Microwave properties of HTS films

High-frequency applications of high- temperature superconductors generally fall into two categories: 1) devices that require low values (relative to Cu) of surface resistance RS in ambient surface magnetic fields Hrf; and 2) devices that require low RS in modest fields (Hrf ∼ 250 Oe). Moreover, many applications can be realized with small- surface-area films (∼ 1 cm2) whereas others require larger areas - radiofrequency (rf) cavities, for example. Regardless of the application, the potential of HTS films is predicted on satisfying one or both of the above-stated requirements. The authors have measured the surface resistance of small-area (1 cm2) and large-area (6.5 cm2) YBa2Cu3O7 (YBCO) films that have been laser ablated onto LaA ell O3 substrates, large-area (5.1 cm2 YBCO films that have been e-beam deposited onto LaA ell O3, and large-area (11.4 cm2) T ell-based films that have been magnetron sputtered onto metallic substrates. The best RS values are obtained from the 1-cm2 laser-ablated films; they are 40 μΩ and 340 μΩ at 4 K And 77 K, respectively (ω/2π = 10 GHz). Comparable values for Cu are 6 and 13 mΩ, respectively. Large-area T ell-based films yield typical RS values of 4 mΩ and 14 mΩ at 4 K and 77 K, respectively (ω/2π = 18 GHz). The dependence of RS on Hrf for these films indicates that surface fields as large as 55 Oe can be achieved with RS increasing only by a factor of 10. This field dependence is associated with c-axis texturing