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We have performed magnetotransport experiments on p+-(Ga,Mn)As/n+-GaAs Esaki diode devices. The spin-valve-like signal was observed in these devices in an in-plane magnetic field configuration due to Tunneling Anisotropic Magnetoresistance effect. The pattern of the observed magnetic reversal process strongly depends on the observed magnetic anisotropy of the (Ga,Mn)As layer - depending on its type the sign of the spin-valve-like signal can be changed by a simple rotation of the magnetic field by 90° or not. The type of the anisotropy is found to be strongly shaped, in a random way, during processing of the wafer.

Carbon-doped high mobility two-dimensional hole gases grown on (110) oriented GaAs substrates have been grown with hole mobilities exceeding 10^6 cm^2/Vs in single heterojunction GaAs/AlGaAs structures. At these high mobilities, a pronounced mobility anisotropy has been observed. Rashba induced spin-splitting in these asymmetric structures has been found to be independent on the transport direction.

Utilizing a carbon filament doping source, we prepared two-dimensional hole gases in a symmetric quantum-well structure in the GaAs/AlGaAs heterosystem. Low-temperature hole mobilities up to 1.2×106 cm2/V s at a density of 2.3×1011 cm–2 were achieved on GaAs (001) substrates. In contrast to electron systems, the hole mobility sensitively depends on variations of the quantum-well width and the spacer thickness. In particular, an increase of the quantum-well width from an optimal value of 15 nm to 18 nm is accompanied by a 35% reduction of the hole mobility. The quality of ultrahigh-mobility electron systems is not affected by the employed carbon-doping source.

It is well-established that the electron–electron interaction strongly affects the compressibility of a two-dimensional electron system at zero magnetic field and even leads to the negative sign of the compressibility. Surprisingly, we find that its contribution is almost identical for the metallic states of electrons at zero magnetic field and of composite fermions at Landau level filling 1/2.

Commensurability effects between the period of an artificial array of scatterers (antidots) in a two-dimensional electron gas and the classical cyclotron radius are found to depend strongly on the size of the antidots in comparison with the period. Magnetotransport experiments on tunable antidot arrays are reported and analyzed. In the region of large depletion zones we find strong deviations of the Hall effect from the Drude slope in magnetic fields up to several teslas. From this, we deduce undepleted regions of less than 60 nm in typical 500-nm period structures. The suppression of the commensurability motion around several antidots observed in the longitudinal magnetoresistance in the case of strong depletion has the same origin and allows us to determine independently and consistently the depletion radius of the antidots.

The authors have measured the density of states in the tails of the Landau levels of GaAs-AlxGa1-xAs heterostructures by analysing the variation of the Fermi level position as a function of the magnetic field. The experimental data are best described by a gaussian-like density of states superimposed on a constant background density of states., Letter