Solid solution decomposition and Guinier-Preston zone formation in Al-Cu alloys: A kinetic theory with anisotropic interactions

Using methods of statistical kinetic theory parametrized with first-principles interatomic interactions that include chemical and strain contributions, we investigated the kinetics of decomposition and microstructure formation in Al-Cu alloys as a function of temperature and alloy concentration. We show that the decomposition of the solid solution forming platelets of copper, known as Guinier-Preston (GP) zones, includes several stages and that the transition from GP1 to GP2 zones is determined mainly by kinetic factors. With increasing temperature, the model predicts a gradual transition from platelet-like precipitates to equiaxial ones and at intermediate temperatures both precipitate morphologies may coexist.Comment: 9 pages, 8 figure

SDRS—an algorithm for analyzing large-scale dose–response data

Summary: Dose–response information is critical to understanding drug effects, yet analytical methods for dose–response assays cannot cope with the dimensionality of large-scale screening data such as the microarray profiling data. To overcome this limitation, we developed and implemented the Sigmoidal Dose Response Search (SDRS) algorithm, a grid search-based method designed to handle large-scale dose–response data. This method not only calculates the pharmacological parameters for every assay, but also provides built-in statistic that enables downstream systematic analyses, such as characterizing dose response at the transcriptome level

Quantum Coherence in a Single Ion due to strong Excitation of a metastable Transition

We consider pump-probe spectroscopy of a single ion with a highly metastable (probe) clock transition which is monitored by using the quantum jump technique. For a weak clock laser we obtain the well known Autler-Townes splitting. For stronger powers of the clock laser we demonstrate the transition to a new regime. The two regimes are distinguished by the transition of two complex eigenvalues to purely imaginary ones which can be very different in magnitude. The transition is controlled by the power of the clock laser. For pump on resonance we present simple analytical expressions for various linewidths and line positions.Comment: 6 figures. accepted for publication in PR

Aerothermal Ground Testing of Flexible Thermal Protection Systems for Hypersonic Inflatable Aerodynamic Decelerators

Flexible TPS development involves ground testing and analysis necessary to characterize performance of the FTPS candidates prior to flight testing. This paper provides an overview of the analysis and ground testing efforts performed over the last year at the NASA Langley Research Center and in the Boeing Large-Core Arc Tunnel (LCAT). In the LCAT test series, material layups were subjected to aerothermal loads commensurate with peak re-entry conditions enveloping a range of HIAD mission trajectories. The FTPS layups were tested over a heat flux range from 20 to 50 W/cm with associated surface pressures of 3 to 8 kPa. To support the testing effort a significant redesign of the existing shear (wedge) model holder from previous testing efforts was undertaken to develop a new test technique for supporting and evaluating the FTPS in the high-temperature, arc jet flow. Since the FTPS test samples typically experience a geometry change during testing, computational fluid dynamic (CFD) models of the arc jet flow field and test model were developed to support the testing effort. The CFD results were used to help determine the test conditions experienced by the test samples as the surface geometry changes. This paper includes an overview of the Boeing LCAT facility, the general approach for testing FTPS, CFD analysis methodology and results, model holder design and test methodology, and selected thermal results of several FTPS layups

High temperature oxidation resistance in titanium-niobium alloys

Titanium alloys are ideally suited for use as lightweight structural materials, but their use at high temperature is severely restricted by oxidation. Niobium is known to confer oxidation-resistance, and here we disprove the normal explanation, that Nb5+ ions trap oxygen vacancies. Using density functional theory calculation, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) we show that Nb is insoluble in TiO2. In fact, the Ti–Nb surface has three-layer structure: the oxide itself, an additional Nb-depleted zone below the oxide and a deeper sublayer of enhanced Nb. Microfocussed X-ray diffraction also demonstrates recrystallization in the Nb-depleted zone. We interpret this using a dynamical model: slow Nb-diffusion leads to the build up of a Nb-rich sublayer, which in turn blocks oxygen diffusion. Nb effects contrast with vanadium, where faster diffusion prevents the build up of equivalent structures

Ground state laser cooling using electromagnetically induced transparency

A laser cooling method for trapped atoms is described which achieves ground state cooling by exploiting quantum interference in a driven Lambda-shaped arrangement of atomic levels. The scheme is technically simpler than existing methods of sideband cooling, yet it can be significantly more efficient, in particular when several motional modes are involved, and it does not impose restrictions on the transition linewidth. We study the full quantum mechanical model of the cooling process for one motional degree of freedom and show that a rate equation provides a good approximation.Comment: 4 pages, 3 figures; v2: minor modifications to abstract, text and figure captions; v3: few references added and rearranged; v4: One part significantly changed, 1 figure removed, new equations; v5: typos corrected, to appear in PR

Destabilization of dark states and optical spectroscopy in Zeeman-degenerate atomic systems

We present a general discussion of the techniques of destabilizing dark states in laser-driven atoms with either a magnetic field or modulated laser polarization. We show that the photon scattering rate is maximized at a particular evolution rate of the dark state. We also find that the atomic resonance curve is significantly broadened when the evolution rate is far from this optimum value. These results are illustrated with detailed examples of destabilizing dark states in some commonly-trapped ions and supported by insights derived from numerical calculations and simple theoretical models.Comment: 14 pages, 10 figure