Temperature dependence of the surface resistance in YBa2Cu3O7-delta single crystals

Microwave surface resistance, R(S), for YBa2Cu3O7-delta single crystals is measured in the range 15-90 K on frequency 10 GHz inside the Nb3Sn cavity with quality factor of 5-10(8). The R(S)(T) dependence turned out to be drastically distinguished from that for conventional superconductors. Two steep falls of R(S) (above 80 and below 20 K) are found with almost unchanged value similar to 170 mu Ohm in between. By means of the Auger spectra study a very thin oxygen-depleted layer (Delta approximate to 3-5 nn) is shown to exist on YBCO single crystal surface. A consistent model is developed which allows to fit the experimental R(S)(T) dependence taking into account an extra contribution of the surface layer into the total value of R(S) congruent to R(SO) + X(S)(2)(Delta/rho(l)). The inductive part, X(S), is affected strongly with the surface layer resistivity, rho(l). The carrier density (and rho(l)) of the surface layer may be varied up to semiconducting or dielectric ones determining the total temperature dependence of R(S).