Frequency Scaling of Microwave Conductivity in the Integer Quantum Hall Effect Minima

We measure the longitudinal conductivity $sigma_{xx}$ at frequencies $1.246 {rm GHz} le f le 10.05$ GHz over a range of temperatures $235 {rm mK} le T le 4.2$ K with particular emphasis on the Quantum Hall plateaus. We find that $Re(sigma_{xx})$ scales linearly with frequency for a range of magnetic field around the center of the plateaus, i.e. where $sigma_{xx}(omega) gg sigma_{xx}^{DC}$. The width of this scaling region decreases with higher temperature and vanishes by 1.2 K altogether. Comparison between localization length determined from $sigma_{xx}(omega)$ and DC measurements on the same wafer show good agreement.