Interface microstructure in epitaxial YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox ramp-type Josephson junctions

The interfaces in ramp-type YBa₂Cu₃O_x/PrBa₂Cu₃O_x/YBa₂Cu₃O_x Josephson junctions with different ramp slopes were characterized electrically and structurally. From the cross-sectional TEM images, it has been found that the epitaxy remains through all layers even on the ramp surface, providing the ramp angle is less than 45°. No big defects such as grain boundaries and secondary phases occurred at the interface. The major defects which appeared at the interface are antiphase boundaries which were formed by lattice shifts with c/3 unit between PrBa₂Cu₃O_x and YBa₂Cu₃O_x. The interface between YBa₂Cu₃O_x base-layer and PrBa₂Cu₃O_x barrier is more defective in comparison with that between PrBa₂Cu₃O_x barrier and top-layer. Alteration of the ramp slope from 10° to 45° leads to a significant increase in the density of defects near the base-YBa₂Cu₃O_x/PrBa₂Cu₃O_x interface. The average roughness of the ramp surface created by ion milling is about 10 nm, which seems insensitive to the ramp slope. The damaged ramp surface could be repaired during the subsequent deposition process, leading to a negligible interface resistance.     

Band structure of YBa2Cu3Ox in relation with the oxygen vacancy distribution

The electronic band structure and the density of states of the YBa₂Cu₃O_x superconductor are computed for x = 7 using the extended Hückel scheme. Two ordered arrangements of the oxygen vacancies in the basal Cu-O planes have been considered. It is shown that the oxygen vacancy distribution plays a major role in the electronic structure of this compound and deeply influences the density of states at the Fermi level.     

Local probe (170Yb3+) measurements of magnetic fields in YBa2Cu3Ox

We introduce the technique of studying the field dependence of the electro-nuclear energy levels of a rare earth to measure the magnetic field present at the rare earth/yttrium site in YBa₂Cu₃O_x. Measurements were made by¹⁷⁰Yb Mössbauer spectroscopy. The hyperfine spectrum of the ground state Kramers doublet for Yb³⁺ ions diluted into this matrix is sensitive to fields in the range 100 to 2000G. Flux penetration and trapping at the local site level have been measured in superconducting samples. A molecular field exists on the rare earth site in non superconducting samples suggesting that the ordered Cu2 magnetic moments are intrinsically non colinear.     

Contributions to photodoping in oxygen-deficient YBa2Cu3Ox films

We report our studies on the superconducting and normal-state properties of metallic thin films ( 52 K) exposed to long-term white-light illumination (photodoping). It was observed that the effects of photoexcitation strongly depended on the temperature at which the photodoping was performed. At low temperatures, both the Hall mobility and the Hall number were photoenhanced, whereas, at temperatures slightly below room temperature, the Hall mobility initially showed an abrupt increase followed by a long-term decrease, and the Hall number increased even stronger than at low temperatures. The enhancement of the film's superconducting transition temperature T_c, caused by photodoping, exhibited the same temperature dependence as the enhancement of the Hall number, being largest ( 2.6 K) at high temperatures. From the asynchronous behavior of the Hall quantities, we conclude that both the photoassisted oxygen ordering and charge transfer mechanisms contribute to photodoping. The relative contributions of both mechanisms and, thus, the electronic properties of the photoexcited state are strongly temperature dependent. Studies of the relaxation of the photoexcited state at 290 K showed an unexpectedly short relaxation time of the Hall mobility after termination of the illumination. The relaxation saturated somewhat below the initial, undoped value, similarly to the decrease of the Hall mobility, observed upon long illumination. These latter findings give evidence for a competition between the oxygen ordering and thermal disordering processes during and after the photoexcitation in the high-temperature range. Received: 13 October 1997 / Accepted: 19 November 1997     

PREPARATION OF a-AXIS ORIENTED YBa2Cu3Ox THIN FILMS

The a-axis oriented YBa₂Cu₃O_x(YBCO) thin films could be grown on (100) SrTiO₃(STO) substrates with STO buffer layers by dc and rf magnetron sputtering either by low-ering the deposition temperature, or by using a self-template technique. For the latter, the resistivity of the thin film at 290K along the substrate [001] direction is about four times larger than that in the [010] direction. The zero resistance temperatures T_c0 are 89 K in both directions. So high quality a-axis oriented YBCO thin films can be prepared by the self-template technique. Also the T_c0 increase monotonously with the reduction of the thickness of the YBCO seed layer.     

Thermoelectric power of YBa2Cu3Ox in equilibrium with gas phases

Thermoelectric power (TP) and electrical conductivity (EC) measurements were performed for YEa₂Cu₃O_x at 1128 K under controlled oxygen partial pressure varying between 50 and 10⁵ Pa. Three regimes are observed for the electrical properties. At low both TP and EC remain constant with . In the medium range sharp changes of both electrical parameters occur; TP changes sign from positive above 4×10² Pa to negative below this value. In the high region (>7.6×10³ Pa) TP vs log exhibits two slopes; 5.1 below 1.5×10⁴ Pa and 8.4 above this value. The slopes can be discussed in terms of the defect structure involving singly and doubly ionized oxygen vacancies below and above 7.6×10³ Pa, respectively.     

采用X-射线衍射测定YBa2Cu3Ox中的氧含量

本文详细介绍了采用X-射线衍射测定YBa2Cu3Ox中氧含量的实验细节并采用此法测定了六个样品的氧含量。结果表明：分别在空气和氧气中生长的名义成分为123 15wt.!1 0.5wt.?O2的样品的氧含量分别为6.63和6.76。经渗氧处理后，氧含量分别增加至6.88和6.94。在名义成分为123 15wt.!1 0.5wt.%Pt的样品中也得到类似的结果。本文对采用X-射线衍射测定YBa2Cu3Ox中氧含量的合理性和测定精度进行了讨论。     

Ultrasonic measurements on polycrystalline YBa2Cu3O7

Temperature dependent ultrasonic attenuation data of polycrystalline YBa₂Cu₃O₇ at 10 MHz show a plateau between 220 and 250 K, and slope changes at 263, 135 and 97 K. The attenuation does not display a sharp decrease at T_c as would be described in the BCS theory. Around 255 K, the sound velocity in the sample increases from its room temperature value by more than 10%.