Triple charmonium production in pQCD Blok, B.; Mehl, J.
The European physical journal. C, Particles and fields,
02/2024, Letnik:
84, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We study the role of
1
→
2
and
1
→
3
processes in triple charmonium production in the leading logarithmic approximation (LLA). We see that the ratio of effective cross sections of TPS and DPS only ...moderately depends on charmonium transverse momenta, but the total DPS and TPS cross sections each separately may have rather strong dependence on charmonia transverse momenta in the central kinematics region that can be studied experimentally.
Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund's rules state that the lowest-energy exciton is a poorly emitting triplet state. For ...inorganic semiconductors, similar rules predict an analogue of this triplet state known as the 'dark exciton'. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX
, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin-orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX
nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.
Display omitted
The Automatic-Flow (AFLOW) standard for the high-throughput construction of materials science electronic structure databases is described. Electronic structure calculations of solid ...state materials depend on a large number of parameters which must be understood by researchers, and must be reported by originators to ensure reproducibility and enable collaborative database expansion. We therefore describe standard parameter values for k-point grid density, basis set plane wave kinetic energy cut-off, exchange–correlation functionals, pseudopotentials, DFT+U parameters, and convergence criteria used in AFLOW calculations.
The recent discovery of superconductivity at 190 K in highly compressed H sub(2)S is spectacular not only because it sets a record high critical temperature, but because it does so in a material that ...appears to be, and we argue here that it is, a conventional strong-coupling BCS superconductor. Intriguingly, superconductivity in the observed pressure and temperature range was predicted theoretically in a similar compound, H sub(3)S. Several important questions about this remarkable result, however, are left unanswered: (1) Does the stoichiometry of the superconducting compound differ from the nominal composition, and could it be the predicted H sub(3)S compound? (2) Is the physical origin of the anomalously high critical temperature related only to the high H phonon frequencies, or does strong electron-ion coupling play a role? We show that at experimentally relevant pressures H sub(2)S is unstable, decomposing into H sub(3)S and S, and that H sub(3)S has a record high T sub(c) due to its covalent bonds driven metallic, which make this compound rather similar to MgB sub(2), but unlike most other good conventional superconductors.
In 1913, x-ray crystallography was first used to determine the crystal structure of diamond. Since that time hundreds of thousands, if not millions, of crystal structures have been determined. ...Published structures require critical review and indexing to make them easily available to other researchers. One of the first systems for doing this was Strukturbericht (Structure Reports), published in Germany from 1931-1943 and covering research from 1913-1939, and now best known for the Strukturbericht symbols which label common crystal structures. A comprehensive history of Strukturbericht has not been written. This brief report sketches the early history of Strukturbericht, as well as its post-World War II successor publications, handbooks, and online resources.
► We present AFLOW (Automatic Flow), a software framework for high-throughput materials. ► We describe standards and automatic correction techniques. ► We describe the software capabilities and the ...on-line interface through aflowlib.org.
Recent advances in computational materials science present novel opportunities for structure discovery and optimization, including uncovering of unsuspected compounds and metastable structures, electronic structure, surface, and nano-particle properties. The practical realization of these opportunities requires systematic generation and classification of the relevant computational data by high-throughput methods. In this paper we present Aflow (Automatic Flow), a software framework for high-throughput calculation of crystal structure properties of alloys, intermetallics and inorganic compounds. The Aflow software is available for the scientific community on the website of the materials research consortium, aflowlib.org. Its geometric and electronic structure analysis and manipulation tools are additionally available for online operation at the same website. The combination of automatic methods and user online interfaces provide a powerful tool for efficient quantum computational materials discovery and characterization.
Abstract
High-entropy ceramics are attracting significant interest due to their exceptional chemical stability and physical properties. While configurational entropy descriptors have been ...successfully implemented to predict their formation and even to discover new materials, the contribution of vibrations to their stability has been contentious. This work unravels the issue by computationally integrating disorder parameterization, phonon modeling, and thermodynamic characterization. Three recently synthesized carbides are used as a testbed: (HfNbTaTiV)C, (HfNbTaTiW)C, and (HfNbTaTiZr)C. It is found that vibrational contributions should not be neglected when precursors or decomposition products have different nearest-neighbor environments from the high-entropy carbide.
First-principles calculations of the energy of various crystal structures of Fe, Ni and ordered Fe–Ni compounds with different stoichiometries have been performed by the linearized augmented plane ...wave (LAPW) method in the generalized gradient approximation. The most stable compounds are L1
2–Ni
3Fe, L1
0–FeNi, C11
f–Ni
2Fe and C11
f–Fe
2Ni. The L1
2–Ni
3Fe compound has the largest negative formation energy, which is consistent with the experimental Fe–Ni phase diagram. The L1
0–FeNi compound has also been observed experimentally in meteorite samples as a metastable phase. It is suggested here that the C11
f compounds could also form in Fe–Ni alloys at low temperatures. A new semi-empirical interatomic potential has been developed for the Fe–Ni system by fitting to experimental data and the results of the LAPW calculations. Recognizing the significance of the covalent component of bonding in this system, the potential is based on the embedded-atom method (EAM) but additionally includes a bond-angle dependence. In comparison with the existing modified EAM method, our potential form is simpler, extends interactions to several (3–5) coordination shells and replaces the screening procedure by a smooth cutoff of the potential functions. The potential reproduces a variety of properties of Fe and Ni with a reasonable accuracy. It also reproduces all stability trends across the Fe–Ni system established by the LAPW calculations. The potential can be useful in atomistic simulations of the phases of the Fe–Ni system.
Identification of fungi and the International Code of Nomenclature underpinning this process, rests strongly on the characterisation of morphological structures. Yet, the value of these characters to ...define species in many groups has become questionable or even superfluous. This has
emerged as DNA-based techniques have increasingly revealed cryptic species and species complexes. This problem is vividly illustrated in the present study where 105 isolates of the Botryosphaeriales were recovered from both healthy and diseased woody tissues of native Acacia
spp. in Namibia and South Africa. Thirteen phylogenetically distinct groups were identified based on Internal Transcribed Spacer (ITS) rDNA PCR-RFLP and translation elongation factor 1-α (TEF1-α) sequence data, two loci that are known to be reliable markers to distinguish species
in the Botryosphaeriales. Four of these groups could be linked reliably to sequence data for formerly described species, including Botryosphaeria dothidea, Dothiorella dulcispinae, Lasiodiplodia pseudotheobromae and Spencermartinsia viticola. Nine groups,
however, could not be linked to any other species known from culture and for which sequence data are available. These groups are, therefore, described as Aplosporella africana, A. papillata, Botryosphaeria auasmontanum, Dothiorella capri-amissi, Do. oblonga,
Lasiodiplodia pyriformis, Spencermartinsia rosulata, Sphaeropsis variabilis and an undescribed Neofusicoccum sp. The species described here could not be reliably compared with the thousands of taxa described in these genera from other hosts and regions, where only morphological
data are available. Such comparison would be possible only if all previously described taxa are epitypified, which is not a viable objective for the two families, Botryosphaeriaceae and Aplosporellaceae, in the Botryosphaeriales identified here. The extent of diversity
of the Botryosphaeriales revealed in this and other recent studies is expected to reflect that of other undersampled regions and hosts, and illustrates the urgency to find more effective ways to describe species in this, and indeed other, groups of fungi.
The validity of the fluctuation theorem for entropy production as deduced from the observation of trajectories implicitly requires that all slow degrees of freedom are accessible. We experimentally ...investigate the role of hidden slow degrees of freedom in a system of two magnetically coupled driven colloidal particles. The apparent entropy production based on the observation of just one particle obeys a fluctuation theorem-like symmetry with a slope of 1 in the short time limit. For longer times, we find a constant slope, but different from 1. We present theoretical arguments for a generic linear behavior both for small and large apparent entropy production but not necessarily throughout. By fine-tuning experimental parameters, such an intermediate nonlinear behavior can indeed be recovered in our system as well.
PDF
