  Rectification of microwave radiation (20-40 GHz) by a line boundary between
two two-dimensional metals on a silicon surface was observed and investigated
at different temperatures, in-plane magnetic fields and microwave powers. The
rectified voltage $V_{dc}$ is generated whenever the electron densities
$n_{1,2}$ of the two metals are different, changing polarity at $n_1 \approx
n_2$. Very strong nonlinear response is found when one of the two 2D metals is
close to the electron density corresponding to the reported magnetic
instability in this system.
