The bimolecular nature of exciton formation from free carriers is demonstrated experimentally in a GaAs asymmetric double-quantum-well tunneling structure. A bimolecular formation coefficient of 6X10-12 cmhigh2/ps is obtained. The electron- and hole-tunneling times are determined simultaneously under the same sample conditions

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We study how impurities influence the current-induced dynamics of magnetic skyrmions moving in a racetrack geometry. For this, we solve numerically the generalized Landau-Lifshitz-Gilbert equation extended by the current-induced spin transfer torque. In particular, we investigate two classes of impurities, nonconducting and magnetic impurities. The first are magnetically rigid objects and yield to an inhomogeneous current density over the racetrack, which we determine separately by solving the fundamental electrostatic equations. In contrast, magnetic impurities leave the applied current density homogeneous throughout the stripe. Depending on the parameters, we observe four different scenarios of skyrmion motions in the presence of disorder, the skyrmion decay, the pinning, the creation of additional skyrmions, and ordinary skyrmion passage. We calculate and discuss phase diagrams in the dependence of the impurity concentration and radii of the impurities.

The carrier velocity is measured in Cr-compensated GaAs in dependence on the applied field strength by picosecond photoconductivity measurements. Two different behaviours are obtained in dependence on the excitation energy. In the case of low excitation a region of negative differential conductivity due to the electron transfer from r-valley into the L-valley exists. I n the case of high excitation the formation of charge domains lowers the mobility.Die Elektronendriftgeschwindigkeit in a-kompensiertem GaAs wird mittels PikosekundenPhotoleitfahigkeit bei unterschiedlichen Feldstiirken nnterwcht. Die gemessene Abhiingigkeit unteracheidet aich bei ~terschiedlichen Anregnngsenergien. h i niedriger Anregung wird ein Gebiet negativer differentieller Leitfiihigkeit gefunden, was sich auf den Elektronentransfer vom I'-Punkt zum L-Punkt des Leitungsbandes zuriickfiihren la&. Bei hoher Anregung verringert das Auftreten von Ladungsdomiinen die Beweglichkeit.

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