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Bilayers as a Prospective Multiferroic System
No full textComposite bilayer multiferroics combining ferroelectric (FE) and ferromagnetic (FM) thin‐film materials in a heterostructure and exhibiting magnetoelectric (ME) coupling effect are of great scientific and technological interest. In particular, electronically driven ME coupling implies that the FE polarization orientation affects the magnetic properties of FM at the interface with FE. Unlike metals, where the electric field penetrates over distances of 1–2 unit cells only, magnetic semiconductors, particularly doped EuS, with a ≈10 nm screening length appear a viable alternative. In addition, EuS exhibits a metal–insulator transition, thus offering new functionalities in nanoelectronics. Meanwhile, ultrathin polycrystalline films of doped HfO, such as HfZrO (HZO), stabilized in the noncentrosymmetric orthorhombic phase, are identified as a novel class of robust FE materials. In this work, FM EuS integrated with FE HZO in a bilayered structure is promoted as a prospective composite multiferroic. The functionality of both ultrathin FM‐EuS and FE‐HZO layers as well as their compatibility in a capacitor configuration is demonstrated. The comprehensive information on the structural, chemical, and electronic properties of EuS/HZO interface endorses it as a promising medium for magnetoelectric coupling phenomena, particularly, the effect of polarization reversal in FE‐HZO on the magnetic and transport properties in EuS
The three-loop single mass polarized pure singlet operator matrix element
No full textWe calculate the massive polarized three-loop pure singlet operator matrix element in the single mass case in the Larin scheme. This operator matrix element contributes to the massive polarized three-loop Wilson coefficient in deep-inelastic scattering and constitutes a three-loop transition matrix element in the variable flavor number scheme. We provide analytic results in Mellin and in space and study the behaviour of this operator matrix element in the region of small and large values of the Bjorken variable
Two-level ablation and damage morphology of Ru films under femtosecond extreme UV irradiation
No full textThe dynamics of a thin ruthenium film irradiated by femtosecond extreme UV laser pulses is studied with a hybrid computational approach, which includes Monte Carlo, two-temperature hydrodynamics and molecular dynamics models. This approach is capable of accurate simulations of all stages of material evolution induced by extreme UV or X-ray photons: from nonequilibrium electron kinetics till complete lattice relaxation. We found that fast energy deposition in a subsurface layer leads to a two-level ablation: the top thin layer is ablated as a gas–liquid mixture due to expansion of overheated material at near and above critical conditions, whereas a thicker liquid layer below is ablated via a cavitation process. The latter occurs due to a thermo-mechanically induced tensile pressure wave. The liquid ablating layer exhibits unstable behaviour and disintegrates into droplets soon after detachment from the rest of the target. Our simulations reveal basic processes leading to formation of specific surface morphologies outside and inside the damage craters. The calculated ablation threshold, crater depth and morphological features are in quantitative agreement with the experimental data, which justifies the applicability of our hybrid model to study laser-induced material damage
Formation Region eEfects in X-ray Transition Radiation from 1 to 6 GeV Electrons in Multilayer Targets
No full textThe formation region effects in x-ray transition radiation have been experimentally investigated. The radiationwas generated using 1–6 GeV electrons impinging on two multilayer targets with considerably different periods.The absolute yield of transition radiation was measured and the wide spectral peak in the range from 10 to30 keV was observed. In the most part of the electron energy range the emission from the short-period radiatorwas expectedly suppressed, compared to the case of the long-period one. But for the electron energy of 1 GeVan opposite effect, though rather small, of the emission enhancement in the short-period radiator was observed.The conditions, under which this effect is much stronger, are derived and its possible practical value is outlined.The theory accounting for an arbitrary transversal shape of the electron beam and the finite size of the detectoris developed. This theory describes rather well the experimental results
Quantifying the Elemental Distribution in Solar Cells from X-Ray Fluorescence Measurements with Multiple Detector Modules
No full textWithin the analysis of solar cells with multi-modalX-ray microscopy, X-ray fluorescence (XRF) measurements havebecome a reliable source for evaluating elemental distributions.While XRF measurements can unveil the elemental distributionat unparalleled sensitivity and spatial resolution, the quantitativeanalysis is challenged by effects such as self-absorption and furthercomplicated by the inclusion of multiple detector modules.Here, we showcase the exemplary analysis of XRF spectraobtained from a Cu(In,Ga)Se2 solar cell utilizing four detectormodules. After cataloging typical features found in XRF spectra,we demonstrate the inclusion of detector modules with individualabsorption correction. This results in quantitative stoichiometricratios of the critical absorber elements Cu, In, and Ga that arein good agreement with the nominal ratios.These results are particularly relevant in view of futuremeasurements at diffraction-limited synchrotron beamlines: inorder to profit from the boost of nano-focused photon flux, XRFmeasurements will require multiple detector modules, for whichwe demonstrate an approach of quantitative analysis
Ptychographic X-ray speckle tracking with multi-layer Laue lens systems
No full textThe ever-increasing brightness of synchrotron radiation sources demands improved X-ray optics to utilize their capability for imaging and probing biological cells, nano-devices and functional matter on the nanometre scale with chemical sensitivity. Hard X-rays are ideal for high-resolution imaging and spectroscopic applications owing to their short wavelength, high penetrating power and chemical sensitivity. The penetrating power that makes X-rays useful for imaging also makes focusing them technologically challenging. Recent developments in layer deposition techniques have enabled the fabrication of a series of highly focusing X-ray lenses, known as wedged multi-layer Laue lenses. Improvements to the lens design and fabrication technique demand an accurate, robust, in situ and at-wavelength characterization method. To this end, a modified form of the speckle tracking wavefront metrology method has been developed. The ptychographic X-ray speckle tracking method is capable of operating with highly divergent wavefields. A useful by-product of this method is that it also provides high-resolution and aberration-free projection images of extended specimens. Three separate experiments using this method are reported, where the ray path angles have been resolved to within 4 nrad with an imaging resolution of 45 nm (full period). This method does not require a high degree of coherence, making it suitable for laboratory-based X-ray sources. Likewise, it is robust to errors in the registered sample positions, making it suitable for X-ray free-electron laser facilities, where beam-pointing fluctuations can be problematic for wavefront metrology
In Situ Formation of Metal Hydrides Inside Carbon Aerogel Frameworks for Hydrogen Storage Applications
No full textNano-confined chemical reactions bear great promise for a wide range of important applications in the near-to-medium term, e.g., within the emerging area of chemical storage of renewable energy. To explore this important trend, in the present work, resorcinol-/formaldehyde-based carbon aerogels were prepared by sol-gel polymerisation of resorcinol, with furfural catalysed by a sodium-carbonate solution using ambient-pressure drying. These aerogels were further carbonised in nitrogen to obtain their corresponding carbon aerogels. Through this study, the synthesis parameters were selected in a way to obtain minimum shrinkage during the drying step. The microstructure of the product was observed using Scanning Electron Microscopy (SEM) and Field Emission Scanning Electron Microscopy (FESEM) imaging techniques. The optimised carbon aerogels were found to have pore sizes of ~21 nm with a specific accessible surface area equal to 854.0 m/g. Physical activation of the carbon aerogel with CO generates activated carbon aerogels with a surface area of 1756 m/g and a total porosity volume up to 3.23 cm/g. The product was then used as a scaffold for magnesium/cobalt-hydride formation. At first, cobalt nanoparticles were formed inside the scaffold, by reducing the confined cobalt oxide, then MgH was synthesised as the second required component in the scaffold, by infiltrating the solution of dibutyl magnesium (MgBu) precursor, followed by a hydrogenation reaction. Further hydrogenation at higher temperature leads to the formation of MgCoH. In situ synchrotron X-ray diffraction was employed to study the mechanism of hydride formation during the heating process
SUGAR: An improved empirical model of Type Ia Supernovae based on spectral features
No full textType Ia Supernovae (SNe Ia) are widely used to measure the expansion of the Universe. Improving distance measurements of SNe Ia is one technique to better constrain the acceleration of expansion and determine its physical nature. This document develops a new SNe Ia spectral energy distribution (SED) model, called the SUpernova Generator And Reconstructor (SUGAR), which improves the spectral description of SNe Ia, and consequently could improve the distance measurements. This model is constructed from SNe Ia spectral properties and spectrophotometric data from The Nearby Supernova Factory collaboration. In a first step, a PCA-like method is used on spectral features measured at maximum light, which allows us to extract the intrinsic properties of SNe Ia. Next, the intrinsic properties are used to extract the average extinction curve. Third, an interpolation using Gaussian Processes facilitates using data taken at different epochs during the lifetime of a SN Ia and then projecting the data on a fixed time grid. Finally, the three steps are combined to build the SED model as a function of time and wavelength. This is the SUGAR model. The main advancement in SUGAR is the addition of two additional parameters to characterize SNe Ia variability. The first is tied to the properties of SNe Ia ejecta velocity, the second is correlated with their calcium lines. The addition of these parameters, as well as the high quality the Nearby Supernova Factory data, makes SUGAR an accurate and efficient model for describing the spectra of normal SNe Ia as they brighten and fade. The performance of this model makes it an excellent SED model for experiments like ZTF, LSST or WFIRST
Covert symmetries in the neutrino mass matrix
No full textThe flavour neutrino puzzle is often addressed by considering neutrino mass matrices m with a certain number of vanishing entries (m = 0 for some values of the indices), since a reduction in the number of free parameters increases the predictive power. Symmetries that can enforce textures zero can also enforce a more general type of conditions f(m) = 0 with f some function of the matrix elements m. In this case m can have all entries non-vanishing with no reduction in its predictive power. We classify all generation-dependent U(1) symmetries which, in the presence of two leptonic Higgs doublets, can reduce the number of independent high-energy parameters of type-I seesaw to the minimum number compatible with non-vanishing neutrino mixings and CP violation. These symmetries are broken above the scale where the effective operator is generated and can thus remain covert, in the sense that no explicit evidence of the symmetry can be read off the neutrino mass matrix, and different symmetries can give rise to the same low-energy structure. We find that only two cases are viable: one yields a structure with two zero-textures already considered in the literature, the other has no zero-textures and has never been considered before. It predicts normal ordering, a lightest neutrino mass ∼ 10 meV, a Dirac phase δ ∼ and definite values for the Majorana phases