Single-ion States Research Articles

Hybrid spin-orbit exciton-magnon excitations in FePS3

FePS3 is a layered van der Waals (vdW) Ising antiferromagnet that has recently been studied in the context of true 2D magnetism and emerged as an ideal material platform for investigating strong spin-phonon coupling, and non-linear magneto-optical phenomena. In this work, we demonstrate an important unresolved role of spin-orbit coupling (SOC) in the ground state and excitations of this compound. Combining first-principles calculations with linear flavor wave theory (LFWT), we find strong mixing and spectral overlap of different spin-orbital single-ion states. Low-lying excitations form hybrid spin-orbit exciton/magnon modes. Complete parameterization of the low-energy model requires nearly half a million coupling constants. Despite this complexity, such a model can be inexpensively derived using local many-body-based approaches, which yield quantitative agreement with recent experiments. The results highlight the importance of SOC even in first-row transition metals and provide essential insight into the properties of 2D magnets with unquenched orbital moments.

Modeling of tungsten impurity transport and distribution in EAST based on multi-fluid and kinetic Monte Carlo simulations

In recent years, the materials of plasma facing components, such as divertor target plates, domes, and outer walls of tokamaks, such as ASDEX Upgrade, WEST, JET, EAST, and ITER, have been changed from carbon to tungsten because of its lower erosion and tritium retention rates. Impurities are produced by interactions between the plasma and the first wall. This study provides an investigation to simulate the transport and distribution of tungsten impurities in the edge plasma on EAST. The 2D multi-fluid edge plasma transport code SOLPS-ITER and 2D kinetic Monte Carlo impurity transport code DIVIMP were used in the simulations. The multi-fluid model in SOLPS-ITER and the kinetic Monte Carlo model in DIVIMP were employed to treat tungsten impurity ions. The 2D density contour distributions in the computational region and the 1D density radial profiles at the inner and outer midplanes of tungsten impurity particles with ionization states (W0–W+74) and the total tungsten particles with all charge states were obtained. With the heating power 1.5 MW and the line-averaged plasma density 2 × 1019 m−3, the results from SOLPS-ITER and DIVIMP show that the maximum density of tungsten ion with single ionization state is about 1014 m−3 and the total density of tungsten impurities with all charge states is about 1015 m−3 at the core boundary. To the best of our knowledge, the simulation results from SOLPS-ITER and DIVIMP are compared for the first time to benchmark SOLPS-ITER with the multi-fluid mode and DIVIMP with the kinetic model for tungsten impurity transport. The density distributions of tungsten impurities with different ionization states from SOLPS-ITER and DIVIMP are highly similar, and good agreement can be found under similar conditions involved in the calculation. From the comparison benchmark between SOLPS-ITER and DIVIMP for tungsten impurity transport, it can be concluded that the impurity transport approximation used by DIVIMP is good.

Quantifying stochasticity-driven uncertainties in H ii region metallicities

ABSTRACT With the advent of integral field units (IFUs), surveys can now measure metallicities across the discs of nearby galaxies at scales ≲100 pc. At such small scales, many of these regions contain too few stars to fully sample all possible stellar masses and evolutionary states, leading to stochastic fluctuations in the ionizing continuum. The impact of these fluctuations on the line diagnostics used to infer galaxy metallicities is poorly understood. In this paper, we quantify this impact for six most commonly used diagnostics. We generate stochastic stellar populations for galaxy patches with star formation rates varying over a factor of 1000, compute the nebular emission that results when these stars ionize gas at a wide range of densities, metallicities, and determine how much inferred metallicities vary with fluctuations in the driving stellar spectrum. We find that metallicities derived from diagnostics that measure multiple ionization states of their target elements (e.g. electron temperature methods) are weakly affected (variation <0.1 dex), but that larger fluctuations (∼0.4 dex) occur for diagnostics that depend on a single ionization state. Scatter in the inferred metallicity is generally largest at low star formation rate and metallicity, and is larger for more sensitive observations than for shallower ones. The main cause of the fluctuations is stochastic variation in the ionization state in the nebula in response to the absence of Wolf–Rayet stars, which dominate the production of ≳2−3 Ryd photons. Our results quantify the trade-off between line brightness and diagnostic accuracy, and can be used to optimise observing strategies for future IFU campaigns.

Coordination of Actinide Single Ions to Deformed Graphdiyne: Strategy on Essential Separation Processes in Nuclear Fuel Cycle.

The coordination of actinides and lanthanides, as well as strontium and cesium with graphdiyne (GDY) was studied experimentally and theoretically. On the basis of experimental results and/or theoretical calculations, it was suggested that Th4+ , Pu4+ , Am3+ , Cm3+ , and Cs+ exist in single-ion states on the special triangular structure of GDY with various coordination patterns, wherein GDY itself is deformed in different ways. Both experiment and theoretical calculations strongly indicate that UO2 2+ , La3+ , Eu3+ , Tm3+ and Sr2+ are not adsorbed by GDY at all. The distinguished adsorption behaviors of GDY afford an important strategy for highly selective separation of actinides and lanthanides, Th4+ and UO2 2+ , and Cs+ and Sr2+ , in the nuclear fuel cycle. Also, the present work sheds light on an approach to explore the unique functions and physicochemical properties of actinides in single-ion states.

Self-assembly of novel manganese (II) compounds based on bifunctional-group ligands: Synthesis, structures, and magnetic properties

Four manganese (II) compounds are obtained by the reaction of manganese salts, triazole-derivatives and auxiliary reagents in aqueous solution or mix-solvents by routine or hydrothermal reactions. X-ray crystal structure analyses reveal that a neutral 0D compound [Mn(Hmctrz)2(H2O)2] (1) (H2mctrz = 1H-1,2,4-triazole-3-carboxylic acid) displays a centro-symmetric mononuclear octahedral entity with two Hmctrz− anions and two water molecules; two neutral 2D clusters [Mn(Hdctrz)(H2O)2]n (2) (H3dctrz = 1H-1,2,4-triazole-3,5-dicarboxylic acid) and [Mn2(pbtrz)(btca)]n·4nH2O (3) (pbtrz = 1,3-bis(1,2,4-triazol-1-yl)-propane&H4btca = benzene-1,2,4,5-tetracarboxylic acid) possess layer structures with Hdctrz2− linkers (2) and Mn(II)-pbtrz-Mn(II) building blocks periodically extended by μ-btca4− connectors (3); [Mn(pbtrz)]n·nOAc·nOH (4) shows a 3D diamond-shaped cationic framework with the anion void volume of 49.2%. Nitrogenous bases are used as the auxiliary ligand in compound 3 and the temple ligand in compounds 1, 2, and 4. Compounds 1–4 show antiferromagnetic coupling that has been fitted by different models with the molecular field approximate with D = − 0.129(1) cm−1 for 1, J = − 0.354(4) cm−1 for 2 and J = − 0.696(6) cm−1 for 3, respectively. The magnetic differences can be related to different superexchange interactions transmitted by the crystal lattice and/or the zero field splitting (ZFS) of the 6A1g single-ion states of 1 and the syn-anti-COO− of 2 as well as the mixed magnetic bridges of μ1-O and μ-pbtrz-μ-COO− of 3.

Alignment dependent enhancement of the photoelectron cutoff for multiphoton ionization of molecules.

The multiphoton ionization rate of molecules depends on the alignment of the molecular axis with respect to the ionizing laser polarization. By studying molecular frame photoelectron angular distributions from N(2), O(2), and benzene, we illustrate how the angle-dependent ionization rate affects the photoelectron cutoff energy. We find alignment can enhance the high energy cutoff of the photoelectron spectrum when probing along a nodal plane or when ionization is otherwise suppressed. This is supported by calculations using a tunneling model with a single ion state.

Low-energy magnetic excitations in Co/CoO core/shell nanoparticles

We have used inelastic neutron scattering measurements to study the magnetic excitations of Co core/CoO shell nanoparticles for energies from 0 to 50 meV. Above the blocking temperature ${T}_{B}$, broad quasielastic scattering is observed, corresponding to the reorientation of the Co core moments and to paramagnetic CoO scattering. Below ${T}_{B}$, two nearly dispersionless inelastic peaks are found, whose energies increase with decreasing temperature as order parameters, controlled by the nanoparticle N\'eel temperature ${T}_{N}=235$ K, and saturating as $T\ensuremath{\rightarrow}0$ at 2.7 and 6.7 meV, respectively. Similar excitations were observed in a powdered single crystal of CoO, indicating that both are intrinsic excitations of CoO, resulting from the exchange splitting of single-ion states for $T\ensuremath{\leqslant}{T}_{N}$. Pronounced finite-size effects are observed for the scattering from the CoO nanoparticle shells, whose thicknesses range from 1.7 to 4.5 nm. These include an enhanced excitation linewidth, as well as a response that is not only spread over a much wider range of wave vectors, but is also significantly more intense in the nanoparticles than in bulk CoO.

Electronic structure and magnetism of Sr 2RuO 4

We present the calculated low-energy electronic structure, in the meV energy scale, of Sr 2RuO 4 that is associated with the single-ion states of the Ru 4+ ion. For description of the d electrons of the Ru 4+ ion we take into account the orbital degrees of freedom, the spin–orbit coupling, and the extremely strong d–d electron correlations. These strong correlations lead to the formation of the highly correlated atomic-like 4d 4 electron system. The derived electronic structure is different from that discussed at present in literature. We take recent Raman experimental data as experimental evidence for the existence of the discrete electronic structure.

Inductively Coupled Plasma-Atomic Emission Spectroscopy: Two Laboratory Activities for the Undergraduate Instrumental Analysis Course

Two inductively coupled plasma–atomic emission spectroscopy exercises are described for use in an undergraduate instrumental analysis course. The first activity looks at the emission signal produced by two different ionization states of the same element. The ionization states are in equilibrium within the plasma, and by observing the emission signal in different spatial regions of the plasma one can deduce information about the predominant ionic state and see suppression of the emission signal from the minor state. In the second exercise, the excitation temperature of the plasma is measured as a function of the power applied to the plasma induction coils. This temperature derives from the Boltzmann distribution, and can be determined by monitoring the intensity of a series of well-characterized emission transitions that emanate from a single ionization state of the element being observed.

Observational Signatures of X‐Ray–irradiated Accretion Disks

Reflection of X-rays from cool material around a black hole is one of the few observational diagnostics of the accretion flow geometry. Models of this reflected spectrum generally assume that the accretion disk can be characterized by material in a single ionization state. However, several authors have recently stressed the importance of the classic ionization instability for X-ray-irradiated gas in hydrostatic balance. This instability leads to a discontinuous transition in the vertical structure of the disk, resulting in a hot ionized skin above much cooler material. If the Compton temperature of the skin is high, then even iron is completely ionized, and the skin does not produce any spectral features. These new models, where the ionization structure of the disk is calculated self-consistently, require an excessive amount of computing power and so are difficult to use in directly fitting observed X-ray spectra. Instead, we invert the problem by simulating X-ray spectra produced by the new reflection models and then fit these with the old, single-zone reflection models, to assess the extent to which the derived accretion geometry depends on the reflection model used. We find that the single-zone ionization models can severely underestimate the covering fraction of the "cold" material as seen from the X-ray source if the optical depth in the ionized skin is of order unity and that this can produce an apparent correlation between the covering fraction and the X-ray spectral index similar in nature to that reported by Zdziarski, Lubiński, and Smith.

Experimental and simulated neon spectra in the 10-nm wavelength region from tokamak and reversed field pinch plasmas

Experimental neon spectra (in the 10-nm region), from the tokamak Tore Supra and the reversed field pinch experiment RFX, have been simulated. The spectra include lines from three neon ionization states, namely Ne(7+), Ne(6+), and Ne(5+) ions. Collisional radiative models have been built for these three Ne ions, considering electron collisional excitation and radiative decay as populating processes of the excited states. These models give photon emission coefficients for the emitted lines at electron density and temperature values corresponding to the experimental situations. Impurity modelling is performed using a one-dimensional impurity transport code, calculating the steady-state radial distribution of the Ne ions. The Ne line brightnesses are evaluated in a post-process subroutine and simulated spectra are obtained. The parts of the spectra corresponding to a single ionization state do not depend on the experimental conditions and show good agreement with the simulated single ionization state spectra. On the other hand, the superposition of the three spectra depends on the experimental conditions, as a consequence of the fact that the ion charge distribution depends not only on the radial profiles of the electron density and temperature, but also of the impurity transport coefficients. Simulations of the Ne spectra (including transport) give confidence in the atomic physics calculations; moreover, they allow the determination of the transport coefficients in the plasma region emitting the considered ionization states.

Pair states and orbital exchange coupling parameters of the dimeric diol complexes [Cr(ox) 2(OH)] 24− and [Cr(gly) 2(OH)] 2

A method for calculating exchange-induced energy-level splittings in polynuclear transition-metal complexes when orbitally unquenched single-ion states are involved is described. The method refers to Dirac's permutation operator in the spin space. Using symmetry-adapted wavefunctions, the pair energies are readily derived, often directly from the diagonal matrix elements of the bilinear exchange hamiltonian ▪ The theoretical results have been applied to absorption and emission spectra from pure spin-flip transitions of two di-μ-hydroxo bridged chromium(III) complexes containing oxalate and glycine chelate ligands. Low-temperature measurements down to 1.9 K yielded highly resolved spectra, in particular in the region of the 4A 2g 4A 2g → 4A 2g 2E g transitions. Pair lines due to inequivalent molecules in the crystal lattice of Na 4[Cr(ox) 2(OH)] 2 were distinguished by site-selective emission spectroscopy. Complete assignments were derived using spin and symmetry selection rules, and pair energies were calculated in terms of orbital exchange coupling constants J ij , which strictly obey the Goodenough-Kanamori rules. The following parameter values were obtained for the binuclear oxalate (glycinate) complex (in cm −1): j 11 = 4 (55), j 12 = − 29 (−150), j 13 = 13 (30), j 33 = −6 (−10). These data show that super-exchange via the bridging OH − ligands dominates direct exchange, resulting m a relatively weak antiferromagnetic coupling for the 4A 2g 4A 2g ground state: J gr ab = −0.3 (−4.2) cm −1. Magneto-structural correlations are found to arise predominantly from the position of the hydrogen atoms within the bridging unit.

On the charge state of anomalous oxygen

With the experiment 1ES024 aboard Spacelab 1 measurements were made of low-energy cosmic radiation within 10-18 MeV nucleon. With the help of a time-resolving unit it was possible to correlate the impact time to the geomagnetical conditions at the impact location for many particles. By means of cutoff rigidity considerations and trajectory tracing calculations it was possible to confirm the predicted single ionization state for the anomalous oxygen component. Furthermore, it was possible to distinguish among an anomalous component, Galactic cosmic rays, and the magnetospherical contribution to the registered oxygen nuclei. For one particle of both carbon and neon, single ionization states could be verified by trajectory tracing calculations.

Normal and superconducting states in Kondo-lattice and resonating-valence-bond systems: Why T c is so low

The Kondo single-ion state is real-space spin-singlet pairing of electrons. The Kondo-lattice (KL) state may be nearly-real-space Kondo pairing of electrons. Such pairing is incoherent and not superconductivity because the number of pairs is locally almost certain, causing the phase of the pairing wave function to be independently uncertain everywhere. Long-range phase coherence and e ∗ = 2 e superconductivity can occur in second-order pairing of two single-particle states determined by the presence of incoherent Kondo pairing. The two-band resonating-valence-bond (RVB) system is essentially equivalent to Kondo pairing in a Kondo lattice. T c for superconductivity due to second-order pairing mediated by magnetic excitations of the KL or RVB normal state would be T KL e -1 λM rather than T KL, or equivalently T RVB, or ω Debye e -1 λph .

Exchange interactions between Cr3+ions in magnesium oxide: I. Theoretical aspects

A theoretical analysis is presented of the consequences of exchange interactions on the spectroscopic properties of Cr3+ dimers. The eigenvalues and eigenvectors of the mod 4A2, 4A2) ground state and mod 2E,4A2) excited states have been calculated assuming the single-ion states to be coupled by an exchange interaction of the form Js1.s2 of arbitrary magnitude relative to the crystal-field splittings. Solution of the appropriate Hamiltonian yields ESR and fluorescence transition energies that are functions of J, the crystal-field splitting Dc and the dipolar coupling DE. The analysis has been applied to ESR and fluorescence spectra of two Cr3+ dimers in MgO with the ionic configuration (Cr.Mg V"Mg Cr.Mg). However, in one case the dimer is linear along a (100) crystal direction, whereas in the other the dimer forms a right angle at the cation vacancy. The particular interest is that for these two species the exchange parameter takes two extreme values J<Dc and J>>Dc. Experiments for accurate determination of the various parameters in the ground and optically excited states are discussed.

Spin-cluster excitations in RbFeCl3·2H2O

Spin-cluster excitations (SCE) from the ground state are observed in the pseudo-one-dimensional canted Ising antiferromagnet RbFe${\mathrm{Cl}}_{3}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O. Since the moments are canted with respect to each other, SCE were observed which show characteristics of a ferromagnetic chain and an antiferromagnetic chain when the external field is along the $c$ or $a$ axis, respectively. The results cannot be interpreted with the simple Ising model as in the case of spin-cluster resonance (SCR) reported earlier. For the small spin clusters observed in the present experiment, it is necessary to include effects from the higher single-ion states. Together with SCR the data provide rather detailed information on the magnetic ground state and the interactions.

Optical spectra of nearest neighbour nickel pairs in KMgF3 and KZnF3 crystals

The absorption, MCD, and emission spectra associated with the nearest neighbour Ni2+ pairs in KMgF3 and KZnF3 have been studied and analysed on the basis of the theory which includes spin dependent excitation transfer as well as spin independent transfer. It is shown that excitation transfers are important in determining the pair energy levels for the case where the singleion states involve orbital degeneracy. The antiferromagnetic J value of the ground state is determined to be 68 cm-1.

Light scattering by magnons in CoO, MnO, andα−MnS

Light scattering in antiferromagnetic crystals CoO, MnO, and $\ensuremath{\alpha}\ensuremath{-}\mathrm{MnS}$ has been studied experimentally in the temperature range from 6 to 300 K. The spectra showed peaks associated with one-magnon and two-magnon excitations. These structures diminished with increasing temperature, becoming unobservable above the N\'eel temperature. Polarization of the scattered light, which is important for the identification of the structures, was investigated for CoO. Raman scattering due to phonons was observed for MnO. The measured spectra for MnO and $\ensuremath{\alpha}\ensuremath{-}\mathrm{MnS}$ also showed photoluminescence, the spectral position of which was found to be independent of the frequency of incident radiation. A theoretical treatment of magnon excitations was worked out which takes into account the existence of many single-ion states. The existing information on magnons in these crystals can be satisfactorily interpreted.

Dispersion of the Magnon and Optical-Exciton States of GdCl3and Gd(OH)3and Their Effect upon the Single-Ion-Induced Absorption Line Shapes

The high-resolution optical-absorption spectra of the magnetic insulators Gd${\mathrm{Cl}}_{3}$ and Gd${(\mathrm{OH})}_{3}$ in a 35-kG magnetic field are used to demonstrate, for the first time, the influence of the magnon dispersion on the line shapes of single-ion-induced optical transitions. The absorption line shapes for transitions from the first thermally populated spin state to several single-ion states of the $^{6}P_{\frac{7}{2}}$ and $^{6}P_{\frac{5}{2}}$ manifolds of the ${\mathrm{Gd}}^{+3}$ ion are calculated. The initial and final states are properly treated as a magnon and exciton, respectively, including the dispersion of both states. The single-ion transition-moment operator is transformed to a basis in the crystal eigenstates and the resulting absorption line shape is shown to depend on a $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$-dependent transition-moment operator weighted by a combined exciton-magnon density of states and the magnon occupation number. The magnon dispersion is calculated from the known ground-state exchange interactions while the exciton dispersion is varied to give a best fit to the observed line shapes. The resulting excited-state exchange parameters are shown to be reasonable. The exciton dispersion is found to be quite significant (up to 2 ${\mathrm{cm}}^{\ensuremath{-}1}$) for one of the excited states. These two materials thus represent the first examples for which the exciton dispersion of the electronically excited states of a rare-earth salt have been directly observed.

Magnetic Field Dependence of Magnon Frequencies in the Canted-Spin Antiferromagnet NiF2

Below TN=73°K, NiF2 is a canted-spin antiferromagnet with the spins lying in the x-y plane. The magnetic excitations (magnons) of the structure form two branches whose Brillouin zone-center frequencies in zero field are 3.3 and 31 cm−1. We have measured the dependence of these frequencies in [100]-directed magnetic fields of up to 100 kOe using inelastic light scattering. The upper branch exhibits negligible field dependence, while the lower branch frequency increases monotonically to 14.4 cm−1 at 95 kOe. Our results join smoothly with previous ir measurements taken at lower magnetic fields. A spin-wave spectrum appropriate to NiF2 for the above field orientation has been derived within a theory which considers the excitations between the lowest pair of single-ion states in the combined crystal, Zeeman, and molecular fields. The excitations on different ions are coupled by the transverse part of the exchange. The calculated zone-center frequencies are in good agreement with the experimental observations over the entire magnetic field range for both branches.