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Reversible coordination-induced spin-state switching in complexes on metal surfaces
No full textMolecular spin switches are attractive candidates for controlling the spin polarization developing at the interface between molecules and magnetic metal surfaces, which is relevant for molecular spintronics devices. However, so far, intrinsic spin switches such as spin-crossover complexes have suffered from fragmentation or loss of functionality following adsorption on metal surfaces, with rare exceptions. Robust metal–organic platforms, on the other hand, rely on external axial ligands to induce spin switching. Here we integrate a spin switching functionality into robust complexes, relying on the mechanical movement of an axial ligand strapped to the porphyrin ring. Reversible interlocked switching of spin and coordination, induced by electron injection, is demonstrated on Ag(111) for this class of compounds. The stability of the two spin and coordination states of the molecules exceeds days at 4 K. The potential applications of this switching concept go beyond the spin functionality, and may turn out to be useful for controlling the catalytic activity of surfaces
Possible storage in the crystal structure of perovskite
No full textThe lower mantle is believed to contain much less hydrogen (or H2O) because of the low storage capacity of the dominant mineral phases, such as bridgmanite and ferropericlase. However, possible hydrogen storage in the third most abundant mineral in the region, CaSiO3 perovskite (Ca-Pv), is not well unknown. We have synthesized Ca-Pv from different starting materials with varying H2O contents at 19–120 GPa and 1400–2200 K in laser-heated diamond-anvil cell. While cubic perovskite structure is stable at the mantle-related pressures-temperatures (P−T) in anhydrous systems, we found non-cubic diffraction peak splitting in Ca-Pv even at high temperatures when it is synthesized from hydrous starting materials. In-situ high-pressure infrared spectroscopy showed OH vibration possibly from Ca-Pv. The unit-cell volume of hydrothermally synthesized Ca-Pv is systematically smaller than that of anhydrous Ca-Pv at high pressures. These observations suggest possible H2O storage in Ca-Pv at mantle-related P−T conditions. We also found the formation of separate δ–AlOOH and Ca-Pv phases from Al-bearing CaSiO3 glass starting materials in an H2O medium at 60 GPa and 1400 K. Ca-Pv still showed non-cubic peak splitting at high temperatures in this experiment. Therefore, it is possible that hydrous phases may coexist together with hydrous Ca-Pv in the lower mantle
In situ compressive investigations on the effects of solid solution Gd on the texture and lattice strain evolution of Mg
No full textThe present work studies the effects of solid solution Gd on the texture and lattice strain evolution of an extruded Mg15Gd alloy under uniaxial compression. In situ experiments were carried out using high energy X-ray diffraction on samples of the investigated materials with three different orientations. The original textures of the pure Mg and the Mg15Gd alloy exhibit basal planes that are preferentially parallel and perpendicular to the extrusion direction (ED), respectively. The c/a ratio of the Mg15Gd alloy decreases with increasing Gd content in the solid solution, leading to a different deformation behavior compared with pure Mg under the compressive load. The addition of Gd enhances the slip and twinning modes. However, prismatic slip is activated earlier in the Mg15Gd alloy due to the lower c/a ratio
Searches for physics beyond the standard model with the variable in hadronic final states with and without disappearing tracks in proton-proton collisions at 13 TeV
No full textTwo related searches for phenomena beyond the standard model (BSM) are performed using events with hadronic jets and significant transverse momentum imbalance. The results are based on a sample of proton–proton collisions at a center-of-mass energy of , collected by the CMS experiment at the LHC in 2016–2018 and corresponding to an integrated luminosity of 137. The first search is inclusive, based on signal regions defined by the hadronic energy in the event, the jet multiplicity, the number of jets identified as originating from bottom quarks, and the value of the kinematic variable for events with at least two jets. For events with exactly one jet, the transverse momentum of the jet is used instead. The second search looks in addition for disappearing tracks produced by BSM long-lived charged particles that decay within the volume of the tracking detector. No excess event yield is observed above the predicted standard model background. This is used to constrain a range of BSM models that predict the following: the pair production of gluinos and squarks in the context of supersymmetry models conserving R-parity, with or without intermediate long-lived charginos produced in the decay chain, the resonant production of a colored scalar state decaying to a massive Dirac fermion and a quark, or the pair production of scalar and vector leptoquarks each decaying to a neutrino and a top, bottom, or light-flavor quark. In most of the cases, the results obtained are the most stringent constraints to date
Search for invisible decays of the Higgs boson produced invector-boson fusion in final states with jets and large missingtransverse energy with the ATLAS detector
No full textThe Standard Model of particle physics is a very successful theory, but it leaves some openquestions. Especially the topic of dark matter is a very active field of research and the discoveryof dark matter candidates might be accessible to modern collider experiments. Answering openquestions of the Standard Model is one of the greater goals of this work.The dark matter candidates might interact with the recently discovered Higgs boson andwould appear invisible to a particle detector. This motivates a search for invisible decays ofthe Higgs boson produced in vector-boson fusion. The search is looking for a pair of wellseparated, highly energetic jets and missing transverse energy in the final state. The analysisuses 36.1 fb−1of proton–proton collision data recorded at a centre-of-mass energy of 13 TeV in2015 and 2016 with the ATLAS experiment at the LHC. The main backgrounds are leptonicallydecaying vector bosons. These backgrounds are constrained in dedicated data control regions.The multijet background is small, since it can only result from mismeasurements of thejet transverse momentum, but it is challenging to quantify. The jet response is a measure forthe mismeasurement of jet transverse momenta. To study how well it is simulated in areasof extreme mismeasurements the non-Gaussian tails of these distributions are quantified in acomparison between data and simulation. This is achieved by modelling the Gaussian corewith fits. In order to see the effect in data the momentum balance of jet pairs is considered byusing an extrapolation to pure dijet events. The effort is undertaken with a new jet definition,particle flow jets, as well as topocluster jets. For both of them simulation and data are in goodagreement. This leads to systematic uncertainties small enough to have a negligible impact onthe analysis.The systematic uncertainty resulting from the jet energy resolution is one of the main limitations to the sensitivity of the search. This is addressed with the global sequential calibration(GSC), a simulation-driven method that removes the dependencies of jet momenta on a selection of detector variables in order to improve the jet resolution. The calibration leads to ajet resolution improvement of up to 20%. The GSC is fully derived for particle flow jets forthe first time, allowing performance comparisons between different kinds of jet reconstructionalgorithms.The search is able to derive a new observed (expected) limit on the Higgs to invisible branching fraction of 0.37 (0.28) at 95% confidence level. The results are also interpreted consideringa Higgs portal model, treating the invisible decay products as dark matter candidates. The resulting limits on the cross-section for the DM candidate to interact with an atomic nucleus isbetween 10−46 cm2and 10−42 cm2at 90% confidence level depending on the DM mass andspin
Cascaded multi-cycle terhertz driven ultrafast electron acceleration and manipulation
No full textTerahertz (THz)-based electron acceleration and manipulation has recently been shown to be feasible and to hold tremendous promise as a technology for the development of next-generation, compact electron sources. Previous work has concentrated on structures powered transversely by short, single-cycle THz pulses, with mm-scale, segmented interaction regions that are ideal for acceleration of electrons in the sub- to few-MeV range where electron velocities vary significantly. However, in order to extend this technology to the multi-MeV range, investigation of approaches supporting longer interaction lengths is needed. Here, we demonstrate first steps in electron acceleration and manipulation using dielectrically-lined waveguides powered by temporally long, narrowband, multi-cycle THz pulses that co-propagate with the electrons. This geometry offers centimeter-scale single stage interaction lengths and offers the opportunity to further increase interaction lengths by cascading acceleration stages that recycle the THz energy and rephase the interaction. We prove the feasibility of THz-energy recycling for the first time by demonstrating acceleration, compression and focusing in two sequential Al2O3-based dielectric capillary stages powered by the same multi-cycle THz pulse. Since the multi-cycle THz energy achievable using laser-based sources is currently a limiting factor for the maximum electron acceleration, recycling the THz pulses provides a key factor for reaching relativistic energies with existing sources and paves the way for applications in future ultrafast electron diffraction and free-electron lasers
Study of Microstructural Development of Bainitic Steel using Eddy Current and Synchrotron XRD in-situ Measurement Techniques during Thermomechanical Treatment
No full textIm Bereich der massiv umgeformten Schmiedebauteile sucht die Maschinenbauindustrie nach Prozessen mit gesteigerter Energie- und Ressourceneffizienz. Die bainitischen Stähle der neuen Generation sind aufgrund der hohen Festigkeit, Zähigkeit sowie hoher Dauerfestigkeit erfolgsversprechend für diese Anwendungen. Um die erwünschten mechanischen Eigenschaften zu erreichen, muss die Entwicklung der bainitischen Gefüge in Abhängigkeit von den Parametern der thermomechanischen Prozesse und von der Abkühlung bekannt sein. In der vorliegenden Arbeit wurden unterschiedliche Messverfahren angewendet, um die Mikrostrukturentwicklung während einer thermomechanischen Behandlung und anschließender kontinuierlicher bainitischer Umwandlung zu untersuchen. Die Prozesse wurden in Dilatometern simuliert und gleichzeitig wurden die Proben durch einen Wirbelstromsensor oder durch röntgenografische In-situ-Messungen am Synchrotron (DESY) untersucht. Die Ergebnisse zeigen, dass der Wirbelstromsensor für die Überwachung der Gefügeentwicklung während der Abkühlung sowie während der Umformung geeignet ist. Aus den Untersuchungen wurden geeignete Prozessparameter ermittelt, um ein möglichst feines bainitisches Gefüge zu erzielen. Die wesentlichen Faktoren waren eine niedrige Umformtemperatur im austenitischen Bereich, eine schnelle Abkühlung (> 2 K/s) bis zum bainitischem Gebiet, bainitische Umwandlung und/oder eine kurze Umformung im unteren Bainitbereich und schließlich eine langsamere Abkühlung auf Raumtemperatur
Finite-Size Dark Matter and its Effect on Small-Scale Structure
No full textIf dark matter has a finite size that is larger than its Compton wavelength, the corresponding self-interaction cross section decreases with the velocity. We investigate the implications of this puffy dark matter for addressing the small-scale problems of the Λ cold dark matter model and show that the way the nonrelativistic cross section varies with the velocity is largely independent of the dark matter internal structure. Even in the presence of a light particle mediating self-interactions, we find that the finite-size effect may dominate the velocity dependence. We present an explicit example in the context of a QCD-like theory and discuss possible ways to differentiate puffy dark matter from the usual light-mediator scenarios. Particularly relevant for this are low-threshold direct-detection experiments and indirect signatures associated with the internal structure of dark matter
A portable on-axis laser-heating system for near-90° X-ray spectroscopy: application to ferropericlase and iron silicide
No full textA portable IR fiber laser-heating system, optimized for X-ray emission spectroscopy (XES) and nuclear inelastic scattering (NIS) spectroscopy with signal collection through the radial opening of diamond anvil cells near 90°with respect to the incident X-ray beam, is presented. The system offers double-sided on-axis heating by a single laser source and zero attenuation of incoming X-rays other than by the high-pressure environment. A description of the system, which has been tested for pressures above 100 GPa and temperatures up to 3000 K, is given. The XES spectra of laser-heated Mg0.67Fe0.33O demonstrate the potential to map the iron spin state in the pressure–temperature range of the Earth's lower mantle, and the NIS spectra of laser-heated FeSi give access to the sound velocity of this candidate of a phase inside the Earth's core. This portable system represents one of the few bridges across the gap between laser heating and high-resolution X-ray spectroscopies with signal collection near 90°