Epitaxial Tl2CaBa2Cu2O8 thin films with low 9.6 GHz surface resistance at high power and above 77 K

Abstract

We report measurements of the dc and microwave properties of epitaxial thin films of Tl₂CaBa₂Cu₂O₈ on LaAlO₃. ac magnetic susceptibility measurements yield critical temperatures at the transition midpoint T_c∼100 K and 10–90% transition widths of ∼1.0 K. We observed a critical current density of 1.06×10⁶ A/cm² at 77 K measured by dc transport. We measured a low‐power surface resistance of R_s=0.2 mΩ at 9.55 GHz and 77 K. This is ∼50 times lower than oxygen‐free high‐conductivity (OFHC) Cu at the same frequency and temperature. At 90 K, R_S rises to 0.4 mΩ, and at 95 K, R_S=0.7 mΩ. We measured the microwave power dependence of R_S at 77, 90, and 95 K. At high microwave field (≳30 G, 77 K) R_s=0.8 mΩ, more than ten times lower than OFHC Cu.