Line impedance and propagation coefficient of narrow superconducting coplanar lines made of YBaCuO

Abstract

The microwave properties of coplanar waveguides with line widths from 1 /spl mu/m to 40 /spl mu/m made of superconducting YBaCuO films with a thickness t=180 nm on LaAlO/sub 3/ are investigated. The line impedance Z/sub L/ and the normalized propagation coefficient /spl beta///spl beta//sub 0/ 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/sub L/ and /spl beta///spl beta//sub 0/ are independent of w for perfectly conducting waveguides. For superconducting waveguides it is found that Z/sub L/ and /spl beta///spl beta//sub 0/ differ from the values of perfectly conducting waveguides. They increase for smaller line widths at a constant temperature. At w=1 /spl mu/m and T=80 K, Z/sub L/ and /spl beta///spl beta//sub 0/ are nearly twice as high as calculated for perfect conductors. Furthermore, Z/sub L/ and /spl beta///spl beta//sub 0/ 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/sub L/ at a given line width w, are calculated.