Title

The role of bromine in restoring superconductivity in YBa2Cu3Oy and weak link studies in YBa2Cu3Oy films

Date of Completion

January 1998

Keywords

Engineering, Chemical|Physics, Condensed Matter|Engineering, Materials Science

Degree

Ph.D.

Abstract

Studies of structures and materials used for the creation of weak links in the high temperature superconductor $\rm YBa\sb2Cu\sb3O\sb{y}$ (YBCO) have led to the study of bromination of deoxygenated YBCO. The question of the location of the bromine after the deoxygenated superconductor has been brominated and its role in the restoration of superconductivity in the resulting material has been addressed.^ $\sp{63,65}$Cu nuclear quadrupole resonance and Br K-edge x-ray absorption fine structure are used to study the local structures of Cu and Br in $\rm YBa\sb2Cu\sb3O\sb{y}$ which is almost fully oxygenated (y $\approx$ 6.9), deoxygenated $\rm YBa\sb2Cu\sb3O\sb{y}$ ($\rm6 \leq y \leq 6.7$), and deoxygenated and brominated $\rm YBa\sb2Cu\sb3O\sb{y}Br\sb{x}$. The results answer the unresolved question of how Br restores superconductivity in deoxygenated YBCO. On the basis of the Cu nuclear quadrupole resonance experiments, it is concluded that the bromine does not occur as a substitutional or interstitial atom in the YBCO, but that nearly fully oxygenated YBCO is present after deoxygenation and bromination. The Br K-edge x-ray absorption fine structure data provides compelling evidence for the formation of nanoscale BaBr$\sb2$ precipitates resulting from local destruction of the YBCO phase. Combined with the nuclear quadrupole resonance results, this indicates that through this nanoscale decomposition reaction, oxygen is liberated which goes into the nearby chain sites of locally undecomposed YBCO, thus restoring superconductivity. ^