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

Abstract

Epitaxial thin films of a Tl(Ba,Sr)2Ca2Cu3Oy (Tl-1223) superconductor with a thickness of 5000 Å 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 filmmicrowave filter. For the annealing conditions followed in this work, we observed twokinds of surface morphology: both of them have a large plate-like morphologywith 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 criticalcurrent density (Jc) for both kinds of films (with and without needles) are about 108 K and at 90 K, respectively. However, the values of the temperature dependence of the microwave surfaceresistance (Rs) of the films with needles are about one order higher than those forthe films without needles, due to the absorption of microwaves bya-axis-orientedneedles. The value of Rs for the films without needles at 90 K and 10 GHz is507 µΩ, which satisfies the requirement for microwave device applications with high performance.