Microwave surface resistance of high T c superconducting films

Abstract

The surface resistance, R<SUB>s</SUB>, at microwave frequencies has been an important qualification parameter for high temperature superconductor (HTS) thin films. HTS thin films with low R<SUB>s</SUB> have been realized on many substrates, and many groups have realized R<SUB>s</SUB> values in the range 300 - 400 (mu) (Omega) at 10 GHz at 77 K with YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-(delta</SUB> ) (YBCO) films on <100> LaAlO<SUB>3</SUB> substrates. Both microstrip resonator and parallel plate resonator techniques are being used to measure R<SUB>s</SUB> values of HTS thin films. It has been observed that the value of R<SUB>s</SUB> at given frequency and temperature critically depends upon the epitaxial quality and granularity of the films. For example, YBCO films grown on <100> MgO have been found to be granular and weak link limited with a significant microwave power dependence of R<SUB>s</SUB>. On the other hand, YBCO films insitu grown on <100> LaAlO<SUB>3</SUB> have shown better epitaxy with low R<SUB>s</SUB>. This is obviously due to the much better lattice match of YBCO with <100> LaAlO<SUB>3</SUB> if the targets used for laser deposition are doped with Ag. Extensive work carried out in our laboratory has shown that a Ag-doping level of around 5 wt.% in YBCO is the optimum which results in YBCO films of much improved quality. We have realized Ag-doped YBCO films with J<SUB>c</SUB> values of 6 - 8 X 10<SUP>6</SUP> Acm<SUP>-2</SUP> at 77 K and a low R<SUB>s</SUB> value of 210 (mu) (Omega) at 10 GHz at 77 K on <100> LaAlO<SUB>3</SUB>. Both these values are the best realized on LaAlO<SUB>3</SUB> to date. What is equally important is the fact that with Ag-doping the reproducibility of the epitaxial quality of the films improves significantly. This has been found to be due to the enhanced oxygenation of films during growth and the surfactant effect of Ag. Experiments have shown that even the optimum temperature for insitu growth in reduced considerably by Ag-doping. It must be mentioned, however, that the only negative aspect of Ag-doping is the higher microwave residual surface resistance, R<SUB>res</SUB>, observed in these films at (very) low temperatures. This is obviously due to the presence of Ag in the grain boundaries which gives rise to a metallic contribution to R<SUB>s</SUB> which appears as R<SUB>res</SUB>. Nevertheless, Ag-doped YBCO films have been found to be superior to undoped films both due to their higher stability and lower R<SUB>s</SUB> at 77 K, a temperature at which HTS devices are expected to operate.