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We investigate interactions between electrons and nuclear spins by using the resistance (R xx ) peak which develops near filling factor ν = 2/3 as a probe. By temporarily tuning ν to a different value, ν temp, with a gate, the R xx peak is shown to relax quickly on both sides of ν temp = 1. This is due to enhanced nuclear spin relaxation by Skyrmions, and demonstrates the dominant role of nuclear spin in the transport anomaly near ν = 2/3. We also observe an additional enhancement in the nuclear spin relaxation around ν = 1/2 and 3/2, which suggests a Fermi sea of partially-polarized composite fermions.

We study charge excitations in quantum Hall ferromagnets realized in a symmetric quantum well. Landau levels (LLs) with different subband and orbital indices crossing at the Fermi level act as up and down pseudospin levels. The activation energy measured as a function of the pseudospin Zeeman energy, D Z , reveals easy-plane and easy-axis ferromagnetism for LL filling of n 3 and 4, respectively, for which the crossing levels have parallel and antiparallel spin. For n 4, we observe a sharp reduction in the gap for D Z ! 0, which we discuss in terms of a topological excitation in domain walls akin to Skyrmions.

We have achieved electron mobilities as high as 1.05×107 cm2/Vs at 1.5 K with an electron density of approximately 3×1011/ cm2 for modulation-doped AlGaAs/GaAs by using high purity layers with a residual acceptor concentration of 1×1013/ cm3, and relatively thick spacer layers ∼(75 nm). We found the electron scattering process caused by spatially separated ionized donors to be most important in limiting the observed low-temperature electron mobility, even in these thick-spacer-layer samples. Theoretical calculation predicts that the mobility caused by this scattering mechanism is approximately 1.6×107 cm2/Vs. The observed electron mobility exhibits an anisotropy with respect to the principal axes ([110] and [*BAR*1*BAR*10] directions) on the (001) surface. The anisotropy is such that the mobility in the [*BAR*1*BAR*10] direction is always higher than that in the [110] direction. Theoretical calculations reasonably explained this anisotropy by assuming the existence of islands at the interface which are longer in the [*BAR*1*BAR*10] direction than in the [110] direction, and revealed that the scattering caused by interface roughness was as important as that caused by ionized donors. Thus, these two major components mainly determine the observed low-temperature electron mobility. We also discuss the mobility expected for residual impurity free limit.

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Quantum point contacts fabricated using a backgated two-dimensional electron-gas system show clear quantized features and a 0.7 anomaly in conductance. Using these density-tunable point contacts, we have studied the behavior of the 0.7 anomaly. The 0.7 step shifts down to around 0.5 as the electron density is decreased under a zero magnetic field. This suggests that electron-electron interactions play an important role in forming the conductance anomalies of quantum point contacts.

By using a back-gate operation, a high-quality two-dimensional electron gas (2DEG) is formed in an undoped GaAs/AlGaAs inverted heterostructure. A high mobility of around 3×106 cm2/V s at 1.6 K is obtained for the structure without any compensating surface doping. The electron density is controllable down to 7×109 cm−2. The relation between electron density and mobility is studied for samples both with and without a surface gate. The obtained results indicate that background impurities and an inhomogeneity of the electric field coming from the surface govern the mobility in a low-electron-density region and that the interface inhomogeneity becomes important at a high electron density.

Ballistic electron transport characteristics are studied using macroscopic four-terminal square structures formed in high-mobility wafers (μ=7.8×106 cm2/V s at 1.5 K). Ballistic transport over 200 μm can be detected as a negative peak in resistance around B=0 T when four-terminal resistance is measured as a function of magnetic field. The ballistic mean free path (lb) of electrons is evaluated from the size dependence of the negative peak height. The estimated lb becomes 86 μm, which is approximately equal to a conventional mean free path calculated from carrier density and mobility of the wafer.