Microwave surface impedance of the cuprate superconductor YBa2Cu3O6.95

Abstract The microwave surface impedance, Z s = R s + i X s , probes important aspects of the superconducting state: X s ( T ) gives the penetration depth λ( T ) and R s ( T ) tracks the conductivity of the thermally excited quasiparticles. Measurements on high-quality crystals of YBa 2 Cu 3 O 6.95 show that both λ and R s have predominantly linear temperature dependencies at low temperatures, strongly suggesting nodes in the gap function. At higher temperatures R s has a nonmonotonic dependence, interpreted as a rapid decrease in the quasiparticle scattering rate below T c . Upon doping with small quantities of Zn, R s ( T ) becomes monotonic, with values similar to those in the best thin films, and λ( T ) becomes quadratic, as observed in most thin films. These results demonstrate that certain aspects of the intrinsic properties of this material are easily masked by low levels of impurities.