Nonlinear surface impedance Z(T, f, H/sub rf/ of Tl-Ba-Ca-Cu-O thin films

Abstract

YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) film quality has reached a stage enabling rf applications at T/spl les/80 K. Higher operating temperatures or better power handling are very much desired. Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ (Tl-2212) is a perfect candidate with its higher transition temperature T/sub c/>100 K. We studied the rf field dependence of the impedance Z/sub s/ of epitaxial Tl-2212, which shows already surface resistances at low fields superior to YBCO and promising power handling with a nonlinearity onset above /spl mu//sub 0/H/sup *//spl sime/3 mT. Defective Tl-2212 films show anomalous nonlinear behavior for /spl mu//sub 0/H/sub rf//spl les//spl mu//sub 0/H/sup *//spl sim/0.3 to 1 mT, extremely pronounced at T<40 K, indicating the presence of very weak-links yielding flux-flow, followed at higher fields /spl mu//sub 0/H/sub rf//spl ges/0.5 mT by hysteresis losses with a linear field dependence /spl delta/R/sub s/(H/sub rf/)/spl infin/H/sub rf/. Better quality Tl-2212 films show /spl delta/R/sub s/(H/sub rf/)/spl infin/H/sub rf//sup 2/ up to fields of about 5 to 10 mT. The /spl delta/R/sub s/(H/sub rf/)/spl infin/H/sub rf//sup n/ (n=1,2) dependencies together with the ratios r(T)=/spl delta/X/sub s///spl delta/R/sub s//spl sim/1 to 10 hint to hysteresis losses of Josephson fluxons enhanced by slits or holes in the films.