Cu63Knight shifts in the superconducting state ofYBa2Cu3O7−δ(Tc=90 K)

The authors have used ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR to measure the internal magnetic field of a sample of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (${\mathit{T}}_{\mathit{c}}$=90 K) in the superconducting state to correct for the effects of the Meissner shielding currents. They used this information to separate the magnetic-shift tensor into orbital (chemical-shift) and spin (Knight-shift) contributions. They find that the temperature dependence of the chain Cu Knight shift fits the classic Yosida function of weak-coupling, orbital-s-state, spin-singlet BCS theory. For the planes, the Knight shift also requires a spin singlet, but with a strong-coupling Yosida function. The best fit is for an orbital s state, but an orbital d state is also possible. They find a zero-temperature gap Δ(0) of 1.76${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$ for the chains. For the planes, Δ(0) can range from 1.9${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$ to 3.1${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$.