The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show ...that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase space for three-phonon scattering processes, combined with the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optic ferroelectric mode.
The interactions between electrons and lattice vibrations are fundamental to materials behaviour. In the case of group IV-VI, V and related materials, these interactions are strong, and the materials ...exist near electronic and structural phase transitions. The prototypical example is PbTe whose incipient ferroelectric behaviour has been recently associated with large phonon anharmonicity and thermoelectricity. Here we show that it is primarily electron-phonon coupling involving electron states near the band edges that leads to the ferroelectric instability in PbTe. Using a combination of nonequilibrium lattice dynamics measurements and first principles calculations, we find that photoexcitation reduces the Peierls-like electronic instability and reinforces the paraelectric state. This weakens the long-range forces along the cubic direction tied to resonant bonding and low lattice thermal conductivity. Our results demonstrate how free-electron-laser-based ultrafast X-ray scattering can be utilized to shed light on the microscopic mechanisms that determine materials properties.
Abstract
The APOGEE Open Cluster Chemical Abundances and Mapping survey is used to probe the chemical evolution of the s-process element cerium in the Galactic disk. Cerium abundances were derived ...from measurements of Ce
ii
lines in the APOGEE spectra using the Brussels Automatic Code for Characterizing High Accuracy Spectra in 218 stars belonging to 42 open clusters. Our results indicate that, in general, for ages < 4 Gyr, younger open clusters have higher Ce/Fe and Ce/
α
-element ratios than older clusters. In addition, metallicity segregates open clusters in the Ce/X–age plane (where X can be H, Fe, or the
α
-elements O, Mg, Si, or Ca). These metallicity-dependent relations result in Ce/Fe and Ce/
α
ratios with ages that are not universal clocks. Radial gradients of Ce/H and Ce/Fe ratios in open clusters, binned by age, were derived for the first time, with
d
Ce/H/
d
R
GC
being negative, while
d
Ce/Fe/
d
R
GC
is positive. Ce/H and Ce/Fe gradients are approximately constant over time, with the Ce/Fe gradient becoming slightly steeper, changing by ∼+0.009 dex kpc
−1
Gyr
−1
. Both the Ce/H and Ce/Fe gradients are shifted to lower values of Ce/H and Ce/Fe for older open clusters. The chemical pattern of Ce in open clusters across the Galactic disk is discussed within the context of s-process yields from asymptotic giant branch (AGB) stars, gigayear time delays in Ce enrichment of the interstellar medium, and the strong dependence of Ce nucleosynthesis on the metallicity of its AGB stellar sources.
The improvement of laboratory diagnosis is a critical step for the reduction of syphilis cases around the world. In this paper, we present the development of an impedance-based method for detecting
...antigens and antibodies as an auxiliary tool for syphilis laboratory diagnosis. We evaluate the voltammetric signal obtained after incubation in carbon or gold nanoparticle-modified carbon electrodes in the presence or absence of Poly-L-Lysine. Our results indicate that the signal obtained from the electrodes was sufficient to distinguish between infected and non-infected samples immediately (T0') or 15 min (T15') after incubation, indicating its potential use as a point-of-care method as a screening strategy.
The emergence of automated manufacturing techniques has allowed the realization of the so-called tow-steered composite laminates, in which the fibers are deposited following continuous curvilinear ...paths. This enables to broaden the design space to satisfy a variety of design objectives. Previous studies have shown that conventional composites can be designed to maximize the modal frequencies and modal damping factors. However, similar investigations have not been devoted to tow-steered composites so far. In this context, the objective of this paper is to investigate the use of multi-objective optimization aiming at simultaneously maximizing the fundamental modal frequency and corresponding specific damping capacity of tow-steered composite laminates. The fiber trajectories are parameterized using two different schemes, and the parameters are taken as design variables. The equations of motion are derived from the combination of the Classical Lamination Theory with the Rayleigh–Ritz method. Damping is modeled by using the Strain Energy Method. Numerical optimization is performed using the evolutionary Direct Multisearch method, which provides optimal solutions forming Pareto fronts. Results obtained from various scenarios, including fully and partially steered laminates, and different boundary conditions, show that fiber steering can indeed improve substantially the dynamic characteristics, including damping, of composite laminates.
Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wave vectors (1/2 1/2 L) in tetragonal notation. On cooling ...below T_{C}, a clear resonance peak is observed at this wave vector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) k_{B}T_{C}. This is in good agreement with the canonical value of 5 k_{B}T_{C} observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c axis, indicating that the magnetic fluctuations are two dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.
We report neutron scattering measurements on single crystals of BaFe1.92Co0.08As2. The magnetic Bragg peak intensity is reduced by 6% upon cooling through TC. The spin dynamics exhibit a gap of 8 meV ...with anisotropic three-dimensional interactions. Below TC additional intensity appears at an energy of approximately 4.5(0.5) meV, similar to previous observations of a spin resonance in other Fe-based superconductors. No further gapping of the spin excitations is observed below TC for energies down to 2 meV. These observations suggest the redistribution of spectral weight from the magnetic Bragg position to a spin resonance, demonstrating the direct competition between static magnetic order and superconductivity.
The origin of the superconducting state in the recently discovered Fe-based materials is the subject of intense scrutiny. Neutron scattering and NMR (ref. 8) measurements have already demonstrated a ...strong correlation between magnetism and superconductivity. A central unanswered question concerns the nature of the normal-state spin fluctuations that may be responsible for the pairing. Here we present inelastic neutron scattering measurements from large single crystals of superconducting and non-superconducting Fe 1+y Te 1-x Se x . These measurements indicate a spin fluctuation spectrum dominated by two-dimensional incommensurate excitations extending to energies greater than 250 meV. Most importantly, the spin excitations in Fe 1+y Te 1-x Se x have four-fold symmetry about the (1, 0) wavevector (square-lattice (π,π) point). Moreover, the excitations are described by the identical wavevector and can be characterized by the same model as the normal-state spin excitations in the high-TC cuprates. These results demonstrate commonality between the magnetism in these classes of materials, which perhaps extends to a common origin for superconductivity. PUBLICATION ABSTRACT
This paper is devoted to the attitude and vibration control of spacecraft containing flexible appendages. It entails an investigation of a passive control strategy which consists in connecting ...piezoelectric transducers bonded to the flexible elements to electric circuits in such a way that the vibration energy, once converted into electrical energy, is transferred and partially dissipated into the electric circuit. This strategy enables to circumvent some difficulties involved in active control such as instability and the necessity of a large amount of hardware, which can be critical in space applications. One considers an artificial satellite model composed of a hub, a reaction wheel used for angular position control and two identical flexible panels, which contain piezoelectric patches symmetrically bonded to their surfaces. The equations of motion are derived based on the Assumed Modes approach, accounting for the electromechanical coupling and the presence of two types of circuits (resistive, and resistive–inductive). The effectiveness of the control strategy suggested is assessed by means of numerical simulations of a satellite undergoing an angular position correction commanded by proportional–derivative torque applied by the reaction wheel. The results demonstrate that the panel vibrations levels and coupling between flexible and rigid-body motions are significantly reduced for both types of circuits considered, such effectiveness being greater for resistive–inductive shunt circuits.
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