Abstract
The nuclear relaxation rate (1/T 1) and Knight shift for Cu of YBa 2Cu 3O 7−y were measured by using the nuclear quadrupole (NQR) and magnetic resonance (NMR) techniques. For 90 K-superconductor with y∼o, 1/T 1 at the CuO 2 plane site decreases markedly without the enhancement just below T c and approximates to a T 3 behavior between 40K and 10 K. Knight shift shows a distinct decreases below T c, giving an evidence for dominant singlet pairing. For an explanation of unconventional nuclear relaxation behavior, it seems plausible to apply a d-wave model with the gap zeros of lines on the Fermi surface. For 60 K-superconductor, we found a surprising result that 1/T 1 at the CuO 2 plane site is by three order of magnitude suppressed as compared to that of 90 K superconductor and shows no anomalies at T c= 60K. This marked difference of relaxation behaviors between 90K and 60K superconductors implies a strong correlation with the mechanism of high T c superconductivity.
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