Spin Distribution Research Articles

Exploring Spin Distribution and Electronic Properties in FeN4-Graphene Catalysts with Edge Terminations.

Understanding the spin distribution in FeN4-doped graphene nanoribbons with zigzag and armchair terminations is crucial for tuning the electronic properties of graphene-supported non-platinum catalysts. Since the spin-polarized carbon and iron electronic states may act together to change the electronic properties of the doped graphene, we provide in this work a systematic evaluation using a periodic density-functional theory-based method of the variation of spin-moment distribution and electronic properties with the position and orientation of the FeN4 defects, and the edge terminations of the graphene nanoribbons. Antiferromagnetic and ferromagnetic spin ordering of the zigzag edges were considered. We reveal that the electronic structures in both zigzag and armchair geometries are very sensitive to the location of FeN4 defects, changing from semi-conducting (in-plane defect location) to half-metallic (at-edge defect location). The introduction of FeN4 defects at edge positions cancels the known dependence of the magnetic and electronic proper-ties of undoped graphene nanoribbons on their edge geometries. The implications of the reported results for catalysis are also discussed in view of the presented electronic and magnetic properties.

The energies and charge and spin distributions in the low-lying levels of singlet and triplet N2V defects in diamond from direct variational calculations of the excited states.

This paper reports the energies and charge and spin distributions of the low-lying excited states in singlet and triplet N2V defects in diamond from direct Δ-SCF calculations based on Gaussian orbitals within the B3LYP, PBE0, and HSE06 functionals. They assign the observed absorption at 2.463eV, first reported by Davies et al. [Proc. R. Soc. London 351, 245 (1976)], to the excitation of a N(sp3) lone-pair electron in the singlet and triplet states, respectively, with estimates of ∼1.1eV for that of the unpaired electrons, C(sp3). In both cases, the excited states are predicted to be highly local and strongly excitonic with 81% of the C(sp3) and 87% of the N(sp3) excited charges localized at the three C atoms nearest neighbor (nn) to the excitation sites. Also reported are the higher excited gap states of both the N lone pair and C unpaired electron. Calculated excitation energies of the bonding sp3 hybrids of the C atoms nn to the four inner atoms are close to that of the bulk, which indicates that the N2V defect is largely a local defect. The present results are in broad agreement with those reported by Udvarhelyi et al. [Phys. Rev. B 96, 155211 (2017)] from plane wave HSE06 calculations, notably for the N lone pair excitation energy, for which both predict an energy of ∼2.7eV but with a difference of ∼0.5eV for the excitation of the unpaired electron.

Modulation of photonic skyrmions in a thin metal film structure.

Photonic skyrmions have been a hot topic in recent years. However, modulating the spin distributions of the skyrmions is still a challenging topic. In this paper, we investigate the detailed spin distributions of photonic skyrmions in thin metal film sandwiched by different dielectrics. We find that the ratios of different spin components can be adjusted by the thickness of the metal film, while the absolute value of total spin can be controlled by the frequency of the light source. Therefore, by choosing proper metal thickness in the preparation process and certain beam frequency in actual experiment, we can get the exact type of spin distribution we prefer. In addition, when the dielectric layers are arranged asymmetrically, the spin distributions can also be modulated significantly by adjustig the ratio of the dielectric constants of the upper and lower dielectric layers. Our results provide a new pathway for the modulation of photonic skyrmions.

Gravitational form factors of the nucleon and one pion graviproduction in chiral EFT

In the framework of chiral effective field theory of delta resonances, nucleons, and pions interacting with background gravitational field we calculate the gravitational form factors of the nucleon up to fourth order in the small scale expansion and obtain the long-range behavior of the corresponding contributions to the energy, spin, pressure, and shear force distributions. By comparing nucleon gravitational form factors with and without delta contributions we conclude that explicit inclusion of deltas plays an important role. Next we explore the Lorentz structure of the N↦Nπ transition matrix element of the conserved symmetric energy-momentum tensor and introduce its parametrization in terms of twelve transition form factors. We use the chiral effective field theory to calculate the tree-order contributions to the gravitational transition form factors of the pion graviproduction off the nucleon up to third order. Published by the American Physical Society 2024

Problem with gradual absorption in MSD/MSC calculations

Replacing classical exciton model with the better founded Multistep Direct (MSD) and Multistep Compound (MSC) mechanisms has been impeded by incapability of the latter models to describe central part of the neutron emission spectra at incident neutron energies of about 14 MeV and above. We have ascribed this deficiency to the decrease of absorption to the MSC mechanism resulting from the concept of gradual absorption. We were able to obtain very good reproduction of experimentally measured neutron spectra using MSD/MSC calculations when this option was turned off. Such treatment is, however, at odds with the fundamental distinction between MSD and MSC mechanisms that should proceed through the chain of open (P-space) and closed (Q-space) configurations respectively. By blocking gradual absorption we allow the first stage of MSC to be fully populated from the incident channel that at higher incident energies is impossible. We discuss various attempts of addressing the problem that, so far, remains open. In addition, we present an evidence for much tighter spin distribution of particle-hole states than normally assumed.

Quantum spin coherence and electron spin distribution channels in vanadyl-containing lantern complexes.

We herein investigate the heterobimetallic lantern complexes [PtVO(SOCR)4] as charge neutral electronic qubits based on vanadyl complexes (S = 1/2) with nuclear spin-free donor atoms. The derivatives with R = Me (1) and Ph (2) give highly resolved X-band EPR spectra in frozen CH2Cl2/toluene solution, which evidence the usual hyperfine coupling with the 51V nucleus (I = 7/2) and an additional superhyperfine interaction with the I = 1/2 nucleus of the 195Pt isotope (natural abundance ca. 34%). DFT calculations ascribe the spin density delocalization on the Pt2+ ion to a combination of π and δ pathways, with the former representing the predominant channel. Spin relaxation measurements in frozen CD2Cl2/toluene-d8 solution between 90 and 10 K yield Tm values (1-6 μs in 1 and 2-11 μs in 2) which compare favorably with those of known vanadyl-based qubits in similar matrices. Coherent spin manipulations indeed prove possible at 70 K, as shown by the observation of Rabi oscillations in nutation experiments. The results indicate that the heavy Group 10 metal ion is not detrimental to the coherence properties of the vanadyl moiety and that Pt-VO lanterns can be used as robust spin-coherent building blocks in materials science and quantum technologies.

Manipulation of directional acoustic spin angular momentum density based on gradient-structured waveguides

In recent years, the discovery of the transverse spin of acoustic wave in a structural acoustic field and acoustic structural surface wave has expanded our knowledge of the basic characteristics of acoustic waves and opened up new avenues for their manipulation. On the structured surface, however, the distribution of acoustic surface waves often presents a uniform distribution, which restricts the local modification of acoustic spin angular momentum and particle manipulation capabilities. In this study, we develop some acoustic waveguides with gradients that are flat, up-convex, and down-concave in order to manipulate the lateral spin distributions of acoustic surface waves. We verify the direction-locking near-field acoustic spin-momentum, explore the pressure field distribution and the spin angular momentum density distribution of a spin acoustic source excited in each of the three gradient structures, and we also show how to manipulate the spin intensity distributions of acoustic surface waves in the gradient waveguides through theoretical analysis and numerical simulation. The numerical calculation results show that when the acoustic surface wave is excited by a clockwise rotating spin source and propagates along the left side of the waveguide, the spin angular momentum density is positive on the upper surface of the structured waveguide and negative on the lower surface. The spin angular momentum distribution and the direction of propagation of acoustic wave are entirely changed when the spin source is rotated counterclockwise. Specifically, an unequal distribution of acoustic spin angular momentum is produced by the upper convex-type waveguide and bottom concave-type waveguide when we convert the flat-type acoustic structure waveguide into a gradient-type waveguide. According to the computation results, the down-concave type waveguide exhibits a stronger density of acoustic spin angular momentum at the end and the acoustic surface waves gather at the end of the constructed waveguide. On the other hand, the waveguide collects acoustic waves close to the structure center when it is an up-convex structural waveguide. The findings can open up new avenues for manipulating particles using acoustic waves, by providing a means for controlling the acoustic spin angular momentum density and improving our understanding of symmetry in acoustic near-field physics.

Microscopic modeling of direct pre-equilibrium emission: Impact on exclusive and inclusive (n,xn) and fission channels

We report on a microscopic modeling of the first order of multistep direct emission based on one phonon excitations, given by the QRPA model, and an effective in-medium nucleon nucleon interaction, described within the JLM folding model. The results of our coupled channels framework for deformed target are illustrated in the case of low energy discrete state excitations in 152Sm. Pre-equilibrium predictions and their impact on inclusive and exclusive (n,xn) cross sections and fission cross sections are discussed for actinides. We discuss the importance of i) collective excitations to describe the neutron emission spectra and of ii) spin distribution of the residual nucleus formed after the neutron pre-equilibrium emission, that is a key ingredient to model the residual nucleus decay.

(Invited) Chirality-Induced Spin Selectivity Enabling Efficient Spin-Dependent Oxygen Evolution Reaction and High-Performance Photoelectrochemical Water Splitting

The sluggish and complex multi-step oxygen evolution reaction (OER) remains an obstacle to bias-free photoelectrochemical water-splitting systems. Several theoretical and experimental studies have suggested that OER efficiency can be significantly enhanced in chiral material-based photoanodes by utilizing the chirality-induced spin selectivity (CISS) phenomenon. The final product at the photoanode is the oxygen molecules, where the spin alignment of intermediate radicals is favorable to the generation of triplet 3O2 (most stable molecular oxygen species). Considering the energy difference between a singlet 1O2 and triplet 3O2, spin-dependent chemical reactions combined with the CISS phenomenon can play a crucial role in the OER process. Moreover, chiral material-based photoanodes are capable of paralleling spin distribution on the catalytic surface. The resulting spin-aligned intermediate radicals can promote the generation of spin-allowed triplet 3O2 over that of singlet oxygen, while hindering the formation of hydrogen peroxide, in turn leading to the enhanced STH efficiency of the overall PEC water-splitting system. In this regard, chiral materials (as a spin-filter layer) have been recognized as promising candidates for achieving a breakthrough in the solar-to-hydrogen efficiency of water-splitting devices. Herein, we report that the CISS phenomena can become an impressive approach by adopting chiral 2D organic-inorganic hybrid perovskites (OIHPs) as a spin-filtering layer on the 3D OIHP photoanode. Our chiral 2D OIHP-based water-splitting device achieved enhanced oxygen evolution performance with a reduced overpotential of 0.2 V, high fill factor, and increased photocurrent of 22 mA cm–2 at 1.2 VRHE compared to a device without a spin-filtering layer. For unbiased water splitting, a perovskite-based photocathode decorated with a spatially decoupled hydrogen evolution reaction catalyst is fabricated. This decoupled geometry enables the physical protection of the perovskite layer from the electrolyte, thus allowing excellent stability for over 100 h. To investigate the overall water splitting performance, this perovskite photocathode is connected in parallel with spin-filtering layer adopted photoanode based on chirality-induced spin selectivity. An unbiased solar water splitting was achieved using all the OIHP photoelectrodes, resulting in a remarkable unassisted solar-to-hydrogen efficiency of ~12% and a continuous 10 h stable operation.

Global comparison between experimentally measured isomeric yield ratios and nuclear model calculations

The level density steers transition probabilities between different states in the decay and de-excitation of excited nuclei. Reliable level density modelling is, therefore, key in describing, e.g., de-excitation of fission fragments, with implications on neutron and gamma-rays multiplicities, and also manifested in the population of isomeric states. We test six currently used level density models and the spin distribution in the level density by comparing calculations with measured isomeric yield ratios. The model calculations are performed with the TALYS code and experimental data for nuclear reactions populating spin isomers are retrieved from the EXFOR database. On average, calculations are in agreement with measured data. However, we find that the population of the high-spin state in an isomeric pair is clearly favoured in all of the six studied level density models. Further studies are then performed on the three used phenomenological level density models, to investigate the significance of their effect. We find that a significant reduction of the spin width distribution improves the agreement between calculated and experimentally observed isomeric yield ratios. This result is independent of the incident particle in the nuclear reaction. The needed reduction of the spin width distribution to comply with empirical data has, e.g., implications for studies in angular momentum generation in fission using isomeric yield rations, calculations of anti-neutrino spectra from nuclear reactors, as well as neutron and gamma-ray multiplicities in nuclear reactor calculations.

Optomechanical effects caused by non-zero field quantities in multiple evanescent waves.

Evanescent waves, with their high energy density, intricate local momentum, and spatial distribution of spins, have been the subject of extensive recent study. These waves offer promising applications in near-field particle manipulation. Consequently, it becomes imperative to gain a deeper understanding of the impacts of scattering and gradient forces on particles in evanescent waves to enhance and refine the manipulation capabilities. In this study, we employ the multipole expansion theory to present analytical expressions for the scattering and gradient forces exerted on an isotropic sphere of any size and composition in multiple evanescent waves. The investigation of these forces reveals several unusual optomechanical phenomena. It is well known that the scattering force does not exist in counter-propagating homogeneous plane waves. Surprisingly, in multiple pairs of counter-propagating evanescent waves, the scattering force can arise due to the nonzero orbital momentum (OM) density and/or the curl part of the imaginary Poynting momentum (IPM) density. More importantly, it is found that the optical scattering force can be switched on and off by simply tuning the polarization. Furthermore, optical forces typically vary with spatial position in an interference field. However, in the interference field generated by evanescent waves, the gradient force becomes a spatial constant in the propagating plane as the particle's radius increases. This is attributed to the decisive role of the non-interference term of the electromagnetic energy density gradient. Our study establishes a comprehensive and rigorous theoretical foundation, propelling the advancement and optimization of optical manipulation techniques harnessed through multiple evanescent waves. Specifically, these insights hold promise in elevating trapping efficiency through precise control and manipulation of optical scattering and gradient forces, stimulating further explorations.

Study of the surrogate ratio method by determination of 56Fe(n,xp) cross sections

In recent years (n,xp) cross sections of some long-lived radionuclei which are of importance in the upcoming fusion reactor technologies have been constrained using the surrogate ratio method. But the (n,xp) cross sections for the stable nuclei for which a great wealth of data is available from the direct measurements have not been constrained using surrogate ratio method yet, so that the consistency of the surrogate ratio method can be checked. In this study we have investigated the validity of surrogate ratio method by constraining 56Fe(n,xp) cross sections and then the results are compared with the available experimental data, evaluated data and the statistical model predictions. In this study 52Cr(n,xp) reaction has been used as the reference reaction in the surrogate ratio method. It is found in this study that (n,xp) cross sections constrained using surrogate ratio method are sensitive to the spin distribution of the compound nuclei populated in the surrogate reactions. It is also presented in this study that the proton emission probabilities for 57Fe and 53Cr are highly spin dependent. It is concluded that the surrogate ratio method is not suitable for constraining the (n,xp) cross sections.

Thermally Induced Persistent Covalent-Organic Frameworks Radicals.

Persistent covalent-organic framework (COF) radicals hold important applications in magnetics and spintronics; however, their facile synthesis remains a daunting challenge. Here, three p-phenylenediacetonitrile-based COFs (named CityU-4, CityU-5, and CityU-6) were synthesized. Upon heat treatment (250 °C for CityU-4 and CityU-5 or 220 °C for CityU-6), these frameworks were brought into their persistent radical forms (no obvious changes after at least one year), together with several observable factors, including color changes, red-shifted absorption, the appearance of electron spin resonance (ESR) signals, and detectable magnetic susceptibility. The theoretical simulation suggests that after heat treatment, lower total energy and nonzero spin density are two main factors to guarantee persistent COFs radicals and polarized spin distributions. This work provides an efficient method for the preparation of persistent COF radicals with promising potentials.

DFT study of sensing properties of defected and transition-metal doped V2CF2 towards CH4

As a new type of gas sensor, MXenes material is of great significance in indoor harmful gas detection. The geometric, electronic, and magnetic properties of CH4 adsorbed on intact, F-vacancy defected, and transition-metal (TMs: Mn, Co, Ni, Zn, Nb, Mo) doped V2CF2 have been investigated based on density functional theory. Perfect V2CF2 and F-vacancy defected substrates exhibited low charge transfer, long adsorption distance, and small adsorption energy for CH4. The adsorption mechanism was physical adsorption. After introducing Co and Ni dopants, the adsorption mechanism of CH4 changed to be chemisorption. Additionally, the asymmetric electron spin distribution between the dopant and V atom caused the substrate to change from being non-magnetic to magnetic, as shown by the electronic density of states diagrams. The hybridization between TM-3d orbitals and the p orbitals of the CH4 molecule significantly improved the adsorption stability. It is hoped that the results could provide ideas for creating new CH4 gas sensors based on MXenes.

The maximum mass of a black hole which can tidally disrupt a star: measuring black hole spins with tidal disruption events

ABSTRACT The tidal acceleration experienced by an object at the event horizon of a black hole decreases as one over the square of the black hole’s mass. As such there is a maximum mass at which a black hole can tidally disrupt an object outside of its event horizon and potentially produce observable emission. This maximum mass is known as the ‘Hills mass’, and in full general relativity is a function of both the black hole’s spin a• and the inclination angle of the incoming object’s orbit with respect to the black hole’s spin axis ψ. In this paper, we demonstrate that the Hills mass can be represented by a simple analytical function of a• and ψ, the first general solution of this problem. This general solution is found by utilizing the symmetries of a class of critical Kerr metric orbits known as the innermost bound spherical orbits. Interestingly, at fixed black hole spin the maximum Hills mass can lie at incoming orbital inclinations outside of the black hole’s equatorial plane ψ ≠ π/2. When compared to previous results in the literature this effect can lead to an increase in the maximum Hills mass (at fixed spin) by as much as a factor of $\sqrt{11/5} \simeq 1.48$ for a maximally rotating black hole. We then demonstrate how Bayesian inference, coupled with an estimate of the mass of a black hole in a tidal disruption event, can be used to place conservative constraints on that black hole’s spin. We provide a publicly available code tidalspin which computes these spin distributions.

Searching for structure in the binary black hole spin distribution

Searching for structure in the binary black hole spin distribution

Tidal disruption rate suppression by the event horizon of spinning black holes

ABSTRACT The rate of observable tidal disruption events (TDEs) by the most massive (≳ few × 107 M⊙) black holes (BHs) is suppressed due to direct capture of stars. This suppression effect depends on the space–time geometry and holds the promise of probing the spin distribution of dormant BHs at the centres of galaxies. By extending the frozen-in approximation commonly used in the Newtonian limit, we propose a general relativistic criterion for the tidal disruption of a star of given interior structure. The rate suppression factor is then calculated for different BH masses, spins, and realistic stellar populations. We find that either a high-BH spin (≳ 0.5) or a young stellar population (≲1 Gyr) allows TDEs to be observed from BHs significantly more massive than $10^8\, \mathrm{M}_{\odot }$. We call this spin-age degeneracy (SAD). This limits our utility of the TDE rate to constrain the BH spin distribution, unless additional constraints on the age of the stellar population or the mass of the disrupted star can be obtained by modelling the TDE radiation or the stellar spectral energy distribution near the galactic nuclei.

Domain structure ordering and magnetic property enhancement of containerlessly processed quaternary Fe-Mo-Ni-Cr alloy

Quaternary Fe-20 %Mo-12 %Ni-6 %Cr alloy with various composite magnetic structures was fabricated through electromagnetic levitation (EML) coupled with multi-mode quenching techniques. The magnetic domain distributions were characterized and related to the macro/micro-magnetic properties of this containerlessly processed alloy. Under EML condition, nano-scale lamellar (αFe) + Fe2Mo eutectoid structures with completely disordered magnetic domains were formed at small undercoolings. But the eutectoid morphology became anomalous and a ferromagnetic (αFe) single-phase zone with clear stripe domains appeared as undercooling increased, resulting in the obvious improvement of saturation magnetization, coercivity and residual magnetization. The (αFe) + Fe2Mo eutectoid structures were entirely suppressed by multiple quenching processes to secure (αFe) single-phase microstructures with ordered domain distributions including the stripe domains with grain boundary contrast, the maze domains with developed branches and their coexistent mixtures. These magnetic structures displayed much higher saturation magnetization, much lower coercivity and almost unchanged residual magnetization as compared with the highly undercooled alloy at EML state, indicating that the soft magnetic property of this alloy was significantly enhanced after rapid quenching. Moreover, the spin distributions and atomic magnetic moments of two constituent phases were analyzed by first-principle calculations, which were in good accordance with the experimental results.

Evidence for a Correlation between Binary Black Hole Mass Ratio and Black Hole Spins

The astrophysical origins of the binary black hole systems seen with gravitational waves are still not well understood. However, features in the distribution of black hole masses, spins, redshifts, and eccentricities provide clues into how these systems form. Much has been learned by investigating these distributions one parameter at a time. However, we can extract additional information by studying the covariance between pairs of parameters. Previous work has shown preliminary support for an anticorrelation between mass ratio q ≡ m 2/m 1 and effective inspiral spin χ eff in the binary black hole population. In this study, we test for the existence of this anticorrelation using updated data from the third gravitational-wave transient catalog and improve our copula-based framework to employ a more robust model for black hole spins. We find evidence for an anticorrelation in (q, χ eff) with 99.7% credibility. This may imply high common-envelope efficiencies, stages of super-Eddington accretion, or a tendency for binary black hole systems to undergo mass-ratio reversal during isolated evolution. Covariance in (q, χ eff) may also be used to investigate the physics of tidal spinup as well as the properties of binary black hole–forming active galactic nuclei.

Double black hole mergers in nuclear star clusters: eccentricities, spins, masses, and the growth of massive seeds

ABSTRACT We investigate the formation of intermediate mass black holes (IMBHs) through hierarchical mergers of stellar origin black holes (BHs), as well as BH mergers formed dynamically in nuclear star clusters. Using a semi-analytical approach that incorporates probabilistic mass-function-dependent double BH (DBH) pairing, binary-single encounters, and a mass-ratio-dependent prescription for energy dissipation in hardening binaries, we find that IMBHs with masses of $\mathcal {O}(10^2)$–$\mathcal {O}(10^4)\, \rm M_\odot$ can be formed solely through hierarchical mergers in time-scales of a few 100 Myrs to a few Gyrs. Clusters with escape velocities ≳400 km s−1 inevitably form high-mass IMBHs. The spin distribution of IMBHs with masses ≳ 103 M⊙ is strongly clustered at χ ∼ 0.15; while for lower masses, it peaks at χ ∼ 0.7. Eccentric mergers are more frequent for equal-mass binaries containing first- and/or second-generation BHs. Metal-rich, young, dense clusters can produce up to 20 per cent of their DBH mergers with eccentricity ≥0.1 at $10\, \rm Hz$, and ∼2–9 per cent of all in-cluster mergers can form at >10 Hz. Nuclear star clusters are therefore promising environments for the formation of highly eccentric DBH mergers, detectable with current gravitational-wave detectors. Clusters of extreme mass (∼108 M⊙) and density (∼108 M⊙ pc−3) can have about half of all of their DBH mergers with primary masses ≥100 M⊙. The fraction of in-cluster mergers increases rapidly with increasing cluster escape velocity, being nearly unity for vesc ≳ 200 km s−1. Cosmological merger rate of DBHs from nuclear clusters varies ⪅0.01–1 Gpc−3 yr−1, where the large error bars come from uncertainties in the cluster initial conditions, number density distribution, and redshift evolution of nucleated galaxies.