Low-energy Structures Research Articles

Microstructural evolution and twins formation under tensile test of Ni-free Mn-N austenitic stainless steel using laser powder bed fusion method

At the initial uniaxial tensile stage, predominant planar dislocation slip occurs to generate dislocation pile-ups and dipole bundles. Low-energy Taylor lattice structure is then formed by co-planar slip bands along the primary slip system along [001]γ and [111]γ directions to form primary twins. Increasing the straining and generation of numerous micro-bands and crystallographic defects led to the strong restriction for dislocation motion, enhancing the formation of secondary twins with stacking fault near the deformation twin's energy. Decreasing the mean slip band and micoband spacing and configuration of complex dislocation arrangement would cause the dislocation slip, generating an extensive number of dislocation walls and cell blocks, resulting in forming a noncrystalline structure with multi-orientation. Molecular dynamics (MD) analyses revealed that deformation began by nucleation of Shockley partial dislocations with a burger vector of 1/6 〈112〉 from none-periodic boundaries. Afterwards, dislocations junctions created Hirth dislocations with a Burger vector of 1/3 〈100〉, which causes strain hardening in the system. At this point, Shockley dislocations nucleated from Hirth dislocations and strain hardening increases.

Identification of Ge≡O Triple Bond in Ge6O– Cluster: Anion Photoelectron Spectroscopy and Theoretical Calculations

Unlike C≡O, which is common in coordination chemistry and organometallic chemistry, little is known about Si≡O or Ge≡O compounds. Here we report a Ge6O- cluster featuring a Ge≡O triple bond. The structural and chemical bonding properties of Ge6O-/0 are investigated using anion photoelectron spectroscopy and theoretical calculations. Two nearly degenerate isomers have been found for Ge6O-. The lowest-energy structure (6A) can be viewed as an O atom bonding with a tetragonal bipyramidal Ge6. The second one (6B) can be considered as an O atom interacting with a capped trigonal bipyramidal Ge6. Chemical bonding analyses reveal that Ge6O- (6A) can be viewed as a Ge≡O unit interacting with a σ antiaromatic C2v symmetric tetragonal pyramidal Ge53- moiety. Comparisons of the chemical bonding in Ge6O- (6A) with that in Ge5CO- and Ge5MnO- indicate the similar behavior of Ge≡O to C≡O and Mn≡O in its bonding to the Ge53- and Ge54- moieties.

Low Energy Isomers and Infrared Spectra Simulations of C4H3N, C4H4N, and C4H5N and Related Ions

The relative stability of pyrrole derivatives were investigated by applying a global minimum (GM) search for the low-lying energy structures of C4HnN (n = 3-5) clusters at neutral, anionic, and cationic states. Several low-energy structures, previously not reported, were identified. The present results reveal a preference for cyclic and conjugated systems for the C4H5N and C4H4N compounds. In particular, the structures of the cationic and neutral C4H3N species are different from the anionic ones. For the neutrals and cations, cumulenic carbon chains were found, while for the anions, conjugated open chains were obtained. Of particular relevance, the GM candidates C4H4N+ and C4H4N are different from those reported previously. For the most stable structures, infrared spectra were simulated and the main vibrational bands were assigned. Also, a comparison with available laboratory data was done aiming to corroborate with experimental detection.

Low Energy Structures in Nuclear Reactions with 4n in the Final State

We investigate a reaction model that describes a fast removal of the α particle from the ^{8}He nucleus with eventual emission of four neutrons. The obtained four neutron energy distribution allows one to explain the sharp low energy peak observed by studying the missing mass spectra of four neutrons in Duer etal. [Nature (London) 606, 678 (2022)NATUAS0028-083610.1038/s41586-022-04827-6], as a consequence of dineutron-dineutron correlations. The phenomenon of the emergence of a sharp low-energy peak in the four-neutron energy distribution should be more general and is expected in the decay of other systems containing a four-neutron halo.

Vanadium-Containing Anionic Chelate for Spectrophotometric Determination of Hydroxyzine Hydrochloride in Pharmaceuticals

Four azo dyes known to form anionic complexes with V(V) were investigated as potential liquid-liquid extraction-spectrophotometric reagents for the antihistamine medication hydroxyzine hydrochloride (HZH). A stable ion-association complex suitable for analytical purposes was obtained with 6-hexyl-4-(2-thiazolylazo)resorcinol (HTAR). The molar absorption coefficient, limit of detection, linear working range, and relative standard deviation in the analysis of real pharmaceutical samples (tablets and syrup) were 3.50 × 104 L mol-1 cm-1, 0.13 μg mL-1, 0.43-12.2 μg mL-1, and ≤2.7%, respectively. After elucidating the molar ratio in the extracted ion-association complex (HZH:V = 1:1), the ground-state equilibrium geometries of the two constituent ions-HZH+ and [VO2(HTAR)]--were optimized at the B3LYP level of theory using 6-311++G** basis functions. The cation and anion were then paired in four different ways to find the most likely structure of the extracted species. In the lowest-energy structure, the VO2 group interacts predominantly with the heterochain of the cation. A hydrogen bond is present (V-O···H-O; 1.714 Å) involving the terminal oxygen of this chain.

Performance of Density Functionals and Semiempirical 3c Methods for Small Gold–Thiolate Clusters

In light of the recent surge in computational studies of gold thiolate clusters, we present a comparison of popular density functionals (DFAs) and three-part corrected methods (3c-methods) on their performance by taking a data set named as AuSR18 consisting of 18 isomers of Aun(SCH3)m (m ≤ n = 1-3). We have compared the efficiency and accuracy of the DFAs and 3c-methods in geometry optimization with RI-SCS-MP2 as the reference method. Similarly, the performance for accurate and efficient energy evaluation was compared with DLPNO-CCSD(T) as the reference method. The lowest energy structure among the isomers of the largest stoichiometry from our data set, AuSR18, i.e., Au3(SCH3)3, is considered to evaluate the computational time for SCF and gradient evaluations. Alongside this, the numbers of optimization steps to locate the most stable minima of Au3(SCH3)3 are compared to assess the efficiency of the methods. A comparison of relevant bond lengths with the reference geometries was made to estimate the accuracy in geometry optimization. Some methods, such as LC-BLYP, ωB97M-D3BJ, M06-2X, and PBEh-3c, could not locate many of the minima found by most of the other methods; thus, the versatility in locating various minima is also an important criterion in choosing a method for the given project. To determine the accuracy of the methods, we compared the relative energies of the isomers in each stoichiometry and the interaction energy of the gold core with the ligands. The dependence of basis set size and relativistic effects on energies are also compared. The following are some of the highlights. TPSS has shown accuracy, while mPWPW shows comparable speed and accuracy. For the relative energies of the clusters, the hybrid range-separated DFAs are the best option. CAM-B3LYP excels, whereas B3LYP performs poorly. Overall, LC-BLYP is a balanced performer considering both the geometry and relative stability of the structures, but it lacks diversity. The 3c-methods, although fast, are less impressive in relative stability.

Robust UV-curable anti-smudge electrodeposition coating for self-cleaning, anti-graffiti and corrosion protection

In this study, a UV-curable anti-smudge cathodic electrophoretic coating was developed by free radical copolymerization of various acrylic monomers using heptadecafluorodecyl acrylate (FOEA) as a functional monomer to provide a low surface energy structure to the acrylate precursor. The precursor was then grafted with the synthetic double-bond terminated IPDI-HEA on the precursor, and finally dipentaerythritol penta/hexaacrylate (5-Acl) was introduced as a crosslinker for the subsequent cathodic electrophoresis and UV-curing process. The chemical structure and surface fluorine content of the fluorinated waterborne polyacrylate (FWPA) were confirmed by 1H NMR, FT-IR, and XPS, respectively. The surface morphology and roughness of the coating were assessed using SEM and AFM instruments. The contact and sliding angles of water, diiodomethane, and hexadecane were determined. The self-cleaning capability was demonstrated by measuring the oil-based and aqueous epoxy paints, as well as carbon powder sliding on the coated tinplate surface. The anti-graffiti property was also integrated with a red marker. The resistance to strong acid, salt spray, impact, friction, and bending were characterized. All these results showed remarkable self-cleaning and anti-graffiti capabilities of the as-prepared UV-curable electrophoretic coating. Meanwhile, the coating displayed exceptional robustness in resisting chemical corrosion and mechanical damage. This newly developed UV-curable anti-smudge cathodic electrophoretic coating has great application potential in various metal protection industries.

Hierarchy of Lifshitz Transitions in the Surface Electronic Structure of Sr_{2}RuO_{4} under Uniaxial Compression.

We report the evolution of the electronic structure at the surface of the layered perovskite Sr_{2}RuO_{4} under large in-plane uniaxial compression, leading to anisotropic B_{1g} strains of ϵ_{xx}-ϵ_{yy}=-0.9±0.1%. From angle-resolved photoemission, we show how this drives a sequence of Lifshitz transitions, reshaping the low-energy electronic structure and the rich spectrum of van Hove singularities that the surface layer of Sr_{2}RuO_{4} hosts. From comparison to tight-binding modeling, we find that the strain is accommodated predominantly by bond-length changes rather than modifications of octahedral tilt and rotation angles. Our study sheds new light on the nature of structural distortions at oxide surfaces, and how targeted control of these can be used to tune density of state singularities to the Fermi level, in turn paving the way to the possible realization of rich collective states at the Sr_{2}RuO_{4} surface.

Vacancy defects in nitrogen doped diamond

The introduction of nitrogen-vacancy (N–V) defects in diamond is important as they have impact on the optical and electronic properties of diamond-based devices. Here, spin polarised density functional theory simulations are used to find the lowest energy structures of V, VV, NN, NV, NV2, N2V and N2V2 and calculate the energies associated with the formation of clusters from point defects. The results show that there is a significant charge transfer from nearest neighbour C to nitrogen. The energy penalties to introduce N–V defects in diamond are very high, much higher than in Si or Ge. Strong exoergic binding energies are calculated to form clusters from isolated defects though formation energies of both the vacancy and N substitutional defects are high, indicating that there is a very strong driving force to form stable defect clusters. Calculated binding energies in Si and Ge are much lower than that calculated in diamond.

Electronic structure and layer-dependent magnetic order of a new high-mobility layered antiferromagnet KMnBi

Room-temperature two-dimensional antiferromagnetic (AFM) materials are highly desirable for various device applications. In this letter, we report the low-energy electronic structure of KMnBi measured by angle-resolved photoemission spectroscopy, which confirms an AFM ground state with the valence band maximum located at −100 meV below the Fermi level and small hole effective masses associated with the sharp band dispersion. Using complementary Raman, atomic force microscope and electric transport measurement, we systematically study the evolution of electric transport characteristics of micro-mechanically exfoliated KMnBi with varied flake thicknesses, which all consistently reveal the existence of a probable AFM ground state down to the quintuple-layer regime. The AFM phase transition temperature ranges from 220 K to 275 K, depending on the thickness. Our results suggest that with proper device encapsulation, multilayer KMnBi is indeed a promising 2D AFM platform for testing various theoretical proposals for device applications.

Atomic and electronic structure of grain boundaries in a-Al2O3: A combination of machine learning, first-principles calculation and electron microscopy

To accurately determine the atomic and electronic structures of symmetric tilt grain boundaries (GBs) in α-Al2O3, this work employed an artificial-neural-network (ANN) interatomic potential, density-functional-theory (DFT) calculation and scanning transmission electron microscopy (STEM) observation. An ANN-based simulated annealing method was demonstrated to efficiently screen candidate low-energy structures with reasonably high accuracy. For Σ7 and Σ31GBs with the [0001] tilt axis, which were absent in the training datasets for the ANN potential, their lowest-energy structures predicted from ANN and DFT calculations were in quantitative agreement with STEM images in terms of both Al- and O-column positions. The exact GB structures have enabled us to analyze quantitatively the relationship between their atomic and electronic structure. This work will be an important model case where a combination of machine-learning, theoretical calculation and experiment has successfully solved the problem of determining complicated GB structures and their electronic structures in α-Al2O3.

Quantum correlation of electron and ion energy in the dissociative strong-field ionization of H2

We report on the strong field ionization of ${\mathrm{H}}_{2}$ by a corotating two-color laser field. We measure the electron momentum distribution in coincidence with the kinetic energy release (KER) of dissociating hydrogen molecules. In addition to a characteristic half-moon structure, we observe a low-energy structure in the electron momentum distribution at a KER of about 3.5 eV. We speculate that the outgoing electron interacts with the molecular ion, despite the absence of classical recollisions under these conditions. Time-dependent density functional theory simulations support our conclusions.

A novel synthetic sRNA promoting protein overexpression in cell-free systems.

Bacterial small RNAs (sRNAs) that regulate gene expression have been engineered for uses in synthetic biology and metabolic engineering. Here, we designed a novel non-Hfq-dependent sRNA scaffold that uses a modifiable 20 nucleotide antisense binding region to target mRNAs selectively and influence protein expression. The system was developed for regulation of a fluorescent reporter in vivo using Escherichia coli, but the system was found to be more responsive and produced statistically significant results when applied to protein synthesis using in vitro cell-free systems (CFS). Antisense binding sequences were designed to target not only translation initiation regions but various secondary structures in the reporter mRNA. Targeting a high-energy stem loop structure and the 3' end of mRNA yielded protein expression knock-downs that approached 70%. Notably, targeting a low-energy stem structure near a potential RNase E binding site led to a statistically significant 65% increase in protein expression (p < 0.05). These results were not obtainable in vivo, and the underlying mechanism was translated from the reporter system to achieve better than 75% increase in recombinant diaphorase expression in a CFS. It is possible the designs developed here can be applied to improve/regulate expression of other proteins in a CFS.

Finding stable multi-component materials by combining cluster expansion and crystal structure predictions

A desired prerequisite when performing a quantum mechanical calculation is to have an initial idea of the atomic positions within an approximate crystal structure. The atomic positions combined should result in a system located in, or close to, an energy minimum. However, designing low-energy structures may be a challenging task when prior knowledge is scarce, specifically for large multi-component systems where the degrees of freedom are close to infinite. In this paper, we propose a method for identification of low-energy crystal structures within multi-component systems by combining cluster expansion and crystal structure predictions with density-functional theory calculations. Crystal structure prediction searches are applied to the Mo2AlB2 and Sc2AlB2 ternary systems to identify candidate structures, which are subsequently used to explore the quaternary (pseudo-binary) (MoxSc1–x)2AlB2 system through the cluster expansion formalism utilizing the ground-state search approach. Furthermore, we show that utilizing low-energy structures found within the cluster expansion ground-state search as seed structures within crystal structure predictions of (MoxSc1–x)2AlB2 can significantly reduce the computational demands. With this combined approach, we not only correctly identified the recently discovered Mo4/3Sc2/3AlB2i-MAB phase, comprised of in-plane chemical ordering of Mo and Sc and with Al in a Kagomé lattice, but also predict additional low-energy structures at various concentrations. This result demonstrates that combining crystal structure prediction with cluster expansion provides a path for identifying low-energy crystal structures in multi-component systems by employing the strengths from both frameworks.

Symmetry fractionalization in the gauge mean-field theory of quantum spin ice

Symmetry fractionalization is a ubiquitous feature of topologically ordered states that can be used to classify different symmetry-enriched topological phases and reveal some of their unique experimental signatures. Despite its vast popularity, there is currently no available framework to study symmetry fractionalization of quantum spin ice (QSI) -- a $U(1)$ quantum spin liquid (QSL) on the pyrochlore lattice supporting emergent photons -- within the most widely used theoretical framework to describe it, gauge mean-field theory (GMFT). In this work, we provide an extension of GMFT that allows for the classification of space-time symmetry fractionalization. The construction classifies all GMFT Ans\"atze that yield physical wavefunctions invariant under given symmetries and a specific low-energy gauge structure. As an application of the framework, we first show that the only two Ans\"atze with emergent $U(1)$ gauge fields that respect all space-group symmetries are the well-known 0- and $\pi$-flux states. We then showcase how the framework may describe QSLs beyond the currently known ones by classifying chiral $U(1)$ QSI. We find two new states described by $\pi/2$- and $3\pi/2$-fluxes of the emergent gauge field threading the hexagonal plaquettes of the pyrochlore lattice. We finally discuss how the different ways translation symmetries fractionalize for all these states lead to unique experimentally relevant signatures and compute their respective inelastic neutron scattering cross-section to illustrate the argument.

Spin-split collinear antiferromagnets: A large-scale ab-initio study

It was recently discovered that, depending on their symmetries, collinear antiferromagnets can actually break the spin degeneracy in momentum space, even in the absence of spin-orbit coupling. Such systems, recently dubbed altermagnets, are signalled by the emergence of a spin-momentum texture set mainly by the crystal and magnetic structure, relativistic effects playing a secondary role. Here we consider all collinear q = 0 antiferromagnetic compounds in the MAGNDATA database allowing for spin-split bands. Based on density-functional calculations for the experimentally reported crystal and magnetic structures, we study more than sixty compounds and introduce numerical measures for the average momentum-space spin splitting. We highlight some compounds that are of particular interest, either due to a relatively large spin splitting, such as CoF2 and FeSO4F, or because of their low-energy electronic structure. The latter include LiFe2F6, which hosts nearly flat spin-split bands next to the Fermi energy, as well as RuO2, CrNb4S8, and CrSb, which are spin-split antiferromagnetic metals.

Effect of substitution of 3d, 4d and 5d elements on structural, electronic, magnetic properties and XMCD spectra of Co-based full Heusler alloys: A DFT study

In the present work, we have probed the role of substitution of 3d, 4d and 5d elements (i.e., with electronic configuration (EC): ndy, where n = 3–5; y = number of electrons in the d level) at the B site on structural, magnetic and electronic properties of a set of full Heusler alloys (FHAs) namely, Co2BAl, using density functional theory (DFT) based electronic structure (ES) calculations. We have optimized all the 30 combinations studied here in Fm3̄m (A2BC type, conventional FHA structure) and F4̄3m (ABAC type, inverse FHA structure) space groups using VASP code. Thereafter, ES calculations have been carried out using DFT based FPLAPW method as implemented in WIEN2k code with an exchange-correlation term (PBE + mBJ + SOC), which is beyond GGA and includes the spin-orbit-coupling (SOC). Furthermore, XMCD calculations have been performed using single particle approximation and Fermi’s Golden rule as implemented in WIEN2k code. FHAs with atoms having EC nd1−6,10 (n = 3–5) at B site have Fm3̄m as the lowest energy structure with the exception of 4d6 and 5d6. Calculated magnetic moments (MM) of these alloys with atoms with EC nd1−5 are found to be in agreement with the Slater-Pauling (SP) rule, with an integer or close to integer value along with 3d6. However, alloys having atoms at B site with nd7−9 (along with 4d6 and 5d6) are found to be in F4̄3m space group and their values of the MM are not in agreement with the SP rule. XMCD spectra corresponding to various atomic sites help in understanding the impact of the local environment. Hence, calculations of the same have been performed to understand the low values of Co MM with atoms at B site having EC 3d3−5 and the negative MM of the Ti and Zr atoms at B site.

Quadrupole and octupole states in 152Sm using the proton–neutron interacting boson model

A scheme of solving the proton–neutron interacting boson model (IBM-2) in terms of the SU(3) basis is introduced, by which the IBM-2 coupled with an octupole boson is applied to describe the low-energy structure of the critical point nucleus, 152Sm. The results indicate that the spectral properties of both the positive-parity bands and negative-parity bands in this nucleus can be well captured by the IBM-2 calculations through a simple Hamiltonian, thus providing an example of the IBM-2 in a unified description of quadrupole and octupole states in a transitional system. In addition, a statistical analysis of the low-spin states in the model is also provided.

Signature of weakly coupled f electrons and conduction electrons in magnetic Weyl semimetal candidates PrAlSi and SmAlSi

Magnetic topological materials are a class of compounds with the underlying interplay of nontrivial band topology and magnetic spin configuration. Extensive interests have been aroused due to their application potential involved with an array of exotic quantum states. With angle-resolved photoemission spectroscopy and first-principles calculations, here we study the electronic properties of two magnetic Weyl semimetal candidates PrAlSi and SmAlSi. Though the two compounds harbor distinct magnetic ground states (ferromagnetic and antiferromagnetic for PrAlSi and SmAlSi, respectively) and 4$f$ shell fillings, we find that they share quite analogous low-energy band structure. By the measurements across the magnetic transitions, we further reveal that there is no evident evolution of the band structure in both compounds and the experimental spectra can be well reproduced by the nonmagnetic calculations, together suggesting a negligible effect of the magnetism on their electronic structures and a possibly weak coupling between the localized 4$f$ electrons and the itinerant conduction electrons. Our results offer essential insights into the interactions between magnetism, electron correlations, and topological orders in the $R$Al$X$ ($R$ = light rare earth and $X$ = Si or Ge) family.

The local structure and electronic properties of Ho3+-doped BaY2F8: A first-principles method

Trivalent rare-earth Ho3+ doped barium yttrium fluoride (BaY2F8) has attracted a great deal of attention because of its excellent properties in many advanced lasers and other new material applications. However, the clear crystal structure of the Ho3+-doped BaY2F8 system (Ho:BYF) is still uncertain. Hence, through the CALYPSO structure search method, the first-principles study has been carried out on the structure evolution of the trivalent holmium ion (Ho3+) doped BaY2F8. The results indicate that the Y3+ ions in the host lattice are likely to be occupied by the doped Ho3+ ions, forming the [HoF8]5- polyhedra with C2 site symmetry. We successfully demonstrate the lowest energy structure of BaY2F8 heavily doped with Ho3+ ions is a brand-new structure having a standardized monoclinic P2 phase for the first time. According to the electronic structure calculation, the band gap value of Ho3+-doped BYF is about 5.73 eV, which is smaller than approximately 7.19 eV of pure BYF, indicating that both of them maintain insulation. The understanding of rare-earth Ho3+ ions doped crystal has been enriched from the perspective of theoretical calculation. The BaY2F8 crystal doped Ho3+ ions is expected to be applied in biomedical, laser communication and other fields. These provide a clear idea for grasping the structural evolution and exploring the properties when meeting other rare-earth-doped materials.