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 YBa₂Cu₃O_7.0. 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 T_c, 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 (YBa ₂Cu₃O_6.7) the relaxation at the Cu(1) sites is only slightly (∼30%) less than observed for YBa₂Cu ₃O_7.0 while the rates for the resonance at the frequency of the Cu(2) sites in YBa₂Cu₃O_7.0 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 susceptibility of YBa₂Cu₃O _7.0 and obtain an upper limit for the electronic density of states.