Study on Electrical Conductivity and Hall Effect Spectra in a Non-Cuprate High-Tc Superconductive Oxide

点击翻译按钮获取中文摘要

The present study was performed with the aim of investigating the mechanisms of semiconductor-superconductor transition and electrical conductivity, electronic state and electronic structure in a non-cuprate superconductive oxide Ba_1-xKxBiO_3 with high-Tc (Tc_<max>=32K) in the superconducting phase. The study includes (1) synthesis and characterization of single crystals and (2) studies of electrical conductivity, Hall effect, optical spectroscopy, magnetic property and photoconductivity and the magnetic field effect.1.Single crystals of Ba_1-xKxBiO_3 with little potassium deficiency in wide composition range covering from semiconductive to superconducting phases were synthesized by an electrochemical method with a composition (x) control technique by changing the amount of water vapor in atmosphere covering melt. We extended our studies to the successful fabrication of tunnel structure all composed of Ba_1-xKxBiO_3 by adopting this x control technique during the crystal growth.2.From … More the detailed analysis of the Hall effect in superconductive Ba_1-xKxBiO_3 (x=0.42), we clarified the importance of optical phonon scattering of electrons in the mechanism of conductivity. Temperature dependence of the Hall effect has been for the first time studied in the semiconductive phase (x<0.3). Activation type behavior of positive hole density was observed for 0*x*0.19. It was found that mass of positive hole is extremely heavy. For x=0.19, the contribution of electrons to conductivity is clearly observed above 200K.Photoconductive signal has been observed for the first time in semiconductive Ba_1-xKxBiO_3. In order to investigate the phenomenon in details, further research is on progress. By analyzing effective charges of ions from the previously reported experimental results of phonon spectrum and XPS,a much more covalent character of BaBiO_3 than expected by the band calculation has been observed. Thus, we reconfirmed that semiconductive nature of BaBiO_3 and semiconductive phase of related superconductors should be explained without the scenario of split-off by 3D-CDW formation of the wide band composed of Bi (6s) -O (2p). Less