Daisuke Shimada

Metal-Insulator Transition in LiTi2-xVxO4

Resistivity, Seebeck coefficient, and the magnetic susceptibility were measured for the sintered pellets to see if a random distribution of Ti and V makes the oxide insulating. The oxide seems to be an insulator in the composition range 0.8 1. It was a sum of the Pauli paramagnetic one of Ti 3+ electrons and the Curie-Weiss one of V 3+ electrons for x <1. A mechanism of the metal-insulator transition was discussed.

Fine Structure in the Tunneling Conductance of a Bi2Sr2CaCu2O8-GaAs Junction

A fine structure was observed above the gap voltage in the tunneling conductance of a Bi 2 Sr 2 CaCu 2 O 8 -GaAs junction at 4.2 K. It is due to electron-phonon interaction, suggesting that this superconductivity is essentially due to phonons.

Temperature dependence of the tunneling conductance of Bi2Sr2CaCu2O8 : phonon contribution to high-Tc superconductivity

Temperature dependence of the tunneling conductance of a Bi 2 Sr 2 CaCu 2 O 8 –GaAs junction was investigated both experimentally and theoretically. The spectral function deduced from the experimental result at 4.2 K is applied to the Eliashberg equations at higher temperatures. The theory well explains the observed smearing of the phonon structures, but there is a small discrepancy between the experimental and the theoretical gap and the former decreases more rapidly than the latter as temperature increases.

Multiphonon Exchange and the High-Tc Superconductivity of the Multilayer Oxide-Superconductor

A multiphonon exchange model is proposed to explain the high critical temperature and its increase with the number of CuO 2 layers between Bi(Tl) 2 O 2 layers in copper oxides. In this model, the Cooper pair is formed through the exchange of multiphonons on different CuO 2 layers.

Tunneling conductance of a Bi2Sr2CaCu2O8-GaAs junction : temperature dependence of the gap

Good correspondence between the structure in the tunneling conductance curve and the phonon density of states shows that they are really due to phonons at least in a single phonon frequency range. The structures disappeared at T c suggesting that they are due to an exchange of phonons and not due to the inelastic tunneling. From the phonon positions, the gap parameter was found to have the BCS like temperature dependence. 2Δ(0)/ k B T c =7.1. It is isotropic in the CuO 2 plane. A conventional method to determine the gap from the conductance peak-to-peak distance is not bad.

Tunneling conductance of a Bi2Sr2CaCu2O8-SnO2 junction along the c-axis

Abstractfine structure was observed in the conductance curve of a tunneling junction composed of a single crystalline Bi2212 and an evaporated SnO2 film. It is similar to those of Bi2212-GaAs mechanical junctions and there is a certain correspondence between the structure and the phonon density of states. Thus the previous conclusion that the structure is due to phonons has been complemented by this work. The energy gap 2Δ was 57 meV at 13 K and Tc was 78 K. 2Δ(0)/kBTc is then 8.3. Δ(T) showed the BCS-like temperature dependence.

Tunneling conductance of a Bi2−xPbxSr2Ca2Cu3O10−y-SnO2 junction

Fine structures were observed in the tunneling conductance of a sintered Bi 2223-SnO2 junction. The structures are weaker than those of a single crystalline Bi2212. They correspond to the Raman spectrum of Bi2223 and approximately to the phonon density of states of Bi2212. The structures must therefore be phonon structures, and phonons may contribute substantially to highTc superconductivity. Multiphonon structures are scarcely discernible. Hence we propose a new model, in place of the prior multiphonon model, to explain the rapid increase inTc with the CuO2 layer number. 2Δ (21 K)=68±4 meV inTc=98±5 K. 2Δ (0)/kBTc is 8.1±0.9. The temperature dependence of the gap was also observed and discussed.

Reproducibility of Phonon Structures in the Tunneling Conductance of Bi2Sr2CaCu2O8.

Reproducibility of phonon structures in the tunneling conductance of Bi 2 Sr 2 CaCu 2 O 8 is discussed in relation to their positions and intensities. As for the positions, correspondences with the phonon density of states are certainly observed. As for the intensity-ratios among structures corresponding to different phonon-modes, the reproducibility is not so certain as for the positions. As for the absolute intensity, which reflects the interaction strength, the apparent reproducibility is quite poor.

Reproducibility of Phonon Structures in the Tunneling Conductance of Bi2-xPbxSr2Ca2Cu3O10-y

Phonon structures appeared in the tunneling conductance of a Bi2-xPbxSr2Ca2Cu3O10-y-GaAs junction. They are similar to the previous structures obtained for SnO2 junctions. There is no large structure other than that of the phonon in a single phonon frequency range. Above that range, tiny peaks appeared up to 180 meV, which may be anharmonic phonon structures. From the phonon positions, 2Δ is determined to be 60±4 meV and from the temperature of disappearance, Tc is estimated to be 95±6 K. Thus, 2Δ/kBTc is 7.3±1.0.

Tunneling Conductance of a Bi2Sr2CaCu2O8-GaAs Junction along thec-Axis

Anisotropy in the energy gap of Bi2212 was investigated by using a point contact tunneling junction and it was noted that there was very little anisotropy. When a GaAs needle was contacted perpendicular to the c -plane, at least two kinds of conductance curves were obtained, one with a gap of 42 meV and the other with a gap of 64 meV. In the latter, phonon structures extended to a higher voltage and a three-phonon process was observed similar to our multiphonon exchange model for superconductivity.