S. Uchida

Extensive study of the optical spectra for high-temperature superconducting oxides and their related materials from the infrared to the vacuum-ultraviolet energy region

Optical reflectivity spectra of (La, Sr)2CuO4 and YBa2Cu3Oy have been measured over a wide spectral range as high as 40 eV. Although overall features of the observed spectra are explained well by the results of the available band calculations, the spectra differ significantly from the band calculation in the low-energy region below 5 eV, where the spectrum would presumably be dominated by the transitions between the states involving Cu 3d and O 2p or the charge-transfer (CT) excitation between the O 2p and Cu 3d states. In contrast to the calculation, the observed absorption that probably corresponds to the Cu–O CT process is weak and broad, even for insulating La2CuO4; thus we cannot firmly determine the CT energy that characterizes the electronic states of these cuprates. To estimate this parameter, the spectra of the cuprates are compared with those for isostructural (La, Sr)2MO4 (with M = Ni, Co, Fe) as well as with A-site-substituted Nd2CuO4. From this and also from the measurement of the anisotropy of the spectrum with respect to the Cu–O planes, we estimate the CT energy to be 2–4 eV for both cuprate superconductors. This value is larger than that predicted by the band theory, thus indicating the breakdown or great modification of the band picture in the low-energy region.

Doping effect on the transport and optical properties of P-type and N-type cuprate superconductors

Abstract We discuss the physical meaning of the doping high T c oxides, P-type La 2−x Sr x CuO 4 and recently discovered N-type Nd 2−x Ce x CuO 4−y , based on the experimental results of the Hall effect and the optical reflectivity spectrum. It is suggested that for light doping the Mott-Hubbard picture gives proper description of the transport properties whereas for heavy doping or for higher frequencies the Fermi lquid picture seems more appropriate for both N- and P-type materials.

Static magnetic order in electron-superconductor related compounds

Abstract Using the technique of muon spin relaxation, we have shown that the parent compounds Ln 2 CuO 4- y where [ Ln =( Nd , Sm or Pr )] of the new electron-superconductors exhibit static magnetic order below 300 K. No static magnetic order with any appreciable moment was observed above 4 K in superconducting samples of Nd 2- χ Ce χ CuO 4− y for χ > .155. Measurements on a single crystal of Nd 2 CuO 4− y show magnetic order with a sharp onset at 250 K, followed by a number of changes in the ordered spin direction at T ∼ 65 K and T ∼ 33 K.

Physical Properties of High-Tc Oxide Superconductors — Effect of Doping on the Electronic State

We discuss the physical meaning of the doping high T c oxides, P-type La2−x Sr x CuO4 and recently discovered N-type Nd2−x Ce x CuO4−y , based on the experimental results of the Hall effect and the plasma reflectivity spectrum. It is suggested that for light doping the Mott-Hubbard picture gives proper description of the transport properties whereas for heavy doping or for higher frequencies the Fermi lquid picture seems more appropriate for both N- and P-type materials.

ac-dc conversion effect in Y-Ba-Cu-O ceramics

In Y‐Ba‐Cu‐O ceramics, the dc voltage (Vdc ) induced by an rf current of 2–20 MHz, which had been previously reported as ‘‘reverse ac Josephson effect,’’ was observed at 77–300 K. The observed Vdc is not found to be due to reverse ac Josephson effect but due to nonlinear current‐voltage characteristics, as in the case of BaPb1−xBixO3. However, behavior of the Vdc is different from that in BaPb1−xBixO3, which was related to bulk superconducting transition. The Vdc is observed only in the sample with particular electrical contacts and only when the rf current can flow through the potential electrical contacts. The origin of the rf‐to‐dc conversion effect in Y‐Ba‐Cu‐O is ascribed to local nonlinear resistance near potential contacts.

Extensive Study of the Optical Spectra for High Tc Cuprates and Related Oxides

The perovskite-related oxides, (La1-XSrx)2CuO4(LSCO) and YBa2Cu3Oy (YBCO), have been established to be high-Tc superconductors, which poses a fundamental question: why do such extraordinarily high values of Tc seem to be restricted to. Cu oxides? It has also been pointed out that these Cu-oxides show quite peculiar behavior in various physical properties, in particular there is generally a metal to semiconductor transition at a certain composition, x ≃ 0.025 for LSCO and y∿6.35 for YBCO /1/.

AC-DC Conversion Effect in Ceramic Superconductor

The dc voltages(Vdc) induced by an rf current(2–2OMHz), which had been previously interpreted as the reverse ac Josephson effect, were investigated in BaPb1-xBixO3 and YBa2Cu3O7-y, by using second harmonic superposition on the rf current and differential resistance measurement. In both cases, the Vdc is not due to the reverse ac Josephson effect but due to nonlinear current-voltage characteristics. In BaPb1-xBixO3, the nonlinear but symmetric I-V curve is induced by the superconducting transition(≃10 K) and thus the Vdc is observed only at around 10 K. In YBa2Cu3O7-y, local nonlinear resistance near two electrical contacts for potential measurement is responsible for this effect and thus the Vdc is observed over wide temperature range up to 300 K.