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Phonon and transport measurements in the fractional quantum Hall effect

The properties of a fractional quantum Hall system has been investigated with phonon experiments and with transport experiments. A detailed description of the phonon absorption technique is given. We measure the relative specific heat in the fractional quantum Hall effect regime at various filling factors. We use phonons to measure the activation gap Δ of the fractional quantum Hall states at constant fillings as a function of the perpendicular magnetic field B. At a given filling factor nearly all gaps measured show a square‐root dependence on the magnetic field and can be described by one single fit parameter related to the composite fermion effective mass. In addition, we present transport experiments to deduce energy gaps. Here, an astonish linear dependence of Δ on B is observed for a number of fractional filling factors. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)