Temperature dependence of the surface resistance in YBa2Cu3O7−δ single crystals

Microwave surface resistance, Rs, for YBa2Cu3O7−δ single crystals is measured in the range 15–90 K on frequency 10 GHz inside the Nb3Sn cavity with quality factor of 5·108. The RS(T) dependence turned out to be drastically distinguished from that for conventional superconductors. Two steep falls of Rs (above 80 and below 20 K) are found with almost unchanged value≈170 μOhm in between. By means of the Auger spectra study a very thin oxygen-depleted layer (Δ∼3–5 nm) is shown to exist on YBCO single crystal surface. A consistent model is developed which allows to fit the experimental RS(T) dependence taking into account an extra contribution of the surface layer into the total value of RS ≅ RSO + XS 2(Δ/ρ1). The inductive part, XS, is affected strongly with the surface layer resistivity, ρ1. The carrier density (and ρ1) of the surface layer may be varied up to semiconducting or dielectric ones determining the total temperature dependence of RS.