Characterization of superconducting properties using internal friction measurement

by Liu, Zhong-ming

(Uncorrected OCR) Abstract of thesis entitled

" CHARACTERIZATION OF SUPERCONDUCTING PROPERTIES USING INTERNAL FRICTION MEASUREMENT II

submitted by LIU Zhong Ming

for the degree of Doctor of Philosophy at the University of Hong Kong

in January 1994

In order to study the superconducting properties 1n hig~ Tc superconduc~ors, internal friction measurement has been

employed

high

Tc

superconductors.

Both

structt:~e

instability and magnetic flux line pinning form two important aspects of superconducting properties,' and have been studied in a quantitative way. The study on structure instability 1S directly related to the theoretical understanding of high Tc superconductivity, and the study on magnetic flux line pinning is important to realize the application of high Tc superconductors.

The X-ray diffraction experiments reveal the existence of

structure

instability

near

superconducting - transition

temperature in high Tc superconductors. The internal friction

measurement shows a static hysteresis type of internal friction peak at the same temperature, which is typical for the occurrence of crystal distortion and also indicates structure instability. Both experimental methods further confirm that structure instability is correlated with the superconducting transi tion in high Tc superconductors. The correlation between superconducting transition and structure instability implies two important facts in high Tc superconductivity, namely (1) the electron-phonon interaction plays an important role; (2) anharmonicity in lattice vibration is prominent. Based on strong phonon mediated coupling and anharmonicity in high Tc superconductivity, a calculation of electron-phonon coupling parameter shows that the electron-phonon coupling could be enhanced significantly if anharmonic effect is incorporated in strong phonon mediated coupling theory. So the anharmonicity could give a possible explanation for both high superconducting transition temperature and structure instability in high Tc superconductivity.

The weak pinning of magnetic flux lines in high Tc

,.

superconductors is one of the important reasons which diminish

critical current density and restrict the application of high Tc superconductors. As a result, the- study of enhancing magnetic flux line pinning is important to realize the application. The internal friction measured using the vibrating reed method is sensitive to the viscous motion of

11

magnetic flux lines, so internal friction method provides an effective way in investigating the magnetic flux line pinning. This method has been used to study neutron irradiation, paramagnetic doping and melt texture growth effects on enhancing the pinning of magnetic flux lines. The experimental results show that different new pinning centers are introduced during neutron irradiation paramagnetic pinning and melt texture growth; these pinning centers could effectively enhance magnetic flux line pinning in high Tc superconductors. The origin of each new pinning centers has been studied in details.

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