Li Chuan-Yi

An ultrasonic study of energy-gap and granular characteristics of Y1Ba2Cu3O7-y superconducting ceramic

Ultrasonic attenuation measurements of Y1Ba2Cu3O7-y have been carried out between 100 and 10 K by using 10-25 MHz longitudinal acoustic waves. It was shown from the experimental results that when T<Tc the attenuation was apparently decreased and the granular superconducting characteristics of the samples were remarkable. It was indicated from the attenuation curves that the superconductivity of Y1Ba2Cu3O7-y is related to the electron pairing states. By means of introducing a transition volume distribution function fT, the calculated attenuation curves fitted the experimental ones very well, leading to a result of 2 Delta (0)=3.5kBTc. This suggests that the existence of a superconducting energy gap and its temperature dependence should be still the same with the BCS theory, although the mechanism of the electron coupling may be quite different.

Critical Scaling of Extended Power Law I−V Isotherms near Vortex Glass Transition

In view of the question about the vortex glass theory of the freezing of disordered vortex matter raised by recent experimental observations, we reinvestigate the critical scaling of high Tc superconductors. It is found that the dc current-voltage characteristic of mixed state superconductors has a general form of extended power law which is based on the Ginzburg?Landau (GL) functional in the similar way as the vortex glass theory. Isotherms simulated from this nonlinear equation fit the experimental I?V data of Strachan et al. [Phys. Rev. Lett.?87?(2001)?067007]. The puzzling question of the derivative plot for the I?V curves and the controversy surrounding the values of critical exponents are discussed.

Thermally Activated Flux Motion in 2223 Phase High Temperature Superconductors

Isothermal V-I characteristics of Bi1.8Pb0.2Sr2Ca2Cu3Oy high temperature superconductors (HTSCs) have been systematically studied. Below the irreversibility temperature Tr(H), two important features were observed: 1) Both positive and negative curvatures can coexist in the same isothermal V-I curve, so it is different from the scaling behavior V ∝ exp(-(J0/J)μ). 2) The activation volume of flux motion is found temperature dependent as (1-T/Tr(H))-2ν with ν about 0.5. This behavior supports the existence of a two-dimensional secondary phase transition below Hc2(T) in this Bi-based HTSCs.

Extended Power Law and Hall Anomaly of High-Temperature Superconductors

Starting from the free energy of a moving vortex, we obtain the extended power law form of longitudinal resistivity ρxx and the analytical form of Hall resistivity ρxy. Based on them, we obtain a scaling relation which agrees well with the experimental data of different kinds of high-temperature superconductors. Furthermore, the theoretical results well fit the scaling results.

Metastable phases in epitaxial Nb-La and Nb-Ce system

Epitaxial metal film sandwiches of Nb-La-Nb and Nb-Ce-Nb have been prepared in an ultra high vacuum environment. In the sandwiches the La layers and Ce layers are found to be fcc La and bcc Ce, respectively, different from that of bulks. The transmission electron microscope (TEM) observations indicate that La or Ce was grown with partial pseudomorphism epitaxy on bcc Nb layers. The origin of the unusual epitaxies has been discussed.