H.Y. Tang

Hall effect and resistivity in metallic Ba1−xKxBiO3 single crystals: Absence of 1/T dependence in Rh and linear-to-quadratic evolution of ϱ(T)

Abstract Normal-state Hall effect and resistivity were measured in single crystals of Ba1−xKxBiO3 (BKBO) grown by an electrochemical process. They were metallic in resistivity ϱ(T), showing a linear-to-quadratic evolution in temperature dependence with the doping. All had negative Hall coefficients Rh with negligible temperature dependence. These results can be described within the Fermi-liquid framework. The evolution in the temperature-dependence of ϱ(T) likens BKBO to the cuprates, while the absence of any 1/T dependence in Rh distinguishes the cubic perovskite from its layered relatives.

A study of Bi-Pb-Sr-Ca-Cu-O/Ag tape prepared by the jelly-roll process

We combined the electrophoretic deposition technique and the jelly-roll process to fabricate Bi-Pb-Sr-Ca-Cu-O (BPSCCO)/Ag tapes. A thick film of BPSCCO on an Ag substrate was prepared by electrophoretic deposition. The BPSCCO/Ag sheet was rolled up to the jelly-roll configuration and then cold rolled to form tape with six superconducting layers. The critical current density of the tape determined by magnetic measurement at 5 K in a 0.2 T field was about 6.5*104 A cm-2 and decreased to 1.3*104 A cm-2 near 5 T.

Crystallization of La2-xNaxCuO4 superconductor by low-temperature electrochemical deposition

Abstract Crystalline La 2- x Na x CuO 4 was obtained by a low-temperature anodic electrocrystallization method in a sodium hydroxide molten flux. A 2 h experiment can produce plate-like crystals with 0.1 mm×0.1 mm facets in a - b -direction and 32 K superconductivity as-grown.

Structural characteristics and phase separation of superconducting Fe1+ySe1−xTex nanowires

Superconducting Fe1+y Se1?x Te x nanowires with considerable differences in the diameter and aspect ratio were synthesized via annealing thin films with particular composition prepared by pulsed laser deposition. Using transmission electron microscopy and related techniques, we found they were all crystallized in a PbO-type tetragonal structure with a growth direction of [010] (equivalent to [100]). All the nanowires were covered by a thin amorphous oxide layer (?5 nm) determined by electron energy loss spectroscopy. Chemical analysis and the calculated lattice constants reveal the fact that these nanowires are different from chemical compositions. High-resolution electron microscopy images show that the two thick nanowires (with diameters ?80 nm) are composed of multiple phases, while the thin nanowires (with diameters of 14 and 22 nm) are single phase. Digital moir? analysis reveals that the thin nanowires contain surface defects, as well as dislocations, inside the nanowires.

Thermopower of overdoped Ba:KBiO and Ni-substituted 214 cuprates

Abstract We have measured the thermoelectric power and resistivity of two different superconductor series as the doping level is increased into the overdoped regime where Tc decreases below its maximum value: Ba1−xKxBiO3 single crystals with K content up to x=0.55, and La2−xSrxCu1−yNiyO4−δ ceramics with Sr content up to x=0.57. For overdoped samples in both series, thermopowers that are small and negative are found. In overdoped BaKBiO, the thermopower is wholly negative from Tc up to the limit of measurement near room temperature, and the temperature coefficient of thermopower is also negative. All the thermopower versus temperature plots for BaKBiO show a characteristic bend around 150 K, with a smaller temperature coefficient above 150 K. As Ni is substituted for Cu in the 214 cuprate series, there is a substantial increase in the thermopower for Sr concentration x=0.15, but no such increase is seen in the overdoped samples x∼0.5.

Thermoelectric power of superconducting Ba-K-Bi-O crystals

Abstract We have measured the thermopower and resistivity of Ba 1- x K x BiO crystals with superconducting transition temperatures T c close to 30 K. The thermopower is very small, decreasing above T c as temperature increases up to about 150 K; at higher temperatures, a flattening of the temperature dependence or an increase is observed. These features are compared to those seen in the cuprate high-temperature superconductors. The resistivitu increases approximately linearly with temperature, but the slope lessens somewhat below about 100 K.

Fracture toughness measurements of the superconductor Ba0.62K0.38BiO3 by use of the indentation technique

The fracture toughness of a single crystal of Ba0.62K0.38BiO3 was determined. Superconducting Ba0.62K0.38BiO3, prepared by the low-temperature electrocrystallization process, leads to millimeter-sized crystals with TC = 32 K. The Vickers microhardness measurements were made at room temperature for test loads between 50 and 2000 g for a peak-load constant of 10 s. Indentation crack length measurements gave a toughness KC = 1.59 ± 0.07 MPa m12, above a threshold contact load for radial crack initiation of approximately 200 g. It can be seen that BKBO is much less tough than traditional structural ceramics such as A12O3. On the other hand, the hardness of the single crystal was 2.8 ± 0.4 GPa.