Impurity effect on the electronic state of the vortex core in the mixed state of YBa2Cu3OY

Microwave complex surface impedance, Z(s), of YBa2(Cu1-xZnx)(3)O-y (YBCZO) and YBa2(Cu1-xNix)(3)O-y (YBCNO) were measured for broad concentration (0 less than or equal to x less than or equal to 6%) of Zn and Ni as a function of magnetic field up to 8 T. Both, the vortex viscosity, eta, and the pinning constant, kappa(p), estimated from Z(s) in YBCZO agreed with those of YBCNO if we assume that the one-third of the total Ni concentration substitute for Cu in the CuO2 plane. This means that the pinning potentials by Zn and Ni have the same strength in the broad range of impurity concentration. It also means that the additional DOS in the vortex core induced by these two impurities are negligible compared with the DOS in the vortex core of undoped material, suggesting that the vortex core in YBCO has the large DOS. This is consistent with the microwave result for undoped material, but is inconsistent with STM measurement. Possible explanations are discussed. (C) 2004 Published by Elsevier B.V.