25,856 research outputs found
A Method to Determine at the Weak Scale in Top Decays at the LHC
Until now, the Cabibbo Kobayashi Maskawa matrix element, , has
always been measured in decays, i.e.~at an energy scale , far below the weak scale. We consider here the possibility of
measuring it close to the weak scale, at , in top decays at the
Large Hadron Collider (LHC). Our proposed method would use data from the LHC
experiments in hadronic top decays , tagged by the semileptonic decay of the associated top. We estimate the
uncertainty of such a measurement, as a function of present and potential
future experimental jet flavour-tagging performances, and conclude that first
measurements using the data collected during 2016 - 2018 could yield a
fractional error on \Vcb\ of order 7\% per experiment. We also give projected
performances at higher luminosities, which could yield sensitivity to any
Standard Model running of \Vcb\ below the weak scale, if present.Comment: 15 pages, 1 figures. Changes for V3: removed earlier Fig. 1,
associated text and citations, added one new citatio
Design and simulation of InGaAs/AlAsSb quantum-cascade lasers for short wavelength emission
The design and simulation of an In-0.53Ga-0.47As/Al-0.56As-0.44Sb quantum-cascade laser emitting in the near infrared is presented. Designed using a self-consistent rate equation solver coupled with an energy balance rate equation, the proposed laser has a calculated population inversion of ~20% at 77 K and sufficient gain to achieve room-temperature laser emission at λ ~2.8 µm. Threshold currents in the range 4–8 kA/cm2 are estimated as the temperature increases from 77 K to 300 K. The output characteristics of the proposed laser are compared to an existing λ ~3.1 µm In-0.53Ga-0.47As/Al-0.56As-0.44Sb quantum-cascade structure presented in the literature
Dilute magnetic semiconductor quantum-well structures for magnetic field tunable far-infrared/terahertz absorption
The design of ZnCdSe–ZnMnSe-based quantum
wells is considered, in order to obtain a large shift of the peak absorption wavelength in the far infrared range, due to a giant Zeeman splitting with magnetic field, while maintaining a reasonably large value of peak absorption. A triple quantum-well structure with a suitable choice of parameters has been found to satisfy such requirements. A maximal tuning range between 14.6 and 34.7 meV is obtained, when the magnetic field varies from zero
to 5 T, so the wavelength of the absorbed radiation decreases from 85.2 to 35.7 μm with absorption up to 1.25% at low temperatures. These structures might form the basis for magnetic field tunable photodetectors and quantum cascade lasers in the terahertz range
Forming simulation of a thermoplastic commingled woven textile on a double dome
This paper presents thermoforming experiments and FE simulations of a commingled glass-PP woven composite on a double dome geometry, with the aim of assessing the correspondence of predicted and experimental shear angles. Large local deformations - especially in-plane shear, i.e. relative rotation between the two yarn families – occur when draping a textile on a three dimensional part and eventually unwanted phenomena like wrinkling or tearing may occur. The macroscopic drape behaviour of a weave is generally subdivided into: 1) The high tensile resistance along the yarn directions, expressed as non-linear stress-strain curves, and 2) The shear resistance, expressed as non-linear shear force versus shear angle curves. The constitutive model is constituted of a dedicated non-orthogonal hypo-elastic shear resistance model, previously described in [1, 2], combined with truss elements that represent the high tensile resistance along the yarn directions. This model is implemented in a user subroutine of the ABAQUS explicit FE solver. The material parameters have been identified via textile biaxial tensile tests at room temperature and bias extension tests at 200°. Thermoforming experiments are performed on a rectangular blank with the warp direction along the second symmetry plane of the tool, with a preheating temperature of 200°C, a constant mold temperature of about 70°C, and a blankholder ring. It was concluded that the shear angles were fairly well predicted for this particular case study, which could be expected in view of the fact that no wrinkles had formed during the thermoforming experiment
Optically pumped terahertz laser based on intersubband transitions in a GaN/AlGaN double quantum well
A design for a GaN/AlGaN optically pumped terahertz laser emitting at 34 µm (ΔE~36 meV) is presented. This laser uses a simple three-level scheme where the depopulation of the lower laser level is achieved via resonant longitudinal-optical-phonon emission. The quasibound energies and associated wave functions are calculated with the intrinsic electric field induced by the piezoelectric and the spontaneous polarizations. The structures based on a double quantum well were simulated and the output characteristics extracted using a fully self-consistent rate equation model with all relevant scattering processes included. Both electron-longitudinal-optical phonon and electron-acoustic-phonon interactions were taken into account. The carrier distribution in subbands was assumed to be Fermi–Dirac-like, with electron temperature equal to the lattice temperature, but with different Fermi levels for each subband. A population inversion of 12% for a pumping flux Φ=10(27) cm(–2) s(–1) at room temperature was calculated for the optimized structure. By comparing the calculated modal gain and estimated waveguide and mirror losses the feasibility of laser action up to room temperature is predicted
Epitaxial strain effects in the spinel ferrites CoFe2O4 and NiFe2O4 from first principles
The inverse spinels CoFe2O4 and NiFe2O4, which have been of particular
interest over the past few years as building blocks of artificial multiferroic
heterostructures and as possible spin-filter materials, are investigated by
means of density functional theory calculations. We address the effect of
epitaxial strain on the magneto-crystalline anisotropy and show that, in
agreement with experimental observations, tensile strain favors perpendicular
anisotropy, whereas compressive strain favors in-plane orientation of the
magnetization. Our calculated magnetostriction constants of
about -220 ppm for CoFe2O4 and -45 ppm for NiFe2O4 agree well with available
experimental data. We analyze the effect of different cation arrangements used
to represent the inverse spinel structure and show that both LSDA+U and GGA+U
allow for a good quantitative description of these materials. Our results open
the way for further computational investigations of spinel ferrites
Designing strain-balanced GaN/AlGaN quantum well structures: Application to intersubband devices at 1.3 and 1.55 mu m wavelengths
A criterion for strain balancing of wurtzite group-III nitride-based multilayer heterostructures is presented. Single and double strain-balanced GaN/AlGaN quantum well structures are considered with regard to their potential application in optoelectronic devices working at communication wavelengths. The results for realizable, strain-balanced structures are presented in the form of design diagrams that give both the intersubband transition energies and the dipole matrix elements in terms of the structural parameters. The optimal parameters for structures operating at lambda ~1.3 and 1.55 µm were extracted and a basic proposal is given for a three level intersubband laser system emitting at 1.55µm and depopulating via resonant longitudinal optical(LO)phonons (h omega(LO)approximate to 90 meV). © 2003 American Institute of Physics
Photoreflectance and surface photovoltage spectroscopy of beryllium-doped GaAs/AlAs multiple quantum wells
We present an optical study of beryllium delta-doped GaAs/AlAs multiple quantum well (QW) structures designed for sensing terahertz (THz) radiation. Photoreflectance (PR), surface photovoltage (SPV), and wavelength-modulated differential surface photovoltage (DSPV) spectra were measured in the structures with QW widths ranging from 3 to 20 nm and doping densities from 2×10(10) to 5×10(12) cm(–2) at room temperature. The PR spectra displayed Franz-Keldysh oscillations which enabled an estimation of the electric-field strength of ~20 kV/cm at the sample surface. By analyzing the SPV spectra we have determined that a buried interface rather than the sample surface mainly governs the SPV effect. The DSPV spectra revealed sharp features associated with excitonic interband transitions which energies were found to be in a good agreement with those calculated including the nonparabolicity of the energy bands. The dependence of the exciton linewidth broadening on the well width and the quantum index has shown that an average half monolayer well width fluctuations is mostly predominant broadening mechanism for QWs thinner than 10 nm. The line broadening in lightly doped QWs, thicker than 10 nm, was found to arise from thermal broadening with the contribution from Stark broadening due to random electric fields of the ionized impurities in the structures. We finally consider the possible influence of strong internal electric fields, QW imperfections, and doping level on the operation of THz sensors fabricated using the studied structures. © 2005 American Institute of Physic
Bandgap Narrowing in Quantum Wires
In this paper we consider two different geometry of quasi one-dimensional
semiconductors and calculate their exchange-correlation induced bandgap
renormalization (BGR) as a function of the electron-hole plasma density and
quantum wire width. Based on different fabrication scheme, we define suitable
external confinement potential and then leading-order GW dynamical screening
approximation is used in the calculation by treating electron-electron Coulomb
interaction and electron-optical phonon interaction. Using a numerical scheme,
screened Coulomb potential, probability of different states, profile of charge
density and the values of the renormalized gap energy are calculated and the
effects of variation of confinement potential width and temperature are
studied.Comment: 17 Pages, 4 Figure
Magnetic field tunable terahertz quantum well infrared photodetector
A theoretical model and a design of a magnetic field tunable CdMnTe/CdMgTe terahertz quantum
well infrared photodetector are presented. The energy levels and the corresponding wavefunctions
were computed from the envelope function Schr¨odinger equation using the effective mass
approximation and accounting for Landau quantization and the giant Zeeman effect induced by
magnetic confinement. The electron dynamics were modeled within the self-consistent coupled rate
equations approach, with all relevant electron-longitudinal optical phonon and electron-longitudinal
acoustic phonon scattering included. A perpendicular magnetic field varying between 0 T and 5 T,
at a temperature of 1.5 K, was found to enable a large shift of the detection energy, yielding a
tuning range between 24.1 meV and 34.3 meV, equivalent to 51.4 μm to 36.1 μm wavelengths. For
magnetic fields between 1 T and 5 T, when the electron population of the QWIP is spin-polarized,
a reasonably low dark current of ≤1.4×10–² A/cm² and a large responsivity of 0.36−0.64 A/W
are predicted
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