861 research outputs found
Author response: India and China in Africa: a comparative perspective of the oil industry by Raj Verma
No full textEarlier this month Ian Taylor reviewed India and China in Africa, a new book about Asian engagement in the West African oil industry. Here, the book’s author Raj Verma responds to Taylor’s comments, outlining the rationale and evidence for the framework used in the study. India and China in Africa: A comparative perspective of the oil industry. Raj Verma. London: Routledge. 2017
Molecular beam epitaxy of InAs Quantum Dot and Quantum Dot molecule heterostructures
No full textVerspannungsgetriebene selbstorganisierte InAs-QDs auf GaAs(100), die über den Stranski-Krastonov-Wachstumsmodus hergestellt wurden, sind wohl das am häufigsten untersuchte QD-System. Für verschiedene Anwendungen werden QD-Dichten in der Größenordnung von 1 pro QD m2 benötigt. Allerdings ist die Herstellung solch geringer Dichten aufgrund der extrem starken Abhängigkeit von der In-Menge recht anspruchsvoll. In dieser Arbeit werden selbstorganisierte InAs-Quantenpunkte (QDs) und Quantenpunktmoleküle (QDMs) durch Molekularstrahlepitaxie auf GaAs(100)-Substrat aufgewachsen. Im ersten Teil werden InAs-QDs mit niedriger Dichte unter Verwendung von zwei In-Deposition-Verfahren hergestellt: Der In-Gradient-Ansatz und der homogene In-Deposition-Ansatz mit anschließendem Tempern, um Dichten in der Größenordnung von 1x 107-108 QDs cm-2 zu erreichen. Beim In-Gradienten-Ansatz wird die Substratrotation während der In-Abscheidung gestoppt, entweder für die vollständige Abscheidung (Vollgradient) oder für die Hälfte der Abscheidungszeit (Halbgradient). Ein Vergleich der beiden In-Deposition-Schemata wurde durchgeführt, um ein besseres Verständnis der Region geringer Flächendichte zu erlangen. Beim Full-Gradienten erstreckt sich der Niedrig-Dichte-Bereich nur über einen kleinen Bereich entlang des Gradienten, was im besten Fall weniger als 10 % eines vollen 3"-Wafers entspricht, während er bei einem Half-Gradienten auf 15 % ansteigt Im Vergleich dazu konnten wir für den homogenen Abscheidungsansatz für mehr als 70 % eines 3"-Wafers eine QD-Dichte zwischen 1107 und 1108 QDs cm-2 erreichen. Dieser Prozess ist jedoch aufgrund der hohen Empfindlichkeit gegenüber der Substrattemperatur und der In-Menge weniger reproduzierbar. Die verschiedenen QD-Wachstumsparameter wie QD-Wachstumstemperatur,Strain-driven self-assembled InAs quantum dots (QDs) on GaAs(100) substrates fabricated via the Stranski-Krastanov (S-K) growth mode is evidently the most widely studied QD system. QD densities in the order of 1 QD per m2 are required for different applications. However, preparing such low densities is challenging due to the extreme dependence on the Indium (In) amount. In this work, self-assembled InAs QDs and quantum dot molecules (QDMs) are grown on GaAs(100) substrate by the molecular beam epitaxy (MBE). In the first part, low-density InAs QDs are fabricated using two In-deposition schemes: the In-gradient approach and the homogenous In-deposition approach with subsequent annealing to attain densities in the order of 1 107-108 QDs cm-2. In the In-gradient approach, the substrate rotation is stopped during In-deposition, either for the full deposition (full-gradient) or for half of the deposition time (half-gradient). A comparison of the two In-deposition schemes has been done to better comprehend low areal density coverage For the full-gradient, the low-density region stretches only over a small area along the gradient, which corresponds in the best case to less than 10 % of a full 3" wafer, whereas for a half-gradient, it increases to 15 %. In comparison, for the homogenous deposition approach, we were able to achieve for more than 70% of a 3" wafer a QD density between 1107 and 1108 QDs cm-2. However, this process is less reproducible due to the high sensitivity to the substrate temperature and the In amount. In the second part various QD growth parameters, such as QD growth temperature, growth interruption, and In-flux, are studied to observe the QD size modification leading to emission wavelength variation. In the last part of this work, QDM heterostructures for electron and hole storage were fabricated. ...by Akshay Kumar Verma ; Doctoral committee: Prof. Dr. Dirk Reuter, Prof. Dr. Cedrik Meier]Tag der Verteidigung: 23.02.2022Universität Paderborn, Dissertation, 202
Sweeping has no effect on renormalized turbulent viscosity
No full textWe perform renormalization group analysis (RG) of the Navier-Stokes equation in the presence of constant mean velocity field , and show that the renormalized viscosity is unaffected by , thus negating the ``sweeping effect", proposed by Kraichnan [Phys. Fluids {\bf 7}, 1723 (1964)] using random Galilean invariance. Using direct numerical simulation, we show that the correlation functions for and differ from each other, but the renormalized viscosity for the two cases are the same. Our numerical results are consistent with the RG calculations
A Unified Shell model for Buoyancy-Driven Turbulence
No full textWe construct a unified shell model for stably stratified and convective turbulence. Shell model simulation of stably stratified flow in turbulent regime exhibit Bolgiano-Obukhbov (BO) scaling in which the kinetic energy spectrum varies as . However, simulation of convective turbulence shows Kolmogorov's spectrum. These results are consistent with the direct numerical simulations of Kumar {\em et al.} [Phys. Rev. E {\bf 90}, 023016 (2014)]. We also observe a dual scaling ( and ) for a limited range of parameters in stably stratified flow
Energy transfers in small-scale and large-scale dynamos
No full textWe study energy transfers during magnetic energy growth in small-scale and large-scale dynamos. We perform direct numerical simulations for magnetic Prandtl number Pm =20 and 0.2 in a periodic box on 1024^3 grid. Energy fluxes and shell-to-shell energy transfers indicate that in small-scale dynamo for Pm =20, the magnetic energy growth takes place due to a non-local energy transfer from large-scale velocity field to small-scale magnetic field. On the other hand, in large-scale dynamo for Pm =0.2, local energy transfers from large-scale velocity field to large-scale magnetic field takes place
Role of the strain-rate tensor in turbulent scalar-transport modeling
No full textWe examine the geometric orientation of the subfilter-scale scalar-flux vector in homogeneous isotropic turbulence. Vector orientation is determined using the eigenframe of the resolved strain-rate tensor. The Schmidt number is kept sufficiently large so as to leave the velocity field, and hence, the strain-rate tensor, unaltered by filtering in the viscous-convective subrange. Strong preferential alignment is observed for the case of Gaussian and box filters, whereas the sharp-spectral filter leads to close to a random orientation. The orientation angle obtained with the Gaussian and box filters is largely independent of the filter-width and the Schmidt number. It is shown that the alignment direction observed numerically using these two filters is predicted very well by the tensor-diffusivity model. Further a-priori tests indicate poor alignment of the Smagorinsky and stretched vortex model predictions with the exact subfilter flux
Systems of Differential Operators and Generalized Verma Modules
No full textIn this paper we close the cases that were left open in our earlier works on the study of conformally invariant systems of second-order differential operators for degenerate principal series. More precisely, for these cases, we find the special values of the systems of differential operators, and determine the standardness of the homomorphisms between the generalized Verma modules, that come from the conformally invariant systems.The author was supported by the Global COE program at the Graduate School of Mathematical
Sciences, the University of Tokyo, Japan. He would like to be thankful for the referees for their
careful reading and invaluable comments
Twisted de Rham Complex on Line and Singular Vectors in sl₂ˆ Verma Modules
No full textWe consider two complexes. The first complex is the twisted de Rham complex of scalar meromorphic differential forms on the projective line, holomorphic on the complement to a finite set of points. The second complex is the chain complex of the Lie algebra of sl₂-valued algebraic functions on the same complement, with coefficients in a tensor product of contragradient Verma modules over the affine Lie algebra sl₂ˆ. In [Schechtman V., Varchenko A., Mosc. Math. J. 17 (2017), 787-802] a construction of a monomorphism of the first complex to the second was suggested, and it was indicated that under this monomorphism, the existence of singular vectors in the Verma modules (the Malikov-Feigin-Fuchs singular vectors) is reflected in the relations between the cohomology classes of the de Rham complex. In this paper, we prove these results.The authors thank V. Schechtman for useful discussions. The second author was supported in part by NSF grant DMS-1665239
Supplemental Material - Disadvantaged communities have lower access to urban infrastructure
No full textSupplemental Material for Disadvantaged communities has lower access to urban infrastructure by Leonardo Nicoletti, Mikhail Sirenko and Trivik Verma in Environment and Planning B: Urban Analytics and City Science</p
Similarities between 2D and 3D convection for large Prandtl number
No full textUsing direct numerical simulations of Rayleigh-B\'enard convection (RBC), we perform a comparative study of the spectra and fluxes of energy and entropy for large and infinite Prandtl numbers in two (2D) and three (3D) dimensions. We observe close similarities between the 2D and 3D RBC, in particular the kinetic energy spectrum , and the entropy spectrum exhibits a dual branch with a dominant spectrum. We showed that the dominant Fourier modes in the 2D and 3D flows are very close
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