Toshio Nagashima

Superconductivity in Tl1.5SrCaCu2Ox

We have discovered superconductivity in the Tl-Sr-Ca-Cu-O system. Depending on the method of sample preparation, a number of superconducting phases with different Tcs appeared. Resistivity data suggest that a Tc of about 100 K is to be attained in Tl1.5Sr1Ca1Cu2Ox.

One-Center Molecular Orbital Wave Function for Methane

The validity of the spherical approximation for the potential due to nuclei used in the calculation of molecules with good symmetry by means of one-center MO method is examined for methane molecule. Molecular orbital wave functions are set up as harmonic expansion about the central carbon nucleus, and energy expectation values are calculated with respect to the exact Hamiltonian. Results obtained by using various approximations are compared. It is shown that while the effect of the angular dependent part of the nuclear potential on the energy value is not so large as is expected, the electron charge distribution has fairly large directional dependence caused by the non-central part of the potential.

A New Family of Copper Oxide Compounds with a Tl-O Monolayer: (Tl1-zPbz)(Sr1-xLax)2(Ln1-yCey)2Cu2O9

We have discovered a new family of copper oxide compounds (Tl1-zPbz)(Sr1-xLax)2(Ln1-yCey)2Cu2O9 (Ln=lanthanoid atoms) with a Tl-O monolayer. The crystal structure of these compounds was determined by Rietveld analysis of X-ray powder diffraction data. Its structure can be derived by stacking TlBa2CaCu2O7- and Nd2CuO4-type slabs alternately along the c-axis; they are joined by conductive CuO2 sheets. These compounds do not exhibit superconductivity down to 10 K.

Improving Superconducting Characteristics of Tl-Sr-Ca-Cu-O by Doping with Pb and/or Rare-Earth Elements

We have investigated the effects of doping Pb and/or rare-earth elements on the Tc of Tl-Sr-Ca-Cu-O. Depending on the amount of Pb doped, at least two superconducting phases were formed: the 1223 and 1212 phase having a Tc of 100 K and 60 K, respectively. The 1223 phase is stabilized by doping and increased in volume by annealing at 550°C. In the case of the rare-earth-doped one, the 1212 phase is stabilized and has a Tc of about 90 K. We also investigated the case of samples doped with both Pb and rare-earth elements. A nominal composition of Tl0.7Pb0.3La0.3SrCaCu1.8Oy has a Tc of above 100 K, which is the highest of materials with 2-CuO2 layers.

Crystal structure and characterization of the new Tl(La, Sr)2CuO5-type superconductor

We have discovered a new superconductor Tl(La, Sr)2CuO5 with transition temperature of about 40 K. The crystal structure of this material was determined by Rietveld analysis of powder X-ray diffraction data. The result indicates Cu-substitution for the Tl site and/or deficiency in the Tl site, and location of La and Sr on the Sr site in the Tl-O monolayer TlSr2CuO5 compound: the compounds have general formulas Tl1-xLaSrCu1+xO5 and Tl1-xLaSrCuO5. In both systems, superconducting properties were improved by Cu-substitution for the Tl site and deficiency of the Tl site.

TEMPERATURE DEPENDENCE OF THE CURRENT DISTRIBUTION IN THE QUANTUM HALL REGIME IN A DISTRIBUTED MAGNETIC FIELD

The temperature dependence of the current distribution in a transition region between the adjacent quantum Hall plateaus has been obtained from an analysis of the temperature dependence of the Hall resistance measured in a distributed magnetic field. The distributed magnetic field was produced by a Bi2Sr2CaCu2O8+δ single crystal platelet placed on top of a GaAs/AlGaAs heterostructure. The profile of the magnetic field was established by measurements using a Hall device with multi-probe pairs. The agreement between the experiment and the calculation confirms that there exists bulk edge current even in the transition region at low temperatures and it spreads out into the interior of the bulk region as the temperature increases.

Current distribution in the quantum Hall regime observed in a distributed magnetic field

The temperature and the current dependence of the Hall resistance in a transition region between the adjacent quantum Hall plateaus has been measured in a distributed magnetic field. We used a single crystal platelet of Bi 2 Sr 2 CaCu 2 O 8+δ high temperature superconductor to make a distributed magnetic field. We have found a shift of the transition region between the adjacent quantum Hall plateaus to higher magnetic fields when the temperature or the current was increased. These experimental results reveal that the current distribution is concentrated near the sample edge at low temperature (current) and spreads out to the bulk region as the temperature (current) is increased. Results of calculation assuming appropriate current and magnetic field distribution have reproduced the temperature and the current dependence of the shift of the transition region.

Magnetization behavior of single crystalline Bi2Sr2CaCu2Oy above 60 K

Abstract Magnetic hysteresis curves have been measured by using a miniature Hall probe on two single-crystal platelets of Bi 2 Sr 2 CaCu 2 O y with the magnetic field applied along the c -axis normal to the plane. With the help of detailed calculations of the critical current distribution in a ribbon-shaped sample, the observed hysteresis curves are reproduced on the assumption of the critical-state model and an exponential field dependence of J c . A tentative B - T vortex phase diagram is constructed, and its implications are discussed.

Some Experiments on Flux Pinning in Pb Doped Bi–Sr–Ca–Cu–O System

In order to investigate the mechanism of energy dissipation by irreversible motion of fluxoids, we performed two different types of experiments; the oscillating-pendulum in magnetic field and the magnetic hysteresis including both major and minor loops. Results obtained for Pb-doped Bi-Sr-Ca-Cu-O system at 77 K are presented and some preliminary discussions are made.

Relaxation of local magnetization of single-crystalline Bi2Sr2CaCu2O8 at low temperatures

Abstract The local magnetization and its relaxation rate of a single crystalline Bi 2 Sr 2 CaCu 2 O 8 was measured by a miniature Hall device with two-dimensional electron gas (2DEG) fabricated from a GaAs/Al x Ga 1− x As heterostructure wafer. We investigated the magnetic field and the temperature dependence of the relaxation rate at low temperatures between 100 mK and 4.2 K under the applied field ranging from 0 to 1.7 T. The obtained results show the anomalous field dependence of the relaxation rate with a large enhancement in the fields between 0.5 and 1 T. The relaxation rate shows the crossover behavior from QCC region to thermally assisted region in temperature dependence. The local relaxation theory of a critical current density based on the critical state model realized the similar anomalous field dependence of the relaxation rate, assuming a constant relaxation rate.