Effect of underdoping on the microwave surface resistance of YBa2Cu3Oz thin films

Abstract

The effect of underdoping on the microwave surface resistance (Rs) and the Hall number (nH) in the Co-substituted YBa2Cu3Oz (YBCO) films grown on LaAlO3 substrates has been investigated, and the results are compared with oxygen-deficient YBCO films. From the analysis of Hall data, it is inferred that the nH is suppressed upon the Co substitution. As the Co concentration increases, the in-plane resistivity (ρ) increases and ρ(T) develops a downward curvature below room temperature, whereas the nH decays exponentially. Microwave studies on both the Co-substituted YBCO as well as the oxygen-deficient YBCO reveal that the reduction in carrier concentration of YBCO, by any means, leads to an undesirable effect on Rs. The Rs(T) value of Co-substituted films increases as the Co concentration is increased. Near the superconducting transition temperature, Tc, the observed high Rs value and a broad transition in the Rs(T) curve in the Co-substituted films are presumed to be due to an increase in the penetration depth (λ), which is caused by the reduced carrier concentration. Moreover, the Rs value of 0.76 mΩ (at 20 K; 22 GHz) for the Co concentration of x=0.15 is very close to the value of oxygen-depleted YBCO examined in the present work. Therefore, the observed high residual Rs value at low temperatures (well below Tc), in both the Co-substituted as well as the oxygen-depleted YBCO systems, is presumably due to (i) the increased penetration depth, λ, and (ii) the poor crystallinity in combination with the deteriorated surface morphology, as evidenced from the results of x-ray diffraction and atomic force microscopy.