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Evidence for the Collective Nature of Radial Flow in Pb+Pb Collisions with the ATLAS Detector
No full textAnisotropic flow and radial flow are two key probes of the expansion dynamics and properties of the quark-gluon plasma (QGP). While anisotropic flow has been extensively studied, radial flow, which governs the system’s radial expansion, has received less attention. Notably, direct experimental evidence for the global and collective nature of radial flow fluctuations has been lacking. This Letter presents the first measurement of transverse momentum (pT) dependence of radial flow fluctuations (v0(pT)) over 0.5<pT<10 GeV and demonstrates its collective nature using a two-particle correlation method in Pb+Pb collisions at sNN=5.02 TeV. The data reveal three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in pT, and centrality-independent shape in pT. The comparison with a hydrodynamic model demonstrates the sensitivity of v0(pT) to bulk viscosity, a crucial transport property of the QGP. These findings establish a new, powerful tool for probing collective dynamics and properties of the QGP
Transforming jet flavour tagging at ATLAS
No full textJet flavour tagging enables the identification of jets originating from heavy-flavour quarks in proton–proton collisions at the Large Hadron Collider, playing a critical role in its physics programmes. This paper presents GN2, a transformer-based flavour tagging algorithm deployed by the ATLAS Collaboration that represents a different methodology compared to previous approaches. Designed to classify jets based on the flavour of their constituent particles, GN2 processes low-level tracking information in an end-to-end architecture and incorporates physics-informed auxiliary training objectives to enhance both interpretability and performance. Its performance is validated in both simulation and collision data. The measured c-jet (light-jet) rejection in data is improved by a factor of 3.5 (1.8) for a 70% b-jet tagging efficiency, compared to the previous algorithm. GN2 provides substantial benefits for physics analyses involving heavy-flavour jets, such as measurements of Higgs boson pair production and the couplings of bottom and charm quarks to the Higgs boson, and demonstrates the impact of advanced machine learning methods in experimental particle physics
Study of at Belle II
No full textThe bottomonium spectrum, consisting of bound states of a quark and an anti- quark, provides an excellent laboratory for probing quantum chromodynamics in the non-perturbative regime. While and -wave bottomonium states are well studied experimentally, information on -wave states remains scarce. We search for -wave bottomonium state via the decay of a vector bottomonium-like state in the reaction , using of data collected with the Belle II detector at center-of-mass energies , and ~GeV, in the vicinity of the resonance. No significant signals are observed. Upper limits at the 90% credibility level are set on the products of the cross sections and branching fractions, and , at each center-of-mass energy
Oxygen vacancy distribution and phase composition in scaled, HfZrO-based ferroelectric capacitors
No full textIn this paper, we address correlations between film thickness, phase composition, and oxygen vacancy (VO) distribution in scaled, hafnia-based ferroelectric capacitors (FeCAPs), necessary to achieve low operating voltages, higher endurance, and advanced node integration. Using x-ray photoelectron spectroscopy, hard x-ray photoelectron spectroscopy, grazing incidence x-ray diffraction, and electrical characterization, we investigate the evolution of phase composition and VO profiles in HfZrO (HZO) films of 6 and 10 nm thickness. We demonstrate that thinner films exhibit a greater fraction of the non-polar tetragonal phase (t-phase), with increased VO concentration at the interface, affecting the device performance. Electrical measurements reveal contrasting wake-up and fatigue behavior between the two thicknesses, with thinner films showing decreased remanent polarization (2PR) due to t-phase dominance and VO redistribution during field cycling. These findings highlight the critical interplay of strain, phase stability, and VO dynamics, providing key insights for the optimization of HZO-based FeCAPs for advanced, low-power memory applications
Methane splitting to hydrogen and base growth carbon nanotubes over Fe-based catalysts
No full textCatalytic methane splitting produces CO2-free H2 from renewable CH4, since the C is captured as a solid. The feasibility of the process heavily relies on the catalyst stability and final application of the carbon. Carbon nanotubes (CNTs) are high value products, decreasing the H2 cost, and can be grown on non-toxic and easily available Fe catalysts. Simultaneously achieving high H2 productivity and controlling the quality of CNTs, which will mark a major advancement in catalytic methane splitting research, is, however, still challenging. The aim of this work is to develop Fe-based catalysts with active species able to grow CNTs from the base to avoid the loss of active phase during the reaction. To achieve this, hydrotalcite-derived catalysts were employed to exploit the metal-support interaction and dispersion of Fe species in an oxidic matrix. A moderately loaded, 20 wt% Fe-based catalyst (FeMgAl), containing a MgAl2O4 spinel with Fe3+ species, obtained by calcination at 800°C is preferred to its counterpart calcined at 700°C with a mixed oxide (MgFeAlOx) structure and performs better than conventional Fe-MgO and Fe-Al2O3 catalysts. Furthermore, reaction conditions have been identified as important for tailoring the catalyst properties and to control CNT growth. The simultaneous production of H2 at ca 5 LH2/gcat h and base-growth multi-walled CNTs is achieved by the adoption of relatively low temperatures (700°C-750°C) and activation of the catalyst under reaction conditions, without the need for H2 pre-reduction. As revealed by in situ XRD, Fe3C forms as the active species, which plays a key role in the activation of CH4, consistent with Density Functional Theory (DFT) results. DFT calculations reveal that the Fe₃C(010) surface lowers the energy barrier of the first dehydrogenation step of CH₄ to 0.66 eV – notably smaller with respect to reported values for well-studied Ni catalysts and the analysis of the adhesion energy between carbon and Fe₃C(010) suggests that, at reaction temperatures, carbon atoms are moderately bound to the surface, which may explain the particles’ resistance to encapsulation. These dual descriptors, corroborated by our experimental data, demonstrates why cementite, and not metallic Fe, emerges as the kinetically dominant phase during base‑growth CNT nucleation
Chemistry and Interfacial Structure Promoting Quasi-van der Waals Epitaxial Growth of WS Nanosheets on Sapphire for Prospective Application in Field-Effect Transistors
No full textHow do chemical and structural modifications to the supporting crystal surface affect the subsequent van der Waals (vdW) or quasi(Q)-vdW epitaxial growth of 2D nanocrystals? Developing an atomic-scale picture of such an interfacial system is crucial for understanding its impact on the physical and chemical properties of the supported 2D materials. The elucidation of the interfacial structure and chemistry needed to promote the Q-vdW epitaxial growth of 2D tungsten disulfide (WS) nanocrystals contributes to the growth mechanism understanding, thus pushing forward the integration of such atomically thin semiconductors toward real field-effect transistor applications. In addition to an atomic-force microscopy top view, we showcase a combination of X-ray techniques for a top-to-bottom investigation of the complexities of the buried interface structures. This approach uses X-ray photoelectron spectroscopy, X-ray standing wave excited X-ray fluorescence, and crystal truncation rod scattering to produce a highly resolved chemical-state-specific 3D atomic map for the extended interface structure of WS/α-AlO(001). Employing these detailed analysis methods, along with density functional theory to further refine the picoscale structure, we demonstrate how two different types of interface engineering during the pregrowth stage lead to significant differences in the chemical and structural modifications to the terminal surface of c-face sapphire, which in turn leads to substantial differences in the submonolayer growth of supported WS 2D nanocrystals in terms of lateral domain sizes, epitaxial registry, vdW gaps, and stability
Hyperfine structure-tunneling coupling in trans -1,2-cyclohexanediamine revealed by rotational spectroscopy
No full textThe microwave spectrum of the non-rigid trans-cyclohexanediamine (CH(NH)) is investigated. It displays a large amplitude interconversion motion during which both amino groups are rotated through 117° leading to tunneling splittings on the order of 21 MHz and line splittings on the order of 42 MHz for b- and c-type transitions. The tunneling is mediated by the quadrupole coupling hyperfine structure arising from both nitrogen atoms which leads to splittings on the same order of magnitude. The frequencies of the rotation-tunneling-hyperfine transitions are analyzed using a new theoretical model in which the large amplitude motion and the quadrupole coupling are treated simultaneously. Hyperfine matrix elements between (within) tunneling sublevels depend on the difference (sum) of the quadrupole coupling of the two nitrogen atoms. Using the theoretical formalism, 249 experimental frequencies are reproduced with an RMS value of 10 kHz, close to the experimental uncertainty. The spectroscopic parameters determined include usual rotational and distortion parameters; tunneling parameters describing the magnitude of the tunneling and its rotational dependence; and various components of the effective quadrupole coupling tensors
Comparative study on 3D morphologies of delignified, single tracheids and fibers of five wood species
No full textWood tracheids and fibers exhibit diverse structures and shapes across plant species. The hierarchical structure and composition of cellulose, hemicelluloses, and lignin enables wood to withstand high stress. This structural resilience makes wood a versatile material for applications ranging from construction to advanced composites. However, a detailed understanding of how delignification affects softwood tracheid and hardwood fiber morphology is crucial for predicting material behavior and developing modified wood products. This study investigated the overall structural changes due to delignification, in five wood species, namely, spruce, beech, balsa, Douglas fir, and poplar. It additionally provides detailed morphology of delignified single tracheids and fibers. Scanning electron microscopy was used to compare the morphology between untreated and delignified fibers and tracheids. X-ray tomography enabled us to reconstruct high-resolution 3D models of delignified single tracheids or fibers, providing information on the pit arrangements. Moreover, delignification resulted in facilitated separation of fibers and tracheids and frayed wall appearance. We observed similar tracheid/fiber diameters and wall thicknesses for all five wood species. These findings enhance our understanding of the wood fiber and tracheid structures across species and the effects of delignification. The 3D models provide a valuable resource for (1) understanding interspecies differences of fibers and tracheids, (2) optimizing the use of delignified wood in industrial applications (including bio-based and bio-inspired materials), and (3) physical modeling of wood regarding questions of wood biomechanics and water management
Tracking the Complex Dynamics of Electron-Transfer-Mediated Decay in Real Space and Time
No full textWhen an electronically excited atom or molecule is embedded in a chemical environment as, e.g., in a liquid or a loosely bound cluster, it can de-excite through mechanisms where neighboring atoms or molecules are actively participating in the decay: either by donating or accepting energy or electrons. For such nonlocal decay channels, nuclear dynamics play a crucial role as they have a direct impact on the decay efficiency itself. Here, we present a detailed study of the electron-transfer-mediated decay in a loosely bound triatomic prototype system, combining experimental results from a 5-fold coincidence measurement and theoretical modeling of the decay process. Depending on the decay time, we find that certain classes of molecular geometries are favored for this type of decay. Our findings provide an intuitive picture of how electron-transfer-mediated decay proceeds. In particular, our results confirm a roaming-like behavior of the atoms of the trimer prior to its decay. Our combined theoretical and experimental approach enables a comprehensive tracing of the real-space properties of the decaying system in the time domain