TEMPERATURE-DEPENDENCE OF THE INPLANE PENETRATION DEPTH OF YBA2CU3O6.95 AND YBA2(CU0.9985ZN0.0015)3O6.95 CRYSTALS FROM T TO T(2)

The low-temperature dependence of the microwave complex surface impeda nce (17 GHz) of pure and Zn-doped high-quality YBCO single crystals is reported. The surface resistance of the pure crystal shows a peaked s tructure around 40 K which disappears with Zn doping. The in-plane pen etration depth varies linearly with temperature below 25 K; a T2 contr ibution appears at the lowest temperature when Zn impurities are added . This suggests that the former temperature dependence is intrinsic to pure crystals whereas the T2 behavior, also observed in thin films, w ould be due to defects. The linear temperature dependence is expected for superconducting gaps with lines of nodes.