Electron g-Factor Anisotropy in Symmetric (110)-oriented GaAs Quantum Wells

We demonstrate by spin quantum beat spectroscopy that in undoped symmetric (110)-oriented GaAs/AlGaAs single quantum wells even a symmetric spatial envelope wavefunction gives rise to an asymmetric in-plane electron Land\'e-g-factor. The anisotropy is neither a direct consequence of the asymmetric in-plane Dresselhaus splitting nor of the asymmetric Zeeman splitting of the hole bands but is a pure higher order effect that exists as well for diamond type lattices. The measurements for various well widths are very well described within 14 x 14 band k.p theory and illustrate that the electron spin is an excellent meter variable to map out the internal -otherwise hidden- symmetries in two dimensional systems. Fourth order perturbation theory yields an analytical expression for the strength of the g-factor anisotropy, providing a qualitative understanding of the observed effects

Fatigue analysis-based numerical design of stamping tools made of cast iron

This work concerns stress and fatigue analysis of stamping tools made of cast iron with an essentially pearlitic matrix and containing foundry defects. Our approach consists at first, in coupling the stamping numerical processing simulations and structure analysis in order to improve the tool stiffness geometry for minimizing the stress state and optimizing their fatigue lifetime. The method consists in simulating the stamping process by considering the tool as a perfect rigid body. The estimated contact pressure is then used as boundary condition for FEM structure loading analysis of the tool. The result of this analysis is compared with the critical stress limit depending on the automotive model. The acceptance of this test allows calculating the fatigue lifetime of the critical zone by using the S–N curve of corresponding load ratio. If the prescribed tool life requirements are not satisfied, then the critical region of the tool is redesigned and the whole simulation procedures are reactivated. This method is applied for a cast iron EN-GJS-600-3. The stress-failure (S–N) curves for this material is determined at room temperature under push pull loading with different load ratios R0σmin/σmax0−2, R0−1 and R00.1. The effects of the foundry defects are determined by SEM observations of crack initiation sites. Their presence in tested specimens is associated with a reduction of fatigue lifetime by a factor of 2. However, the effect of the load ratio is more important

Rapid Synthesis of Sub-10 nm Hexagonal NaYF4-Based Upconverting Nanoparticles using Therminol® 66

We report a simple one-pot method for the rapid preparation of sub-10 nm pure hexagonal (β-phase) NaYF4-based upconverting nanoparticles (UCNPs). Using Therminol® 66 as a co-solvent, monodisperse UCNPs could be obtained in unusually short reaction times. By varying the reaction time and reaction temperature, it was possible to control precisely the particle size and crystalline phase of the UCNPs. The upconversion (UC) luminescence properties of the nanocrystals were tuned by varying the concentrations of the dopants (Nd3+ and Yb3+ sensitizer ions and Er3+ activator ions). The size and phase-purity of the as-synthesized core and core–shell nanocrystals were assessed by using complementary transmission electron microscopy, dynamic light scattering, X-ray diffraction, and small-angle X-ray scattering studies. In-depth photophysical evaluation of the UCNPs was pursued by using steady-state and time-resolved luminescence spectroscopy. An enhancement in the UC intensity was observed if the nanocrystals, doped with optimized concentrations of lanthanide sensitizer/activator ions, were further coated with an inert/active shell. This was attributed to the suppression of surface-related luminescence quenching effects

Cauchy boundaries in linearized gravitational theory

We investigate the numerical stability of Cauchy evolution of linearized gravitational theory in a 3-dimensional bounded domain. Criteria of robust stability are proposed, developed into a testbed and used to study various evolution-boundary algorithms. We construct a standard explicit finite difference code which solves the unconstrained linearized Einstein equations in the 3+1 formulation and measure its stability properties under Dirichlet, Neumann and Sommerfeld boundary conditions. We demonstrate the robust stability of a specific evolution-boundary algorithm under random constraint violating initial data and random boundary data.Comment: 23 pages including 3 figures and 2 tables, revte

Observation of the spin-charge thermal isolation of ferromagnetic Ga_{0.94}Mn_{0.06}As by time-resolved magneto-optical measurement

The dynamics of magnetization under femtosecond optical excitation is studied in a ferromagnetic semiconductor Ga_{0.94}Mn_{0.06}As with a time-resolved magneto-optical Kerr effect measurement with two color probe beams. The transient reflectivity change indicates the rapid rise of the carrier temperature and relaxation to a quasi-thermal equilibrium within 1 ps, while a very slow rise of the spin temperature of the order of 500ps is observed. This anomalous behavior originates from the thermal isolation between the charge and spin systems due to the spin polarization of carriers (holes) contributing to ferromagnetism. This constitutes experimental proof of the half-metallic nature of ferromagnetic Ga_{0.94}Mn_{0.06}As arising from double exchange type mechanism originates from the d-band character of holes

Principals of the theory of light reflection and absorption by low-dimensional semiconductor objects in quantizing magnetic fields at monochromatic and pulse excitations

The bases of the theory of light reflection and absorption by low-dimensional semiconductor objects (quantum wells, wires and dots) at both monochromatic and pulse irradiations and at any form of light pulses are developed. The semiconductor object may be placed in a stationary quantizing magnetic field. As an example the case of normal light incidence on a quantum well surface is considered. The width of the quantum well may be comparable to the light wave length and number of energy levels of electronic excitations is arbitrary. For Fourier-components of electric fields the integral equation (similar to the Dyson-equation) and solutions of this equation for some individual cases are obtained.Comment: 14 page

Variation in nucleotide homology obtained by amplification, cloning and sequencing of complete S1 gene from field samples of avian infectious bronchitis virus.

Projeto/Plano de Ação: 02.09.01.030

Numerical Relativity Using a Generalized Harmonic Decomposition

A new numerical scheme to solve the Einstein field equations based upon the generalized harmonic decomposition of the Ricci tensor is introduced. The source functions driving the wave equations that define generalized harmonic coordinates are treated as independent functions, and encode the coordinate freedom of solutions. Techniques are discussed to impose particular gauge conditions through a specification of the source functions. A 3D, free evolution, finite difference code implementing this system of equations with a scalar field matter source is described. The second-order-in-space-and-time partial differential equations are discretized directly without the use first order auxiliary terms, limiting the number of independent functions to fifteen--ten metric quantities, four source functions and the scalar field. This also limits the number of constraint equations, which can only be enforced to within truncation error in a numerical free evolution, to four. The coordinate system is compactified to spatial infinity in order to impose physically motivated, constraint-preserving outer boundary conditions. A variant of the Cartoon method for efficiently simulating axisymmetric spacetimes with a Cartesian code is described that does not use interpolation, and is easier to incorporate into existing adaptive mesh refinement packages. Preliminary test simulations of vacuum black hole evolution and black hole formation via scalar field collapse are described, suggesting that this method may be useful for studying many spacetimes of interest.Comment: 18 pages, 6 figures; updated to coincide with journal version, which includes some expanded discussions and a new appendix with a stability analysis of a simplified problem using the same discretization scheme described in the pape

British Society of Gastroenterology guidelines for the management of hepatocellular carcinoma in adults

\ua9 Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.Deaths from the majority of cancers are falling globally, but the incidence and mortality from hepatocellular carcinoma (HCC) is increasing in the United Kingdom and in other Western countries. HCC is a highly fatal cancer, often diagnosed late, with an incidence to mortality ratio that approaches 1. Despite there being a number of treatment options, including those associated with good medium to long-term survival, 5-year survival from HCC in the UK remains below 20%. Sex, ethnicity and deprivation are important demographics for the incidence of, and/or survival from, HCC. These clinical practice guidelines will provide evidence-based advice for the assessment and management of patients with HCC. The clinical and scientific data underpinning the recommendations we make are summarised in detail. Much of the content will have broad relevance, but the treatment algorithms are based on therapies that are available in the UK and have regulatory approval for use in the National Health Service

Frictional drag between quantum wells mediated by phonon exchange

We use the Kubo formalism to evaluate the contribution of acoustic phonon exchange to the frictional drag between nearby two-dimensional electron systems. In the case of free phonons, we find a divergent drag rate ($\tau_{D}^{-1}$). However, $\tau_{D}^{-1}$ becomes finite when phonon scattering from either lattice imperfections or electronic excitations is accounted for. In the case of GaAs quantum wells, we find that for a phonon mean free path $\ell_{ph}$ smaller than a critical value, imperfection scattering dominates and the drag rate varies as $ln (\ell_{ph}/d)$ over many orders of magnitude of the layer separation $d$. When $\ell_{ph}$ exceeds the critical value, the drag rate is dominated by coupling through an electron-phonon collective mode localized in the vicinity of the electron layers. We argue that the coupled electron-phonon mode may be observable for realistic parameters. Our theory is in good agreement with experimental results for the temperature, density, and $d$-dependence of the drag rate.Comment: 45 pages, LaTeX, 8 postscript file figure