High frequency conductivity in the quantum Hall e!ect

We present high frequency measurements of the diagonal conductivity of a two-dimensional electron system in the integer quantum Hall regime. The width of the peaks between QHE minima is analyzed within the framework of scaling theory using both temperature (1"100}700 mK) and frequency ( f)6 GHz) in a two parameter scaling ansatz. For the plateau transition width we "nd scaling behaviour for both its temperature dependence as well as its frequency dependence. However, the corresponding scaling exponent for temperature ( "0.42) signi"cantly di!ers from the one deduced for frequency scaling (c"0.6). Additionally, we use the high frequency experiments to suppress the contact resistances that strongly in#uences DC measurements. We "nd an intrinsic critical conductivity "0.17e /h, virtually independent of temperature and "lling factor, and deviating signi"cantly from the proposed universal value 0.5e /h. 2001 Elsevier Science B.V. All rights reserved.