Abstract
Doppler-broadened annihilation radiation (DBAR) spectra have been measured in the high- T c orthorhombic YBa 2Cu 3O 7−x (superconductor) and a similar tetragonal YBCO compound (non-superconductor) as a function of temperature between 77 K and 300 K. Analyses of these spectra bring out certain interesting features. Two components are present in both the cases. A narrow component of near constant width ∼2.4 keV for the superconductor and ∼1.9 keV for the nonsuperconductor are seen to arise due to trapped positrons in defects and oxygen vacancies. In addition, a broad component of width ∼3.6 keV arises due to annihilation with 2s+2p electrons of oxygen ions in the non-superconductor at all temperatures and in the superconductor above 105 K. In the superconducting state two drastic changes occur: (a) width of the broad component shows a sharp increase on cooling from 105 K to 77 K, showing an increase in momentum of the oxygen ion electrons; (b) the intensity of the narrow component increases with decreasing temperature, probably due to a structural change leading to enhancement of trapping sites and/or increase in electron density at the vacancy sites. These changes seem to be closely associated with the superconducting phase transition.
Published Version
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