Ultrafast optical pulses are used to initiate and measure free-induction decays of coherent conduction electron spins and of embedded magnetic Mn 2ϩ ions in a series of magnetic-semiconductor quantum wells. These time-resolved Faraday rotation experiments in transverse applied magnetic fields complement previous studies of spin dynamics in longitudinal fields by unambiguously distinguishing between the spin relaxation of electrons and holes, and by identifying a mechanism by which angular momentum is transferred from spinpolarized carriers to the sublattice of local moments. In transverse fields ͑Voigt geometry͒, the precession of the photoexcited spins about the field axis can be measured as an oscillatory induced Faraday rotation signal. We observe the THz free-induction decay of spin-polarized electrons in modest ͑Ͻ4 T͒ magnetic fields and separately identify the more rapid spin relaxation of the holes as functions of field and temperature. The g factors of the electrons and holes are accurately measured as a function of well width. The role of quantum confinement on the stability of the hole spin is discussed, with particular attention given to the observed ability of the transient hole-exchange field to coherently rotate a macroscopic ensemble of local Mn 2ϩ moments. This ''tipping pulse'' initiates a free-induction decay in the sublattice of Mn 2ϩ spins and enables electron paramagnetic resonance ͑EPR͒ studies of the fractional monolayer magnetic planes. These time-domain EPR measurements reveal a significant magnetic field dependence of the Mn transverse spin relaxation time.

No abstract

Significant new mechanical and electronic phenomena can arise in single-crystal semiconductors when their thickness reaches nanometer dimensions, where the two surfaces of the crystal are physically close enough to each other that what happens at one surface influences what happens at the other. We show experimentally that, in silicon nanomembranes, through-membrane elastic interactions cause the double-sided ordering of epitaxially grown nanostressors that locally and periodically highly strains the membrane, leading to a strain lattice. Because strain influences band structure, we create a periodic band gap modulation, up to 20% of the band gap, effectively an electronic superlattice. Our calculations demonstrate that discrete minibands can form in the potential wells of an electronic superlattice generated by Ge nanostressors on a sufficiently thin Si(001) nanomembrane at the temperature of 77 K. We predict that it is possible to observe discrete minibands in Si nanoribbons at room temperature if nanostressors of a different material are grown.

No abstract

The resolution and wear properties of carbon nanotube and etched-silicon atomic force microscopy probes are compared in intermittent-contact mode. Carbon nanotube probes have at least 20 times the life of etched-silicon probes and provide better resolution at all stages. Sample wear is minimized with carbon nanotube probes. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1452782͔Since the discovery of carbon nanotubes ͑CNTs͒ in 1991, 1 much has been done to characterize their properties and explore their potential applications. Although many of these potential uses are still in the nascent stage, it has become clear that CNTs, because of their geometry and unique mechanical properties, are very well suited for scannedprobe microscopy probes, 2 in particular atomic-force microscopy ͑AFM͒, 3 but also magnetic-force microscopy 4 and electric-force microscopy. 5 CNTs are preferred probes for topographic imaging because they ͑1͒ provide improved resolution ͑uniformly good end form͒, ͑2͒ allow investigating deep and/or narrow surface features ͑very high aspect ratio and long length͒, ͑3͒ enable probing sensitive and/or easily damaged surface features ͑mechanical properties of the CNT͒, ͑4͒ have long life ͑anecdotal evidence of a factor of 10 increased life without degradation of resolution͒, and ͑5͒ provide enhanced capabilities for scanning in water ͑hydro-phobicity of the CNT͒. 6 In this letter we quantify the improved resolution, the long life and the superior imaging properties of CNT probes on fragile samples. We compare the wear and degradation of conventional commercial etched-silicon ͑ES͒ probes with those of multiwall CNTs during intermittent-contact imaging. We confirm at least an order of magnitude longer life of the CNT without degradation of resolution; in fact we are not able to find a reduction of resolution or any wear of the CNT even after more than two meters of scanning ͑over 1000 2 mϫ2 m images͒. Samples imaged with CNT probes also show negligible wear compared with those imaged with ES probes. Figure 1 shows a comparison of the resolution achievable with CNT ͑a͒ and ES ͑b͒ tips, using 10 nm Co spheres on Si͑111͒. The end form of tips made from multiwall CNTs or CNT bundles is invariably better than 20 nm in diameter, and can be made as small as 3 nm, providing resolution of this order. Although new ES tips can sometimes achieve the resolution of a CNT ͑we have never observed it to be better than that of a CNT͒, their resolution begins to degrade within two or three scans in most cases. Figure 1 also shows a comparison of the ability of the CNT tips to probe deep features ͑c͒ versus that of an ES tip ͑d͒. Because they are long, narrow tubes, CNTs have a high aspect ratio ͑tubes with lengths from nanometers to several micrometers can be fabricated as metrology probes͒. Conventional ͑ES͒ probes have a pyramidal shape, which is apparent in the AFM image of a deep trench. Thus, CNT probes can profile morphologies that are inaccessible to ES probes.The wear characteristics of conventional ES tips ...