Investigation of persistent photoconductivity in Si-dopedn-Al x Ga1−x as grown by molecular beam epitaxy

The persistent photoconductivity effect in Si-dopedn-Al _x Ga_1?x As layers grown by molecular beam epitaxy on (100)GaAs substrates has been investigated by detailed Halleffect and capacitance measurements at 10–300 K. In the alloy composition range 0.25<x <0.40 the electrical properties ofn-Al _x Ga_1?x As are governed by a deep electron trap having an emission barrier of 0.34–0.40 eV (depending on the doping concentration), as determined by admittance measurements. The concentration of deep electron traps, deduced from low-temperature capacitance measurements, is found to coincide with the amount of persistent photoconductivity observed in the material. Consequently, the earlier proposed population of two-dimensional subbands at the Al _x Ga_1?x As/GaAs-substrate hetero-interface, i.e. charge separation bymacroscopic barriers, can not account for the measured high overall number of persistent photoexcited carriers. Instead, the vanishing small capture rates of photoexcited electrons result frommicroscopic capture barriers. The dominant deep electron trap, which we attribute to deep donor-type (DX) centers, is found to be homogeneously distributed throughout the Al _x Ga_1?x As layer depth. From our Hall effect measurements a trap depth of 0.05–0.12 eV (depending on the doping concentration) below the conduction band is derived. The capture barrier is thus in the order of 0.30 eV. This value is in excellent agreement with data obtained from liquid phase epitaxially grown Si-dopedn-Al _x Ga_1?x As.     

Unfolding the singularities in superspace

A method is described for unfolding the singularities in superspace, \(\mathcal{G} = \mathfrak{M}/\mathfrak{D}\) , the space of Riemannian geometries of a manifoldM. This unfolded superspace is described by the projection $$\mathcal{G}_{F\left( M \right)} = \frac{{\mathfrak{M} \times F\left( M \right)}}{\mathfrak{D}} \to \frac{\mathfrak{M}}{\mathfrak{D}} = \mathcal{G}$$ whereF(M) is the frame bundle ofM. The unfolded space \(\mathcal{G}_{F\left( M \right)}\) is infinite-dimensional manifold without singularities. Moreover, as expected, the unfolding of \(\mathcal{G}_{F\left( M \right)}\) at each geometry [g _o] ∈ \(\mathcal{G}\) is parameterized by the isometry groupIg _o (M) of g₀. Our construction is natural, is generally covariant with respect to all coordinate transformations, and gives the necessary information at each geometry to make \(\mathcal{G}\) a manifold. This construction is a canonical and geometric model of a nonrelativistic construction that unfolds superspace by restricting to those coordinate transformations that fix a frame at a point. These particular unfoldings are tied together by an infinite-dimensional fiber bundleE overM, associated with the frame bundleF(M), with standard fiber \(\mathcal{G}_{F\left( M \right)}\) , and with fiber at a point inM being the particular noncanonical unfolding of \(\mathcal{G}\) based at that point. ThusE is the totality of all the particular unfoldings, and so is a grand unfolding of \(\mathcal{G}\) .     

Problems, Models and Complexity. Part II: Application to the DLSP

In Part I of this study, we suggest to identify an operations research (OR) problem with the equivalence class of models describing the problem and enhance the standard computer-science theory of computational complexity to be applicable to this situation of an often model-based OR context. The Discrete Lot-sizing and Scheduling Problem (DLSP) is analysed here in detail to demonstrate the difficulties which can arise if these aspects are neglected and to illustrate the new theoretical concept. In addition, a new minimal model is introduced for the DLSP which makes this problem eventually amenable to a rigorous analysis of its computational complexity.     

Influence of crystal defects on the irreversibility line of YBa2Cu3O7-x single crystals

High quality flux grown and defect enriched peritectically grown YBa₂Cu₃O_7–x single crystals have been investigated by an ac-susceptibility technique. This method allows to determine an irreversibility line from the temperature and field dependence of the peaked imaginary part of the susceptibility, which is due to magnetic losses. For magnetic fieldsH _ac perpendicular to thec-axis of the crystal, the irreversibility line of the defect enriched crystal shows a shift to higher field values as compared to the perfect crystal, a sign that crystal defects like Y₂BaCuO₅(211)-precipitates and microcracks act as strong pinning centers. ForH _ac parallel to thec-axis no clear evidence for a stronger pinning is found. From these results we conclude that different pinning mechanisms are dominating at different field orientations.     

High-spin structure of N\simeq Z nuclei around the A = 72 region

High-spin states have been studied in ⁷²Kr and ⁷²Br using the ⁴⁰Ca + ⁴⁰Ca and ³⁶Ar + ⁴⁰Ca reactions at 164 and 145 MeV, respectively. The properties and configurations of the high-spin bands observed have been interpreted using unpaired cranked Nilsson-Strutinsky (CNS), and for ⁷²Kr, paired cranked relativistic Hartree-Bogoliubov (CRHB) calculations. In ⁷²Kr a new band has been identified that has the properties expected for the doubly aligned S-band configuration. In ⁷²Br the previously known bands have been extended to higher spin. This has lead to a re-interpretation of the configurations.Received: 31 October 2002, Published online: 24 February 2004PACS: 21.10.Re Collective levels - 23.20.Lv transitions and level energies - 27.50. + e