Nonlinear surface impedance of Tl2Ba2CaCu2O8 thin films as a function of temperature, frequency, and magnetic field

Abstract

YBa 2 Cu 3 O 7 epitaxial film quality has reached a stage where rf applications at T⩽80 K are carried through. Higher-operating temperatures or better power handling are very much desired. Tl2Ba2CaCu2O8 (TBCCO) is an excellent candidate with its higher-transition temperature Tc≈100–105 K. We study the rf field dependence of surface impedance Zs of epitaxial TBCCO films using dielectric resonators with TE011-modes at 8.5 and 15.3 GHz at 4.2 K<T<100 K and at 0.01 mT⩽Hrf⩽10 mT in the frequency and time domain. High-quality films show surface resistances at low fields Rs(77 K) superior to YBCO and promising power handling with a nonlinearity onset above turn-up field μ0H*⩾3 mT. Defective TBCCO films show anomalous nonlinear behavior δRs(Hrf)∝√Hrf for μ0Hrf⩽μ0H*∼0.3–1 mT, extremely pronounced at T<40 K, indicating the presence of very weak links yielding flux flow, followed at higher fields μ0Hrf≳0.5 mT by hysteresis losses with a linear field dependence δRs(Hrf)∝Hrf. Better quality TBCCO films show δRs(Hrf)∝Hrf2 up to fields of about 5–10 mT. The δRs(Hrf)∝Hrfn (n=1,2) dependencies together with ratios r(T)=δXs/δRs∼1–10 hint to hysteresis losses of Josephson fluxons enhanced by slits or holes in the films.