New substrates for HTSC microwave devices

A new substrate material LSAT, a 30/70 mole % solid solution between LaA10₃ and Sr₂AITaO₆, has been prepared for the purposes of eliminating twinning, strain and non-isotropic microwave properties found in pure LaA10₃ substrates. At 300 K and 30/70 mole %, LSAT is cubic with lattice parameter of 7.737 Å. The dielectric properties of single crystal LSAT (30/70 mole %) substrates have been measured at 10 GHz and 300 K and determined to be: dielectric constant ε=22.5; and, loss tangent tan(δ) ⩽ 10^-3. High quality c -axis oriented high critical temperature superconducting (HTSC), YBa₂Cu₃0_7-δ (YBCO), thin films have been deposited on (001) single crystal LSAT substrates. The crystal quality of these films is excellent as evidenced by the full width half maximum (FWHM) rocking curve widths of typically 300 arc-seconds. Critical current densities (as measured using magnetization) are about 4 × 10⁶ A/cm² at 10 K. Microwave film properties include an onset transition temperature (T_c) higher than 91 K, transition width (ΔT_c) less than 5 K, surface resistance R_S lower than copper (30 mω) at 85 K and 35 GHz for a film of thickness 2500 Å.