Line impedance and propagation coefficient of narrowsuperconducting coplanar lines made of YBaCuO

The microwave properties of coplanar waveguides with line widths from 1 μm to 40 μm made of superconducting YBaCuO films with a thickness t=180 nm on LaAlO₃ are investigated. The line impedance Z_L and the normalized propagation coefficient β/β₀ of these waveguides are measured between 45 MHz and 26.5 GHz at temperatures between 77.4 K and 92 K. The ratio of the line width w to the distance of the ground layers d is constant with w/d=0.2. Therefore, Z_L and β/β₀ are independent of w for perfectly conducting waveguides. For superconducting waveguides it is found that Z_L and β/β₀ differ from the values of perfectly conducting waveguides. They increase for smaller line widths at a constant temperature. At w=1 μm and T=80 K, Z_L and β/β ₀ are nearly twice as high as calculated for perfect conductors. Furthermore, Z_L and β/β₀ increase with the temperature. It is shown that these effects are attributed to an increase of the inductance per unit length L' due to the superconducting material, whereas the capacitance per unit length C' behaves like C' of perfectly conducting waveguides. Using these results, the dimensions of the superconducting waveguides, which are necessary to obtain a desired Z_L at a given line width w, are calculated