EPR Study of the High Tc Superconductor YBa2Cu3O7-y Doped with Palladium or Zinc

摘要

EPR spectra of the high Tc superconductor YBa2Cu3O7-y (YBCO) doped with Pd2+ or Zn2+ have been measured at several temperatures and dopant concentrations. The spectral intensity of YBa2(Cu1-xZnx)3O7-y is proportional to the dopant concentration. The behavior of YBa2(Cu1-xZnx)3O7-y is quite different: the spectral intensity remains almost constant up to x=0.10 and then increases rapidly above x=0.10. The results are interpreted in terms of localized and antiferromagnetically spin-paired d holes in both uO chain and planes. The Pd2+ ion substitutes on the CuO chain consisting of "CuOCu dimers", and a Cu2+ ion with an unpaired spin is gene rated for each Pd2+ ion substituted. On the other hand, Zn2+ substitutes on the CuO planes, and all or most of the spins in the two-dimensional plane manage to pair up in the region of low dopant concentration. When the dopant concentration exceeds a certain limit, it becomes more difficult for the spins to find partners, and the number of unpaired spins increases rapidly with increasing dopant concentration. The Zn2+ ion is more effective than the Pd2+ ion in suppressing the superconductivity of YBCO. This is attributed to the fact that Zn2+ substitutes on the CuO planes which are mainly responsible for the superconductivity, while Pd2+ substitutes on the CuO chain which is of secondary importance in the superconductivity.