Illumination and annealing characteristics of two-dimensional electron gas systems in metal-organic vapor-phase epitaxy grown AlGaN/AlN/GaN heterostructures

We studied the persistent photoconductivity (PPC) effect in AlGaN/AlN/GaN heterostructures with two different Al-compositions (x=0.15 and x=0.25). The two-dimensional electron gas formed at the AlN/GaN heterointerface was characterized by Shubnikov-de Haas and Hall measurements. Using optical illumination, we were able to increase the carrier density of the Al0.15Ga0.85N/AlN/GaN sample from 1.6x10^{12} cm^{-2} to 5.9x1012 cm^{-2}, while the electron mobility was enhanced from 9540 cm2/Vs to 21400 cm2/Vs at T = 1.6 K. The persistent photocurrent in both samples exhibited a strong dependence on illumination wavelength, being highest close to the bandgap and decreasing at longer wavelengths. The PPC effect became fairly weak for illumination wavelengths longer than 530 nm and showed a more complex response with an initial negative photoconductivity in the infrared region of the spectrum (>700 nm). The maximum PPC-efficiency for 390 nm illumination was 0.011% and 0.005% for Al0.25Ga0.75N/AlN/GaN and Al0.15Ga0.85N/AlN/GaN samples, respectively. After illumination, the carrier density could be reduced by annealing the sample. Annealing characteristics of the PPC effect were studied in the 20-280 K temperature range. We found that annealing at 280 K was not sufficient for full recovery of the carrier density. In fact, the PPC effect occurs in these samples even at room temperature. Comparing the measurement results of two samples, the Al0.25Ga0.75N/AlN/GaN sample had a larger response to illumination and displayed a smaller recovery with thermal annealing. This result suggests that the energy scales of the defect configuration-coordinate diagrams for these samples are different, depending on their Al-composition.Comment: 27 pages, 8 figure

Analysis on the hull girder ultimate strength of a bulk carrier using simplified method based on an incremental-iterative approach

The hull girder ultimate strength of a typical bulk carrier is analyzed using a simplified method based on an incremental-iterative approach. First, vertical bending moment is examined by seven different methods. The moment versus curvature curves and the values of the ultimate longitudinal moments at collapse states are determined for both hogging and sagging cases. Second, the ultimate strength under coupled vertical and horizontal bending moment is accounted. An interaction curve is obtained, which corresponds to the results of series of calculation for the ship hull subject to bending conditions with different angles of curvature. It is found that the interaction curve is asymmetrical because the hull cross section is not symmetrical with respect to the horizontal axis and the structural response of the elements under compression is different from that under tension due to nonlinearity caused by buckling. The angles of the resultant bending moment vector and that of the curvature vector are different in investigated cases. The interaction design equations proposed by other researches are also addressed to discuss the results presented by this study

Weak antilocalization and zero-field electron spin splitting in AlGaN/AlN/GaN heterostructures with a polarization induced two-dimensional electron gas

Spin-orbit coupling is studied using the quantum interference corrections to conductance in AlGaN/AlN/GaN two-dimensional electron systems where the carrier density is controlled by the persistent photoconductivity effect. All the samples studied exhibit a weak antilocalization feature with a spin-orbit field of around 1.8 mT. The zero-field electron spin splitting energies extracted from the weak antilocalization measurements are found to scale linearly with the Fermi wavevector with an effective linear spin-orbit coupling parameter 5.5x10^{-13} eV m. The spin-orbit times extracted from our measurements varied from 0.74 to 8.24 ps within the carrier density range of this experiment.Comment: 16 pages, 4 figure

Direct measurement of molecular stiffness and damping in confined water layers

We present {\em direct} and {\em linear} measurements of the normal stiffness and damping of a confined, few molecule thick water layer. The measurements were obtained by use of a small amplitude (0.36 $\textrm{\AA}$), off-resonance Atomic Force Microscopy (AFM) technique. We measured stiffness and damping oscillations revealing up to 7 layers separated by 2.56 $\pm$ 0.20 $\textrm{\AA}$. Relaxation times could also be calculated and were found to indicate a significant slow-down of the dynamics of the system as the confining separation was reduced. We found that the dynamics of the system is determined not only by the interfacial pressure, but more significantly by solvation effects which depend on the exact separation of tip and surface. Thus ` solidification\rq seems to not be merely a result of pressure and confinement, but depends strongly on how commensurate the confining cavity is with the molecule size. We were able to model the results by starting from the simple assumption that the relaxation time depends linearly on the film stiffness.Comment: 7 pages, 6 figures, will be submitted to PR

Spectral characterizations and antibacterial effect of 2-(5-R-1H-benzimidazol-2-YL)-4-methyl/bromo-phenols and some metal complexes

2-(5-H/Cl/Me/NO2-1H-benzimidazol-2-yl)-4-Me/Br-phenols (HL1–HL5) were synthesized. HL1 complexes with Cu(NO3)2, AgNO3, Zn(ClO4)2 and; HL4, HL5 complexes with Zn(ClO4)2 were prepared. The structures of the compounds were confirmed on the basis of elemental analysis, molar conductivity, magnetic moment, FT-IR, 1H- and 13C-NMR. Antibacterial activity of the ligands and the complexes were evaluated using the disk diffusion method in dimethyl sulfoxide (DMSO) as well as the minimal inhibitory concentration (MIC) dilution method, against nine bacteria, and the results were compared with penicillin–G and oxytetracycline. While HL1 ligand has considerable antibacterial activity on B. cereus only; it’s Ag(I) complex show antibacterial effect toward almost all the bacteria. It is highly interesting that HL5 and [Zn(HL5)(L5)]ClO4 exhibit considerable high antibacterial activity toward K. pneumoniae, B. cereus, S. epidermidis and B. subtilis. KEY WORDS: Benzimidazole, Phenol, Metal complexes, Antibacterial activity  Bull. Chem. Soc. Ethiop. 2010, 24(3), 391-400

Fast and frugal heuristics for portfolio decisions with positive project interactions

Funding: ID is supported in part by funding from the National Research Foundation of South Africa (Grant ID 90782, 105782).We consider portfolio decision problems with positive interactions between projects. Exact solutions to this problem require that all interactions are assessed, requiring time, expertise and effort that may not always be available. We develop and test a number of fast and frugal heuristics – psychologically plausible models that limit the number of assessments to be made and combine these in computationally simple ways – for portfolio decisions. The proposed “add-the-best” family of heuristics constructs a portfolio by iteratively adding a project that is best in a greedy sense, with various definitions of “best”. We present analytical results showing that information savings achievable by heuristics can be considerable; a simulation experiment showing that portfolios selected by heuristics can be close to optimal under certain conditions; and a behavioral laboratory experiment demonstrating that choices are often consistent with the use of heuristics. Add-the-best heuristics combine descriptive plausibility with effort-accuracy trade-offs that make them potentially attractive for prescriptive use.PostprintPeer reviewe

Equivalence of matter-type modified gravity theories to general relativity with nonminimal matter interaction

We show that gravity models, such as $f(\mathcal{L}_{\rm m})$, $f(g_{\mu\nu} T^{\mu\nu})$ and $f(T_{\mu\nu} T^{\mu\nu})$, that modify the introduction of the material source in the usual Einstein-Hilbert action by adding only matter-related terms to the matter Lagrangian density $\mathcal{L}_{\rm m}$ are equivalent to general relativity with nonminimal interactions. Through the redefinition $\mathcal{L}_{\rm m}+f \rightarrow \mathcal{L}_{\rm m}^{\rm tot}$, these models are exactly GR, yet the usual material field $T_{\mu\nu}$ and its accompanying partner, viz., the modification field $T_{\mu\nu}^{\rm mod}$ interact nonminimally. That is, $\nabla^{\mu}T_{\mu\nu}=-Q_{\nu}=-\nabla^{\mu}T_{\mu\nu}^{\rm mod}$, where $Q_{\nu}$ is the interaction kernel that governs the rate of energy transfer. We focus on the particular model, the energy-momentum squared gravity, where the usual material field $T_{\mu\nu}$ brings in an accompanying energy-momentum squared field , $T_{\mu\nu}^{\rm emsf}$ along with a sui generis nonminimal interaction between them. Compared to usual phenomenological nonminimal interaction models in the literature, EMSF gives rise to more intricate interaction kernels having covariant formulation even with simple forms of the $f$ function. We elaborate upon EMSF via some different aspects: a DE component induced from the interaction of sources such as cold dark matter and relativistic species with their accompanying EMSFs generating interacting DE-DM models, mimicking noncanonical scalar field, etc., or a Hoyle-type creation field generating steady-state universe models extended to fluids other than dust and a mimicker of modified generalized Chaplygin gas. We also demonstrate the proper calculation of second metric variation of $\mathcal{L}_{\rm m}$, as well as in models that contain scalars like $g_{\mu\nu} T^{\mu\nu}\,,R_{\mu\nu}T^{\mu\nu}$ and $G_{\mu\nu} T^{\mu\nu}$.Comment: 16 pages, no figures and table