Microwave frequency dependence of integer quantum Hall effect: Evidence for finite-frequency scaling

We present ac measurements of the diagonal conductivity σ_xx, in the integer quantum Hall regime for frequency f between 0.2 and 14 GHz and temperature T≥50 mK. Re(σ_xx) was obtained from the measured attenuation of a coplanar transmission line on the sample surface. For f≳1 GHz, σ_xx peaks between IQHE minima broaden as f increases, roughly as (ΔB)∝f^γ, where ΔB is the peak width. γ=0.41±0.04 for spin-split peaks, and for spin-degenerate peaks γ=0.20±0.05. We interpret our data in terms of dynamic scaling. Our γ, when compared with existing estimates of the localization length exponent ν, is consistent with assigning a dynamic exponent z=1.