We have investigated the exciton spin dynamics in quantum wells. In nonresonant excitation conditions, the initial degree of circular polarization of the exciton luminescence is higher than the excitation light polarization. This demonstrates that excitons are formed from nongeminate electron-hole pairs.The understanding of the exciton formation process allows us to propose a method to measure directly the hole spin-relaxation time (7. & j in intrinsic quantum wells. We show how the analysis of the total photoluminescence intensity under circularly and linearly polarized excitation allows us to measure vz, independently of the other spin-flip processes.
We report studies of strained-layer semiconductor superlattice structures under nanoindentation. Coherency strain reduces the yield stress at room temperature, from 6 GPa in unstrained material to 3 GPa in the most highly strained structures. The dependence of the yield stress on the design parameters of the superlattice structures shows that the onset of plastic deformation under an inhomogeneous stress is a cooperative process that takes place simultaneously across a nite volume more than 150 nm across. In this way, we demonstrate a new yield criterion, of which the key feature is that it is to be averaged over a nite volume. This provides a natural explanation of the indentation-hardness size e¬ect.
Multiple-quantum-well structures show thermally activated quenching of the photoluminescence.There are also small changes in photoluminescence intensity as carriers emitted from one well are retrapped in another. We present a coupled-well rate-equation theory which successfully models these changes in intensity in In~Ga&~As/GaAs and In~Ga&~As/GaAs/Al"Ga& "As structures. In both structures, the dominant nonradiative carrier loss from the wells is due to thermal excitation to the barriers; retrapping in the wells can be observed and is included in the model. In contrast, in In~Ga&~As/Al Ga, "As quantum wells, a defect-related nonradiative mechanism dominates even with an Al content in the Al"Gal As of only 5%.
Studies of the mechanical properties of single-walled carbon nanotubes are hindered by the availability only of ensembles of tubes with a range of diameters. Tunable Raman excitation spectroscopy picks out identifiable tubes. Under high pressure, the radial breathing mode shows a strong environmental effect shown here to be largely independent of the nature of the environment . For the G-mode, the pressure coefficient varies with diameter consistent with the thick-wall tube model. However, results show an unexpectedly strong environmental effect on the pressure coefficients. Reappraisal of data for graphene and graphite gives the G-mode Grüneisen parameter = 1.34 and the shear deformation parameter = 1.34.
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