New substrates for HTSC microwave devices

A new substrate material LSAT, a 30/70 mole % solid solution between LaA10
₃
and Sr
₂
AITaO
₆
, has been prepared for the purposes of eliminating twinning, strain and non-isotropic microwave properties found in pure LaA10
₃
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), YBa
₂
Cu
₃
0
_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 × 10
⁶
A/cm
²
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.