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Bond Formation upon Water Removal in an Unusual “Pseudo”-Topotactic Reaction Investigated by Single-Crystal Structure and in Situ Synchrotron X-ray Powder Diffraction Analysis
No full textThe new compound [Cu(cyclam)(H2O)]{[Cu(cyclam)]2[HTiNb9O28]}·26H2O (1) (cyclam = 1,4,8,11-tetraazacyclotetradecane) was obtained under solvothermal conditions. Its crystal structure contains a monotitano-nonaniobate anion in which one position is equally occupied by Nb(V) and Ti(IV). The anions are expanded by [Cu(cyclam)]2+ cations via Nb–O–Cu bridges generating {[Cu(cyclam)]2[HTiNb9O28]}2– cluster units, which are arranged into layers. Between these layers there are additionally isolated [Cu(cyclam)(H2O)2]2+ cations as well as hydrate water molecules. Storage of 1 at room temperature leads to loss of ∼13 water molecules, and a new crystalline phase (2) crystallizes that, with heating, transforms into the anhydrate. The reversibility of this reaction was investigated by thermogravimetry and X-ray powder diffraction (XRPD). Temperature-dependent in situ synchrotron XRPD investigations prove an abrupt phase transition, in which especially the a axis is dramatically shortened and the {[Cu(cyclam)]2[HTiNb9O28]2–} cluster is rearranged. Single-crystal X-ray diffraction of 2 reveals that, despite the unusual large shrinking of the unit cell volume, the domains formed by water removal exhibit some preferred orientation close to that expected for a topotactic reaction, which allowed the performance of a structure analysis. In the structure of 2, the two water molecules of the isolated [Cu(cyclam)(H2O)2]2+ cation in 1 are replaced by two terminal cluster O atoms, leading to the formation of chains via Nb–O–Cu bonds, and this phase transition is accompanied by an ordering of one of the two cyclam ligands
Follow-up structural evolution of Ni/Ti reactive nano and microlayers during diffusion bonding of NiTi to Ti6Al4V in a synchrotron beamline
No full textReaction-Assisted Diffusion Bonding (RADB) of NiTi to Ti6Al4V using either magnetron sputtered Ni/Ti nanomultilayersor Ni/Ti commercial microfoils as filler material was studied. The joining process takes advantage ofthe exothermal reactive character of the Ni-Ti system to provide extra energy during the bonding process.Therefore, sound joints could be achieved at lower thermal conditions. The oven with load capabilities at theHigh Energy Materials Science beamline (P07) of the Deutsch Synchrotron (DESY) is ideal to follow the structuralevolution of the materials involved in the bonding process. Prior to RABD, Ni/Ti multilayers with a 2.5 μmtotal thickness and with 12 or 25 nm of modulation period were deposited onto the materials being joined. Inalternative, up to 20 alternated thin μ-foils were placed in between the base materials. The materials were heatedby induction to the selected temperature during 30 min and quenched to room temperature by blowing helium.During the thermal cycle a 10 MPa pressure was applied. Using thin μ-foils, 650 °C was required to promotejoining, while using multilayer coated materials sound joints were obtained at 600 °C. Such low temperatures areattractive from the application/economic point of view, and are crucial to reduce the formation of undesiredintermetallic phases, such as NiTi2. The nanoindentation experiments of the joints processed using Ni/Ti nanomultilayersconfirm that the presence of the NiTi2 phase is more pronounced at 650 °C than when the jointsare processed at 600 °C
Baryogenesis from axion inflation
No full textThe coupling of an axion-like particle driving inflation to the Standard Model particle content through a Chern-Simons term generically sources a dual production of massless helical gauge fields and chiral fermions. We demonstrate that the interplay of these two components results in a highly predictive baryogenesis model, which requires no further ingredients beyond the Standard Model. If the helicity stored in the hyper magnetic field and the effective chemical potential induced by the chiral fermion production are large enough to avoid magnetic diffusion from the thermal plasma but small enough to sufficiently delay the chiral plasma instability, then the non-vanishing helicity survives until the electroweak phase transition and sources a net baryon asymmetry which is in excellent agreement with the observed value. If any of these two conditions is violated, the final baryon asymmetry vanishes. The observed baryon asymmetry can be reproduced if the energy scale of inflation is around Hinf ~ 1010–1012 GeV with a moderate dependence on inflation model parameters
The effective field theory of large scale structure at three loops
No full textWe study the power spectrum of dark matter density fluctuations in the framework of the Effective Field Theory of Large Scale Structures (EFTofLSS) up to three loop orders. We show that the extra coefficients in the EFT are sufficient to match numerical simulations with percent accuracy when a generic renormalization prescription is implemented (allowing for running of the individual counter-terms). We show that the level of accuracy increases with respect to the two loop results, up to k sime 0.4 h Mpc−1 at redshift z=0, although the overall improvement is somewhat marginal. At the same time, we argue there is evidence that the behavior of the loop expansion in the EFTofLSS is typical of an asymptotic series, already on the brink of its maximum predictive power (at z=0). Hence, the inclusion of higher orders will likely deteriorate the matching to data, even at moderate values of k. Part of the reason for this behavior is due to large contributions to the (renormalized) power spectrum at three loop order from mildly non-linear scales, even after the UV counter-terms are included. In conclusion, the EFTofLSS to three loop orders provides the best approximation to the (deterministic part of the) power spectrum in the weakly non-linear regime at z=0, and higher loops are not expected to improve our level of accuracy
Supercrystal Formation of Gold Nanorods by High Pressure Stimulation
No full textWe demonstrate the pressure-induced formation of supercrystalsmade from PEGylated gold nanorods (NRs) in aqueous suspension.Utilizing the combined effect of hydrostatic pressure and salt on the solubilityof the organic poly(ethylene glycol) (PEG) shell that passivates the NRs, thereversible formation of two-dimensional hexagonal supercrystals has beenobserved by means of small-angle X-ray scattering. The pressure dependenceof the crystal lattice’s structural parameters is determined. By time-resolvedmeasurements performed after a pressure jump, the growth process of thecrystals is found to be completed already after a few seconds. The presented results demonstrate that by PEGylatingnanoparticles, pressure-induced homogeneous supercrystals can be formed for different particle shapes, in particular, anisotropicNRs, which determine the resulting lattice type
Defect-rich GaN interlayer facilitating the annihilation of threading dislocations in polar GaN crystals grown on (0001)-oriented sapphire substrates
No full textThe interaction of microstructure defects is regarded as a possible tool for the reduction of the defect density and improvement of thecrystal quality. In this study, this general approach is applied to reduce the density of threading dislocations in GaN crystals grown usinghigh-temperature vapor phase epitaxy directly on (0001)-oriented sapphire substrates. The GaN crystals under study were deposited in threesteps with different process temperatures, growth rates, and ammonia flows. The first GaN layer accommodates the lattice misfit betweensapphire and gallium nitride. Thus, it contains a high number of randomly distributed threading dislocations. The next GaN layer, which isinternally structured and defect-rich, bends and bunches these dislocations and facilitates their annihilation. The uppermost GaN layermainly contains bunched threading dislocations terminating large areas of almost defect-free GaN. In order to be able to visualize and toquantify the microstructure changes in individual parts of the sandwich-like structure, the samples were investigated using nanofocused synchrotrondiffraction, confocal micro-Raman spectroscopy, and transmission electron microscopy. The transmission electron microscopy providedinformation about the kind of microstructure defects and their mutual interaction. The synchrotron diffraction and the micro-Ramanspectroscopy revealed the depth profiles of dislocation density and lattice parameters
Plasma-photonic spatiotemporal synchronization of relativistic electron and laser beams
No full textModern particle accelerators and their applications increasingly rely on precisely coordinated interactions of intense charged particle and laser beams. Femtosecond-scale synchronization alongside micrometre-scale spatial precision are essential e.g. for pump-probe experiments, seeding and diagnostics of advanced light sources and for plasma-based accelerators. State-of-the-art temporal or spatial diagnostics typically operate with low-intensity beams to avoid material damage at high intensity. As such, we present a plasma-based approach, which allows measurement of both temporal and spatial overlap of high-intensity beams directly at their interaction point. It exploits amplification of plasma afterglow arising from the passage of an electron beam through a laser-generated plasma filament. The corresponding photon yield carries the spatiotemporal signature of the femtosecond-scale dynamics, yet can be observed as a visible light signal on microsecond-millimetre scales
Vanishing carrier-envelope-phase-sensitive response in optical-field photoemission from plasmonic nanoantennas
No full textAt the surfaces of nanostructures, enhanced electric fields can drive optical-field photoemission and thereby generate and control electrical currents at frequencies exceeding 100 THz (refs. 1,2,3,4,5,6,7,8,9,10,11). A hallmark of such optical-field photoemission is the sensitivity of the total emitted current to the carrier-envelope phase (CEP)1,2,3,7,11,12,13,14,15,16,17. Here, we examine CEP-sensitive photoemission from plasmonic gold nanoantennas excited with few-cycle optical pulses. At a critical pulse energy, which we call a vanishing point, we observe a pronounced dip in the magnitude of the CEP-sensitive photocurrent accompanied by a sudden shift of π radians in the photocurrent phase. Analysis shows that this vanishing behaviour arises due to competition between sub-optical-cycle electron emission events from neighbouring optical half-cycles and that both the dip and phase shift are highly sensitive to the precise shape of the driving optical waveform at the surface of the emitter. As the mechanisms underlying the dip and phase shift are a general consequence of nonlinear, field-driven photoemission, they may be used to probe sub-optical-cycle emission processes from solid-state emitters, atoms and molecules. Improved understanding of these CEP-sensitive photocurrent features will be critical to the development of optical-field-driven photocathodes for time-domain metrology and microscopy applications demanding attosecond temporal and nanometre spatial resolution
Measurement of the single top quark and antiquark production cross sections in the channel and their ratio in proton-proton collisions at 13 TeV
No full textMeasurements of the cross sections for the production of single top quarks and antiquarks in the channel, and their ratio, are presented for proton-proton collisions at a center-of-mass energy of 13 TeV. The data set used was recorded in 2016 by the CMS detector at the LHC and corresponds to an integrated luminosity of 35.9 fb. Events with one muon or electron are selected, and different categories of jet and b jet multiplicity and multivariate discriminators are applied to separate the signal from the background. The cross sections for the -channel production of single top quarks and antiquarks are measured to be 136 1 (stat) 22 (syst) pb and 82 1 (stat) 14 (syst) pb, respectively, and their ratio is 1.66 0.02 (stat) 0.05 (syst). The results are in agreement with the predictions from the standard model