Nuclear Magnetic Resonance and Nuclear Quadrupole Resonance Study of Atomic Motion in Yttrium BARIUM(2) COPPER(3) OXYGEN(7)

The evolution of the ^{63 }Cu nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) spectra over a temperature range from -100^circC to 200 ^circC was studied in samples close to the composition YBa_2CU _3O_7. For both the Cu(1) and Cu(2) sites there is an unexpected loss of signal intensity at higher temperatures. The onset of this loss occurs at higher temperatures in samples of higher quality as indicated by their room temperature NQR spectra. This loss was determined to be caused by an extra component to the transverse relaxation. The results are interpreted in terms of the effect that oxygen motion in the chain layer would have on the NQR and NMR spin echoes. The jump rate showed an activation energy of 0.64 eV and an attempt frequency of 10^ {11} s^{-1} for the higher quality sample. The lower quality sample showed a thermal activation energy of 0.2 eV and an attempt frequency of 10^6 s^ {-1}. The difference in attempt frequencies and activation energies is discussed in terms of terms of a transition from individual to correlated oxygen jumps in the chain layer.