R. F. Bell

Nuclear relaxation and Cu spin dynamics in YBa2Cu3O6+x

Abstract We report the 63 Cu NQR spectrum and nuclear relaxation rates for Cu(2) sites in 63 Cu-enriched YBa 2 Cu 3 O 6.64 (T c ∼ 60 K). Nuclear spin-lattice relaxation rates exhibit a strong reduction of (T 1 T) −1 between 100 K and T c . Cu(2) Knight shifts measured in a YBa 2 Cu 3 O 6.7 sample of natural isotopic abundance are reduced substantially relative to YBa 2 Cu 3 O 7.0 and show little change below T c .

NMR and NQR studies of high Tc superconductors

Abstract 63,65 Cu NMR and NQR studies of YBa 2 Cu 3 O 7−δ are described for compositions, δ, of 0.0, 0.3, and 0.9. Spin-lattice relaxation rates in YBa 2 Cu 3 O 7 reveal significantly different electron dynamics at the two Cu sites in both the normal and superconducting states. In YBa 2 Cu 3 O 6.7 , the relaxation rates of the sites giving a 31 MHz NQR line are dramatically lower (by a factor ∼ 1/1500) than in YBa 2 Cu 3 O 7 indicating the essential vanishing of mobile carrier density on these sites. In contrast, relaxation at the 22 MHz sites is reduced only about 30%. Interpretation of these and other NMR and NQR data is discussed in relation to the question of assignment of the NQR lines to the appropriate crystalline sites.

NMR and NQR studies of copper oxide superconductors (invited)

63,65Cu NMR and NQR measurements reveal remarkably different electronic character at the Cu(1) chain and Cu(2) plane sites in YBa2Cu3O7.0. In the normal state, spin‐lattice relaxation rates at both sites are strongly enhanced relative to estimated d‐band Korringa (T1) rates, but have sharply different temperature dependencies. Moreover, T1 anisotropy is greater than an enhanced Korringa model can account for. Below Tc, the temperature dependencies reveal significantly different spectra for quasiparticle excitation at the two sites with the strongest apparent energy gap occurring for the planar Cu(2) sites. At reduced oxygen content (YBa2Cu3O6.7) the relaxation at the Cu(1) sites is only slightly (∼30%) less than observed for YBa2Cu3O7.0 while the rates for the resonance at the frequency of the Cu(2) sites in YBa2Cu3O7.0 exhibit a large (∼2000 ×) decrease for a minority of planar Cu(2) sites indicates that these sites are electronically inactive. NMR shift measurements are used to analyze the susceptibilit...