Dynamic RHEED observations of the MBE growth of GaAs

Detailed measurements have been made of the specular beam intensity in RHEED patterns from static and growing GaAs surfaces. The basic parameters investigated were substrate temperature and electron beam azimuth. The results have provided further understanding of growth dynamics and surface disorder, respectively. There is a significant trend away from two-dimensional growth at the higher temperatures, which also correspond to more Ga-rich surface structures. Conversely, surface disorder is apparently greater during growth at the lower temperatures, where the structure is As-rich. The static As-stable 2×4 surface is, however, the most ordered and the most closely two-dimensional. It has also been shown that ordered, two-dimensional growth can be initiated from excess Ga adatom populations.     

Effects of diffraction conditions and processes on rheed intensity oscillations during the MBE growth of GaAs

The RHEED intensity oscillation technique has received wide-spread attention for the study of MBE growth dynamics, but insufficient consideration has been given to the diffraction conditions and processes involved. We report here a systematic investigation of the intensity oscillation behaviour as a function of diffraction parameters (azimuth, incidence angle, specular and non-specular beams), with constant growth conditions for GaAs films on GaAs (001) substrates.We show that many reported anomalies attributed to growth effects, such as phase differences and periodicity variations, can be accounted for entirely by diffraction events, provided it is realised that multiple scattering processes are the dominant cause of RHEED intensity variations during growth.The technique can provide valuable information on growth behaviour, but only if diffraction-dependent effects are first eliminated.     

On the complexity of a bundle pricing problem

We consider the problem of pricing items in order to maximize the revenue obtainable from a set of single minded customers. We relate the tractability of the problem to structural properties of customers’ valuations: the problem admits an efficient approximation algorithm, parameterized along the inhomogeneity of the valuations.     

Complex plasmas in external fields: the role of non-hamiltonian interactions

Dedicated experiments with strongly coupled complex plasmas in external electric fields were carried out under microgravity conditions using the PK-4 dc discharge setup. The focus was put on the comparative analysis of the formation of stringlike anisotropic structures due to reciprocal (hamiltonian) and nonreciprocal (non-hamiltonian) interactions between microparticles (induced by ac and dc fields, respectively). The experiments complemented by numerical simulations demonstrate that the responses of complex plasmas in these two regimes are drastically different. It is suggested that the observed difference is a manifestation of intrinsic thermodynamic openness of driven strongly coupled systems.     

Optical guided waves at graded metal-dielectric interfaces

Waveguides at interfaces with a linearly graded permittivity between a metal and a dielectric are studied. Analytic expressions for the dispersion relations, modes, losses, and cutoff wavelengths are derived and agree well with simulations. The gradation results in anomalous dispersion, a reduction in the energy velocity, and increased field confinement in the metal-dielectric transition region. These effects lead to increased propagation losses, which are sensitive to the spatial extent of the interface gradation.     

Iron(II)-catalyzed amidation of aldehydes with iminoiodinanes at room temperature and under microwave-assisted conditions

A method for the amidation of aldehydes with PhI=NTs/PhI=NNs as the nitrogen source and an inexpensive iron(II) chloride + pyridine as the in situ formed precatalyst under mild conditions at room temperature or microwave assisted conditions is described. The reaction was operationally straightforward and accomplished in moderate to excellent product yields (20-99%) and with complete chemoselectivity with the new C-N bond forming only at the formylic C-H bond in substrates containing other reactive functional groups. By utilizing microwave irradiation, comparable product yields and short reaction times of 1 h could be accomplished. The mechanism is suggested to involve insertion of a putative iron-nitrene/imido group to the formylic C-H bond of the substrate via a H-atom abstraction/radical rebound pathway mediated by the precatalyst [Fe(py)(4)Cl(2)] generated in situ from reaction of FeCl(2) with pyridine.     

Experimental Investigation of Gas Dynamic Effects Using Nanoporous Synthetic Materials as Tight Rock Analogues

Transport in Porous Media - To improve the understanding of gas transport processes in tight rocks (e.g., shales), systematic flow tests with different gases were conducted on artificial micro- to...     

Exotic phases of finite temperature gauge theories

We calculate the phase diagrams at high temperature of SU(N) gauge theories with massive fermions by minimizing the one-loop effective potential. Considering fermions in the adjoint (Adj) representation at various N we observe a variety of phases when N_f2 Majorana flavours and periodic boundary conditions are applied to fermions. Also the confined phase is perturbatively accessible. For N=3, we add Fundamental (F) representation fermions with antiperiodic boundary conditions to adjoint QCD to show how the Z(3)-symmetry breaks in the confined phase.