Effect of surface needles on microwave surface resistance in Tl(Ba,Sr)2Ca2Cu3Oy superconductor films on a LSAT substrate

Epitaxial thin films of a Tl(Ba,Sr)2Ca2Cu3Oy (Tl-1223) superconductor with a thickness of 5000 A were grown on a moderate dielectric constant (ε = 23.8) substrate (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) in order to study the miniaturization of a superconducting thin film microwave filter. For the annealing conditions followed in this work, we observed two kinds of surface morphology: both of them have a large plate-like morphology with pits but they differ from each other by the presence of needle shaped (a-axis-oriented) grains on top of the plate-like surface. The superconducting transition temperature (Tc) and critical current density (Jc) for both kinds of films (with and without needles) are about 108 K and ∼ 0.8 MAcm-2 at 90 K, respectively. However, the values of the temperature dependence of the microwave surface resistance (Rs) of the films with needles are about one order higher than those for the films without needles, due to the absorption of microwaves by a-axis-oriented needles. The value of Rs for the films without needles at 90 K and 10 GHz is 507 μΩ, which satisfies the requirement for microwave device applications with high performance