operator Valued Measure (POVM) of quantum physics. In addition, we re-interpret the spectral properties of time operators from the standpoint of the Copenhagen Interpretation, especially, of W.Heisenberg.Comment: 9 pages, The Ryukoku Journal of Humanities and Science, Vol.35 No.2, March, 201

Metal-to-Insulator Transition, Spin Gap Generation, and Charge Ordering in Geometrically Frustrated Electron Systems

We investigate a (semi-)metal to insulator transition (MIT) realized in geometrically frustrated electron systems on the basis of the Hubbard model on a three-dimensional pyrochlore lattice and a two-dimensional checkerboard lattice. Using the renormalization group method and mean field analysis, we show that in the half-filling case, MIT occurs as a result of the interplay between geometrical frustration and electron correlation. In the insulating phase, which has a spin gap, the spin rotational symmetry is not broken, while charge ordering exists. The charge ordered state is stabilized so as to relax the geometrical frustration in the spin degrees of freedom. We also discuss the distortion of the lattice structure caused by the charge ordering. The results are successfully applied to the description of the MIT observed in the pyrochlore system ${\rm Tl_2Ru_2O_7}$.Comment: 12 pages, 10 figure

Stable photosensor amplifiers

Minimization of common mode effects in differential amplifier arrangement which processes signals from two high impedance photosensors is achieved by connecting one photosensor in feedback loop of amplifier and using field effect transistors in the input circuit

Pseudogap Phenomena in the BCS Pairing Model

We investigate pseudo-gap phenomena realized in the BCS pairing model with a long but finite interaction range. We calculate the single-particle self-energy in all order exactly in the temperature range where the superconducting fluctuation propagator is Gaussian-like. It is found that vertex corrections to the self-energy which are discarded in the previous studies are crucially important for the pseudo-gap of the single-particle density of states in higher order calculations.Comment: 7 pages, 3 figures, references are added, to appear in J. Phys. Soc. Jpn. vol.71, no.5 (2002