210563 research outputs found
Sort by
Thermal stability of MgSiSn for thermoelectric applications
No full textUnderstanding the thermal stability of the Mg(Si,Sn) system is essential to define their safe tempera- tures of service. Despite its good thermoelectric performance, Mg(Si,Sn) is subject to a phase separation during thermal cycling due to the miscibility gap, which leads to a degradation of its thermoelectric properties and affects its performance during device operation. Isothermal annealing at 500°C and 750°C were performed with different annealing time to investigate thermal stability of Mg(Si,Sn). During the heat treatment, two phases were formed associated with porosity in the matrix. In addition, thickness of specimen was tracked and a significant expansion was detected. This phenomenon is attributed to the Kirkendall effect. The composition and the structure of the two forming phases were investigated by electron probe microanalysis and X-ray diffraction. Finally, the optimized thermal treatment allowed to stabilize the Mg(Si,Sn) without porosity and the presence of two thermody- namically stabilized phase (MgSiSn and MgSiSn) leading to a better reliability of the silicide thermoelectric modules
Evidence for electroweak production of four charged leptons and two jets in proton-proton collisions at 13 TeV
No full textEvidence is presented for the electroweak (EW) production of two jets (jj) in association with two Z bosons and constraints on anomalous quartic gauge couplings are set. The analysis is based on a data sample of proton-proton collisions at 13 TeV collected with the CMS detector in 2016-2018, and corresponding to an integrated luminosity of 137 fb. The search is performed in the fully leptonic final state ZZ, where e, . The EW production of two jets in association with two Z bosons is measured with an observed (expected) significance of 4.0 (3.5) standard deviations. The cross sections for the EW production are measured in three fiducial volumes and the result is (pp ZZjj jj) = 0.33 (stat) (syst) fb in the most inclusive volume, in agreement with the standard model prediction of 0.275 0.021 fb. Limits on anomalous quartic gauge couplings are derived in terms of the effective field theory operators T0, T1, T2, T8, and T9
Dalitz analysis of decays at Belle
No full textWe present the results of the first Dalitz plot analysis of the decay . The analysis is performed on a data set corresponding to an integrated luminosity of 953 fb collected by the Belle detector at the asymmetric-energy ee KEKB collider. The Dalitz plot is well described by a combination of the six resonant decay channels *(892)0η, a(980)+, a(1320)+, ¯*(1410)0η, *(1680)-π+ and (1980)-π+, together with π and η S-wave components. The decays *(1680)-→η and (1980)→η are observed for the first time. We measure ratio of the branching fractions, =0.500±0.002(stat)±0.020(syst)±0.003(). Using the Dalitz fit result, the ratio is measured to be 0.11±0.02(stat)(syst)±0.04(); this is much lower than the theoretical expectations (≈1) made under the assumption that K*(1680) is a pure 1 state. The product branching fraction →[(1980)→η]π+)=(2.2)×10 is determined. In addition, the πη′ contribution to the a(980) resonance shape is confirmed with 10.1σ statistical significance using the three-channel Flatté model. We also measure →*(892)η)=(1.41)%. This is consistent with, and more precise than, the current world average (1.02±0.30)%, deviates with a significance of more than 3σ from the theoretical predictions of (0.51–0.92)%
Observation of an Excitonic Mott Transition Through Ultrafast Core- cum -Conduction Photoemission Spectroscopy
No full textTime-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe. We present a many-body approximation for the Green’s function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions
Benchmarking a new segmented K-band chirped-pulse microwave spectrometer and its application to the conformationally rich amino alcohol isoleucinol
No full textIsoleucinol, a potential precursor to the essential α-amino acid isoleucine, has been studied usingmicrowave spectroscopy from 2-26 GHz. The measurements between 18-26 GHz were performed witha newly developed segmented chirped-pulse Fourier transform microwave spectrometer, which hasreduced the cost of the instrument by half compared to a direct excitation and detection chirped-pulsemicrowave spectrometer in the same frequency range. The performance of the instrument has beendemonstrated and found to be comparable to the previous design. For isoleucinol, the flexibility of thesec-butyl side chain (R = -CH(CH)CHCH) can result in more than 200 different conformers fromits five dihedral angles, and experimentally, seven conformers have been assigned. A fit includingthe hyperfine splitting due to nitrogen nuclear quadrupole coupling for the rotational transitionsis reported for all conformers, along with the experimental structures of the three lowest energyconformers. The observed conformers have intramolecular N· · ·H-O hydrogen bond interactions,similar to the second energetically favorable conformer of the analogous amino acid, isoleucine. Acomplete linelist has been provided to facilitate a search for isoleucinol in the interstellar medium
Direct time-domain determination of electron-phonon coupling strengths in chromium
No full textWe report the results of an ultrafast, direct structural measurement of optically pumped phonons in a Cr thin film using ultrashort x-ray pulses from a free-electron laser. In addition to measuring and confirming the known long-wavelength dispersion relation of Cr along a particular acoustic branch, we are able to determine the relative phase of the phonons as they are generated. The Cr sample exhibits two generation mechanisms for the phonons: the releasing of a preexisting charge density wave at higher frequencies, and the creation of an acoustic strain pulse via laser heating that dominates at lower frequencies. For the latter mechanism, we are able to measure the frequency dependence of the time required to generate the phonons. To explain the observed magnitude and slope of the delays, we perform first-principles simulations in the framework of density functional perturbation theory and ab initio molecular dynamics to fit anharmonic phonon models. These results show that the wave-vector dependence of the electron-phonon coupling is the driving mechanism behind the delay times: Phase-space limitation leads to higher times near the zone center. The absolute magnitudes of the delay times measured are found to be much shorter than the equilibrium electron-phonon coupling times we compute, indicating that the coupling strength is greatly enhanced when the electronic system is out of equilibrium with the lattice, as has been seen in bismuth and other systems
Molecular electronic decoherence following attosecond photoionisation
No full textAttosecond pulses can be used to generate coherent superpositions of cationic electronic states in molecules through photoionisation. These can drive coherent electronic dynamics, which may decay within a few femtoseconds due to nuclear motion. In this work, we study the impact of the photoelectron on decoherence in the valence electron system of molecules following attosecond photoionisation. To this end, we include the photoelectron as a classical point charge in a quantum–classical simulation of light-induced ultrafast molecular dynamics and consider ionisation by sub-femtosecond pulses with distinct qualities. By disentangling the contributions of photoelectron and nuclei to the overall electronic decoherence, we find that the photoelectron causes partial decoherence within the first 50 attoseconds. This timescale is noticed to be independent of the ionising pulse. Full electronic decoherence is only seen when the spatial extension of the nuclear wave packet is considered
Additive Manufacturing of Co-Ni-Ga High-Temperature Shape Memory Alloy: Processability and Phase Transformation Behavior
No full textCo-Ni-Ga high-temperature shape memory alloy is additively processed by selective laser melting for the first time. Reversible martensitic transformation of the as-built material is proven by differential scanning calorimetry. Microstructural analysis reveals a columnar-grained microstructure resulting from epitaxial solidification. Columnar-grained microstructures are characterized by a very low degree of constraints being beneficial for superior functional performance in numerous shape memory alloys. However, process-induced crack formation remains a challenge towards robust realization of adequate conditions showing good mechanical properties
Soft magnetic amorphous alloys in X-ray light: Insights from ultra-fast Joule heating experiments
No full textIn this contribution a novel setup for studying rapid crystallization of metallic glasses using a time resolved in situ X-ray diffraction combined with a direct current fast Joule heating (flash-annealing) is presented. The setup was implemented and successfully tested at the P02.1 beamline of the PETRA III storage ring (DESY Hamburg, Germany). Its potential use is demonstrated by studying rapid crystallization of soft magnetic FeCuNbSiB (at.%) metallic glass prepared by melt spinning technique. Flash-annealing experiment is realized by bursting 20 rectangular current pulses with a fixed amplitude of 1.5 A and pulse length is varied (30, 40 and 50 ms). A single pulse with duration of 30 ms causes temperature to rise to 770 °C with an average heating rate of 4200 K/s. Phase composition of crystallized material consist of major FeSi phase and small traces of boride phase FeB. Consecutive pulses result in cyclic thermal expansion of a crystal lattice, which appears fully reversible. Increasing pulse width to 50 ms causes temperature to increase up to 1020 °C with an average heating rate of 5600 K/s. Differentiation of the temperature profile yields exceptionally high value of heating rate 10800 ± 2400 K/s
Mixed higher-order anisotropic flow and nonlinear response coefficients of charged particles in collisions at and 5.02
No full textAnisotropies in the initial energy density distribution of the quark-gluon plasma created in high energy heavy ion collisions lead to anisotropies in the azimuthal distributions of the final-state particles known as collective anisotropic flow. Fourier harmonic decomposition is used to quantify these anisotropies. The higher-order harmonics can be induced by the same order anisotropies (linear response) or by the combined influence of several lower order anisotropies (nonlinear response) in the initial state. The mixed higher-order anisotropic flow and nonlinear response coefficients of charged particles are measured as functions of transverse momentum and centrality in collisions at nucleon-nucleon center-of-mass energies and 5.02 with the CMS detector. The results are compared with viscous hydrodynamic calculations using several different initial conditions, as well as microscopic transport model calculations. None of the models provides a simultaneous description of the mixed higher-order flow harmonics and nonlinear response coefficients