Papers by Author: Masato Kakihana

Abstract: We investigated the influence of photoexcited nonequilibrium electrons in Bi4Ti3O12-d which contains a high density of oxygen vacancies. As increasing oxygen deficiency, the photocurrent undergoes a rapid enhancement, accompanied by a remarkable change of kinetics. In heavily oxygen deficient Bi4Ti3O12-d (d=0.45), we observed dispersive photoconductivity, a behavior resembles with persistent photoconductivity phenomena in some perovskites. We argue that oxygen vacancies acting as trapping centers for electrons are linked to dispersive diffusion in the perovskite blocks, which control carrier recombination.

Abstract: We have performed neutron powder diffraction (NPD) experiments on polycrystalline powders with nominal compositions (Ca_0.5La_0.5)(Ba_1.25La_0.75)Cu₃O₇- δ , (Ca_0.6La_0.4)(Ba_1.15La_0.85)Cu₃O₇-δ and (Ca_0.8La_0.2)(Ba_0.95La_1.05)Cu₃O₇-δ . The diffraction patterns, analysed by the Rietveld method, show that all samples consist mainly of a tetragonal Y-123 type phase. Unit cell parameters a and c shorten as the calcium content increases: a = 3.8660(2), 3.8634(3), and 3.8624(5) Å; c = 11.6325(11), 11.6143(14), and 11.5822(20) Å for x-values 0.5, 0.6, and 0.8, respectively. For the x = 0.6 and 0.8 samples the Rietveld refinement of calcium occupancies and EDX analysis suggest that the actual composition is closer to x ≈ 0.5. However, since the lattice parameters do change, it is also suggested that at these higher doping levels calcium does enter the Y- site to a larger extent than for the x = 0.5 composition. This is also in accordance with previously reported values for the T_c, which decreases slighty as x changes from 0.5 to 0.6 and has a pronounced change from 80 K to 73 K for x = 0.6 and 0.8, respectively.