Direct Measurement of the g-Factor of Composite Fermions

The activation gap $\Delta$ of the fractional quantum Hall states at constant fillings $\nu =2/3$ and 2/5 has been measured as a function of the perpendicular magnetic field $B$. A linear dependence of $\Delta$ on $B$ is observed while approaching the spin polarization transition. This feature allows a direct measurement of the $g$-factor of composite fermions which appears to be heavily renormalized by interactions and strongly sensitive to the electronic filling factor.Comment: 4 pages, 4 figures Changed content: Fokus more on g-factors (and less on other details

Specific Heat of a Fractional Quantum Hall System

Using a time-resolved phonon absorption technique, we have measured the specific heat of a two-dimensional electron system in the fractional quantum Hall effect regime. For filling factors $\nu = 5/3, 4/3, 2/3, 3/5, 4/7, 2/5$ and 1/3 the specific heat displays a strong exponential temperature dependence in agreement with excitations across a quasi-particle gap. At filling factor $\nu = 1/2$ we were able to measure the specific heat of a composite fermion system for the first time. The observed linear temperature dependence on temperature down to $T = 0.14$ K agrees well with early predictions for a Fermi liquid of composite fermions.Comment: 4 pages, 4 figures (version is 1. resubmission: Added a paragraph to include the problems which arise by the weak temperature dependence at \nu = 1/2, updated affiliation

Relative specific heat at v = 1/2 measured in a phonon absorption experiment

In general it is far from being straightforward to measure the specific heat of a single two-dimensional electron system directly, since it is strongly dominated by the contribution of the substrate. Using a time-resolved phonon absorption technique, we have directly measured the specific heat C of composite fermions at half Landau level filling v = 1/2. We find a nearly linear dependence of C on temperature down to T = 0.1 K. © 2005 American Institute of Physics

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

Phonon excitations of composite-fermion Landau levels

The phonon excitations of fractional quantum Hall (FQH) states at filling factors ν =1÷3,2÷5, 4÷7, 3÷5, 4÷3, and 5÷3 were investigated. The excitations were found to be based on Landau-level transitions of composite fermions (CF). It was observed that at filling factor ν = 2÷3, a linear field dependence of the excitation energy in the high field regime showed a spin transition, which was excited by the phonons. All the transport gaps, except the one measured at ν = 2÷3, were found to be lower than the phonon excitation gaps due to disorder effects and because temperature was also coupled to spin flip excitations

Nonequilibrium localization in quantum Hall systems at very low frequencies

We have measured the conductivity σxx of GaAs GaAlAs Corbino devices under nonequilibrium conditions in the quantum Hall regime at low frequencies from dc up to 10 kHz. At very low frequencies up to 10-20 Hz, σxx shows a steep decrease from its dc value towards a reduced saturation value. Measurements of the temperature dependence of σxx from 70 mK to 4.2 K revealed a dominance of a conductivity with a temperature dependence σxx ∼exp {- (T0 T)1 2 } typical for variable-range hopping (VRH) at temperatures below 1 K and an additional contribution of thermal activation above 2 K. The characteristic temperature T0 as determined from the VRH-like temperature plot of the conductivity showed also an increase with the frequency and a pronounced decrease with increasing voltage below the breakdown of the quantum Hall effect (QHE). We attribute these results to an effective suppression of the delocalization of electrons in alternating external fields due to a reduced heating and scattering between localization centers. The time scale for this process can be as long as 100 ms. Our data show that the background conductivity σxx at subcritical voltages affects the critical voltage for the breakdown of the QHE. © 2005 The American Physical Society