Determination of the doping dependence of the penetration depth using YBCOmicrostrip ring resonators

We present a method to determine the temperature dependent London penetrati on depth measured by a microstrip ring resonator for YBCO thin films with v arious oxygen contents. It allows us to get the following important results of the superconducting properties in the underdoped cuprate superconductor s. With increasing delta (e.g. delta = 0.05, 0.2, and 0.4), the superfluid density, lambda (2)(5 K) /lambda (2)(T), is linear in the low temperature r ange (T < T-c/3), and the slope changes with increasing oxygen deficiency. After the temperature normalization with respect to the critical temperatur e, the temperature dependence of the superfluid density is shown to be inde pendent of doping concentration. Moreover the maximum energy gap can be est imated from the model of the d-wave pairing. We found that the ratio Delta /T-c is nearly fixed value, 3.0 +/- 0.4. (C) 2001 Elsevier Science B.V. All rights reserved.