Light Railways for Tropical Africa

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Lepton - Chargino Mixing and R-Parity Violating SUSY

We present a study of charged lepton mass matrix diagonalization in R-parity violating SUSY. The case in which the bilinear couplings $\mu_i$ have large values is given special attention.Comment: 7 pages, LaTeX, 1 postscript figure, uses aipproc.sty; presented at the 21th annual MRST meeting on High-Energy Physics, Ottawa, Canada, 10-12 May 199

The Decompostion of Potassium Chlorate in the Presence of Iron Oxide

The decomposition temperature of potassium chlorate is known to be lowered by the presence of varying quantities of different substances, especially oxides, which act in the capacity of catalytic agents in causing the evolution of oxygen. In most instances chlorine is also liberated. This investigation on the effect of iron oxide was begun as one of a series of experiments to determine the effectiveness of various oxides in catalyzing the decomposition of potassium chlorate

Effect of surface tension on the growth mode of highly strained InGaAs on GaAs(100)

We have investigated the molecular beam epitaxy growth of highly strained InGaAs on GaAs(100) as a function of the anion to cation flux ratio. Using reflection high energy electron diffraction the evolution of the film morphology is monitored and the surface lattice constant is measured. It is found that the cation to anion flux ratio dramatically affects the growth mode. Under arsenic‐rich conditions, growth is characterized by a two‐dimensional (2D) to three‐dimensional (3D) morphological transformation. However, for cation‐stabilized conditions, 3D islanding is completely suppressed, and 2D planar growth is observed. We associate these differences in the growth mode with corresponding changes in the surface tension of the overlayer. A high surface tension stabilizes 2D growth. An analysis which relates surface tension to a critical thickness for the onset of coherent island formation supports this view.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70295/2/APPLAB-62-1-46-1.pd

Directed network of substorms using SuperMAG ground‐based magnetometer data

We quantify the spatio‐temporal evolution of the substorm ionospheric current system utilizing the SuperMAG 100+ magnetometers. We construct dynamical directed networks from this data for the first time. If the canonical cross‐correlation (CCC) between vector magnetic field perturbations observed at two magnetometer stations exceeds a threshold, they form a network connection. The time lag at which CCC is maximal determines the direction of propagation or expansion of the structure captured by the network connection. If spatial correlation reflects ionospheric current patterns, network properties can test different models for the evolving substorm current system. We select 86 isolated substorms based on nightside ground station coverage. We find, and obtain the timings for, a consistent picture in which the classic substorm current wedge (SCW) forms. A current system is seen pre‐midnight following the SCW westward expansion. Later, there is a weaker signal of eastward expansion. Finally, there is evidence of substorm‐enhanced convection

A combined molecular‐beam epitaxy and scanning tunneling microscopy system

A combined molecular‐beam epitaxy and scanning tunneling microscopy system has been constructed. The design has been optimized for the study of III‐V semiconductors with the goal of examining the surface with both in situ scanning tunneling microscopy (STM) and reflection high‐energy electron diffraction (RHEED). Using this system, it is possible to quench the growth and produce real‐space images of the surface as it appeared during deposition. Measurements obtained with both RHEED and STM are presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70399/2/RSINAK-62-6-1400-1.pd

Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows

The nature of dynamo action in shear flows prone to magnetohydrodynamic instabilities is investigated using the magnetorotational dynamo in Keplerian shear flow as a prototype problem. Using direct numerical simulations and Newton's method, we compute an exact time-periodic magnetorotational dynamo solution to the three-dimensional dissipative incompressible magnetohydrodynamic equations with rotation and shear. We discuss the physical mechanism behind the cycle and show that it results from a combination of linear and nonlinear interactions between a large-scale axisymmetric toroidal magnetic field and non-axisymmetric perturbations amplified by the magnetorotational instability. We demonstrate that this large scale dynamo mechanism is overall intrinsically nonlinear and not reducible to the standard mean-field dynamo formalism. Our results therefore provide clear evidence for a generic nonlinear generation mechanism of time-dependent coherent large-scale magnetic fields in shear flows and call for new theoretical dynamo models. These findings may offer important clues to understand the transitional and statistical properties of subcritical magnetorotational turbulence.Comment: 10 pages, 6 figures, accepted for publication in Physical Review

CCD‐based reflection high‐energy electron diffraction detection and analysis system

A CCD‐based, computer controlled RHEED detection and analysis system that utilizes an on‐chip integration technique and on‐board data manipulation is described. The system is capable of in situ time‐resolved measurements of specular and integral‐order intensity oscillations, their phase differences, streak linewidths, and epitaxial layer lattice constants. The digital RHEED techniques are described in the context of Co/Au bilayer, GaAs/GaAs, and InxGa1−xAs/GaAs MBE growth. The system is compared to other RHEED detection devices.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70680/2/RSINAK-62-5-1263-1.pd

Linear stability, transient energy growth and the role of viscosity stratification in compressible plane Couette flow

Linear stability and the non-modal transient energy growth in compressible plane Couette flow are investigated for two prototype mean flows: (a) the {\it uniform shear} flow with constant viscosity, and (b) the {\it non-uniform shear} flow with {\it stratified} viscosity. Both mean flows are linearly unstable for a range of supersonic Mach numbers ($M$). For a given $M$, the critical Reynolds number ($Re$) is significantly smaller for the uniform shear flow than its non-uniform shear counterpart. An analysis of perturbation energy reveals that the instability is primarily caused by an excess transfer of energy from mean-flow to perturbations. It is shown that the energy-transfer from mean-flow occurs close to the moving top-wall for ``mode I'' instability, whereas it occurs in the bulk of the flow domain for ``mode II''. For the non-modal analysis, it is shown that the maximum amplification of perturbation energy, $G_{\max}$, is significantly larger for the uniform shear case compared to its non-uniform counterpart. For $\alpha=0$, the linear stability operator can be partitioned into ${\cal L}\sim \bar{\cal L} + Re^2{\cal L}_p$, and the $Re$-dependent operator ${\cal L}_p$ is shown to have a negligibly small contribution to perturbation energy which is responsible for the validity of the well-known quadratic-scaling law in uniform shear flow: $G(t/{\it Re}) \sim {\it Re}^2$. A reduced inviscid model has been shown to capture all salient features of transient energy growth of full viscous problem. For both modal and non-modal instability, it is shown that the {\it viscosity-stratification} of the underlying mean flow would lead to a delayed transition in compressible Couette flow

Estimating the duration of speciation from phylogenies

Speciation is not instantaneous but takes time. The protracted birth-death diversification model incorporates this fact and predicts the often observed slowdown of lineage accumulation toward the present. The mathematical complexity of the protracted speciation model has barred estimation of its parameters until recently a method to compute the likelihood of phylogenetic branching times under this model was outlined (Lambert et al. ). Here, we implement this method and study using simulated phylogenies of extant species how well we can estimate the model parameters (rate of initiation of speciation, rate of extinction of incipient and good species, and rate of completion of speciation) as well as the duration of speciation, which is a combination of the aforementioned parameters. We illustrate our approach by applying it to a primate phylogeny. The simulations show that phylogenies often do not contain enough information to provide unbiased estimates of the speciation-initiation rate and the extinction rate, but the duration of speciation can be estimated without much bias. The estimate of the duration of speciation for the primate clade is consistent with literature estimates. We conclude that phylogenies combined with the protracted speciation model provide a promising way to estimate the duration of speciation.</p