Charge-spin excitations of the Ising-type fractional quantum Hall ferromagnets

We study the low-lying excitations from the fractional quantum Hall states at filling factors 2/3 and 2/5 and argue that the charge-carrying excitations involve spin flips, and, in particular, possibly more than one. Energies obtained by exact diagonalization and transport activation gaps measured over a wide range of magnetic fields are invoked. We discuss the relevance of the noninteracting composite-fermion picture where both fractions correspond to the same filling factor 2 of the composite fermions. © 2009 The American Physical Society

Transport gap in a v=1/3 quantum Hall system: a probe for skyrmions

Transport measurements of the activation gap at fractional filling factor 1/3 are compared to results of exact diagonalization, allowing identification of a small anti-skyrmion as the lowest excitation in the low-field regime. In agreement with theory, a crossover to spinless excitations at higher electron densities is observed. Two samples of different quality are investigated. A detailed description of the theoretical calculation of activation gaps is presented and features which should be taken into account are summarized: finite thickness, Landau level mixing, and comparison between different sizes of the model system and-whenever possible-also between different geometries (torus and sphere). Within the chosen model of disorder (entailing a single fit parameter) we obtain a good agreement between calculated energies and experimental results

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

A comparison: 2D electron- and hole systems in the fractional quantum Hall regime

We investigated two-dimensional electron- (2DES) and hole systems (2DHS) in the fractional quantum Hall regime for filling factors ν=1/3 and ν=2/3. Due to a metallic top gate we are able to vary the electron-/hole density of the samples over a wide range. Measuring activated transport on these systems with perpendicular magnetic fields up to 18 T and temperatures down to about 30 mK allows to get an insight into the excitation spectra. Although the bandstructure of holes is complicated compared to those of electrons, the theoretical description of the fractional quantum Hall effect in the composite Fermion picture should hold for both types of particles. We analyzed and compared the results for the different filling factors and systems