Single-ion Model Research Articles

The Role of Electrons and Helium Atoms in Global Modeling of the Heliosphere

We present a new three-dimensional, MHD-plasma/kinetic-neutrals model of the solar wind (SW) interaction with the local interstellar medium (LISM), which self-consistently includes neutral hydrogen and helium atoms. This new model also treats electrons as a separate fluid and includes the effect of Coulomb collisions. While the properties of electrons in the distant SW and in the LISM are mostly unknown due to the lack of in situ observations, a common assumption for any global, single-ion model is to assume that electrons have the temperature of the ion mixture, which includes pickup ions. In the new model, electrons in the SW are colder, which results in a better agreement with New Horizons observations in the supersonic SW. In the LISM, however, ions and electrons are almost in thermal equilibrium. As for the plasma mixture, the major differences between the models are in the inner heliosheath, where the new model predicts a charge-exchange-driven cooling and a decrease of the heliosheath thickness. The filtration of interstellar neutral atoms at the heliospheric interface is discussed. The new model predicts an increase in the H density by ∼2% at 1 au. However, the fraction of pristine H atoms decreases by ∼12%, while the density of atoms born in the outer and inner heliosheath increases by 5% and ∼35%, respectively. While at 1 au the density of He atoms remains unchanged, the contribution from the “warm breeze” increases by ∼3%.

Electronic structure of the frustrated diamond lattice magnet NiRh2O4

The $A$-site spinel ${\mathrm{NiRh}}_{2}{\mathrm{O}}_{4}$ is the only known realization of a spin-1 diamond lattice magnet and is predicted to host unconventional magnetic phenomena driven by frustrated nearest and next-nearest neighbor exchange as well as orbital degeneracy. Previous works found no sign of magnetic order but found a gapped dispersive magnetic excitation indicating a possible valence bond magnetic ground state. However, the presence of many competing low energy degrees of freedom and limited empirical microscopic constraints complicates further analysis. Here we carry out resonant inelastic x-ray scattering (RIXS), x-ray absorption spectroscopy (XAS), and inelastic neutron scattering (INS) to characterize the local electronic structure and lattice dynamics of ${\mathrm{NiRh}}_{2}{\mathrm{O}}_{4}$. The RIXS data can be partly described by a single-ion model for tetrahedrally coordinated ${\mathrm{Ni}}^{2+}$ and indicates a tetragonal distortion $\mathrm{\ensuremath{\Delta}}{t}_{2}=70$ meV that splits the ${t}_{2}$ orbitals into a high energy orbital singlet and lower energy orbital doublet. We identify features of the RIXS spectra that are consistent with a Rh-Ni two-site excitation indicating strong metal-metal hybridization mediated by oxygen in ${\mathrm{NiRh}}_{2}{\mathrm{O}}_{4}$. We also identify signatures of electron-phonon coupling through the appearance of phonon sidebands that dress crystal field excitations. These results establish the key energy scales relevant to the magnetism in ${\mathrm{NiRh}}_{2}{\mathrm{O}}_{4}$ and further demonstrate that covalency and lattice dynamics play essential roles in controlling the magnetic ground states of $A$-site spinels.

Elastic study of electric quadrupolar correlation in the paramagnetic state of the frustrated quantum magnet Tb2+δTi2−δO7

The electric quadrupolar state in the frustrated quantum magnet ${\mathrm{Tb}}_{2+\ensuremath{\delta}}{\mathrm{Ti}}_{2\ensuremath{-}\ensuremath{\delta}}{\mathrm{O}}_{7}$ has been studied by means of ultrasonic and magnetostriction measurements. A single crystal showed an elastic anomaly around 0.4 K, manifesting a long-range quadrupole ordering. By investigating the anisotropy of the magnetoelastic responses, we found a crossover temperature for the strongly correlated quadrupole state, below which the experimental data of the elastic constant and magnetostriction become qualitatively different from their calculations based on a single-ion model. We suppose that relatively high onset temperature of the quadrupole correlation compared with the transition temperature is ascribed to the geometrical frustration effect, and this correlated state seems to be responsible for the unusual properties in the paramagnetic state of ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$.

Giant modulation of optical nonlinearity by Floquet engineering.

Strong periodic driving with light offers the potential to coherently manipulate the properties of quantum materials on ultrafast timescales. Recently, strategies have emerged to drastically alter electronic and magnetic properties by optically inducing non-trivial band topologies1-6, emergent spin interactions7-11 and even superconductivity12. However, the prospects and methods of coherently engineering optical properties on demand are far less understood13. Here we demonstrate coherent control and giant modulation of optical nonlinearity in a van der Waals layered magnetic insulator, manganese phosphorus trisulfide (MnPS3). By driving far off-resonance from the lowest on-site manganese d-d transition, we observe a coherent on-off switching of its optical second harmonic generation efficiency on the timescale of 100 femtoseconds with no measurable dissipation. At driving electric fields of the order of 109 volts per metre, the on-off ratio exceeds 10, which is limited only by the sample damage threshold. Floquet theory calculations14 based on a single-ion model of MnPS3 are able to reproduce the measured driving field amplitude and polarization dependence of the effect. Our approach can be applied to a broad range of insulating materials and could lead to dynamically designed nonlinear optical elements.

Selective probing of magnetic order on Tb and Ir sites in stuffed Tb2Ir2O7 using resonant x-ray scattering

We study the magnetic structure of the ‘stuffed’ (Tb-rich) pyrochlore iridate Tb2+xIr2−xO7−y (x ∼ 0.18), using resonant elastic x-ray scattering (REXS). In order to disentangle contributions from Tb and Ir magnetic sublattices, experiments were performed at the Ir L3 and Tb M5 edges, which provide selective sensitivity to Ir 5d and Tb 4f magnetic moments, respectively. At the Ir L3 edge, we found the onset of long-range magnetic order below K, consistent with the expected signal of all-in all-out (AIAO) magnetic order. Using a single-ion model to calculate REXS cross-sections, we estimate an ordered magnetic moment of at 5 K. At the Tb M5 edge, long-range magnetic order appeared below ∼ K, also consistent with an AIAO magnetic structure on the Tb site. Additional insight into the magnetism of the Tb sublattice is gleaned from measurements at the M5 edge in applied magnetic fields up to 6 T, which is found to completely suppress the Tb AIAO magnetic order. In zero applied field, the observed gradual onset of the Tb sublattice magnetisation with temperature suggests that it is induced by the magnetic order on the Ir site. The persistence of AIAO magnetic order, despite the greatly reduced ordering temperature and moment size compared to stoichiometric Tb2Ir2O7, for which K and , indicates that stuffing could be a viable means of tuning the strength of electronic correlations, thereby potentially offering a new strategy to achieve topologically non-trivial band crossings in pyrochlore iridates.

Polarization-resolved Cu L3 -edge resonant inelastic x-ray scattering of orbital and spin excitations in NdBa2Cu3O7−δ

High resolution resonant inelastic x-ray scattering (RIXS) has proven particularly effective in the determination of crystal field and spin excitations in cuprates. Its strength lies in the large Cu $L_{3}$ resonance and in the fact that the scattering cross section follows quite closely the single-ion model predictions, both in the insulating parent compounds and in the superconducting doped materials. However, the spectra become increasingly broader with (hole) doping, hence resolving and assigning spectral features has proven challenging even with the highest energy resolution experimentally achievable. Here we have overcome this limitation by measuring the complete polarization dependence of the RIXS spectra as function of momentum transfer and doping in thin films of NdBa$_{2}$Cu$_{3}$O$_{7-\delta}$. Besides confirming the previous assignment of $dd$ and spin excitations (magnon, bimagnon) in the antiferromagnetic insulating parent compound, we unequivocally single out the actual spin-flip contribution at all dopings. We also demonstrate that the softening of $dd$ excitations is mainly attributed to the shift of the $xy$ peak to lower energy loss. These results provide a definitive assessment of the RIXS spectra of cuprates and demonstrate that RIXS measurements with full polarization control are practically feasible and highly informative.

Effect of (1 1 1)-oriented strain on the structure and magnetic properties of La0.7Sr0.3MnO3 thin films

Using strain, i.e. subtle changes in lattice constant in a thin film induced by the underlying substrate, opens up intriguing new ways to control material properties. We present a study of the effects of strain on structural and ferromagnetic properties of (1 1 1)pc-oriented La0.7Sr0.3MnO3 epitaxial thin films grown on NdGaO3, SrTiO3, and DyScO3 substrates. (The subscript pc denotes the pseudo-cubic symmetry.) The results show that La0.7Sr0.3MnO3 assumes a monoclinic unit cell on NdGaO3 and DyScO3 and a rhombohedral unit cell on SrTiO3. For La0.7Sr0.3MnO3 on NdGaO3 and DyScO3 a uniaxial magnetic anisotropy is found, while La0.7Sr0.3MnO3 on SrTiO3 is magnetically isotropic. The Néel model is used to explain the anisotropy of the thin films on NdGaO3 and SrTiO3, however, for La0.7Sr0.3MnO3 on DyScO3 the effect of octahedral rotations needs to be included through the single ion model. Through examination of the Curie temperature of the strained films we suggest that (1 1 1)-strain has a different effect on the Jahn–Teller splitting of eg and t2g electron levels than what is seen in (0 0 1)pc-oriented La0.7Sr0.3MnO3 thin films.

High-pressure insulator-to-metal transition in Sr3Ir2O7 studied by x-ray absorption spectroscopy

High-pressure x-ray absorption spectroscopy was performed at the Ir $L_3$ and $L_2$ absorption edges of Sr$_3$Ir$_2$O$_7$. The branching ratio of white line intensities continuously decreases with pressure, reflecting a reduction in the angular part of the expectation value of the spin-orbit coupling operator, $\left\langle {\bf L} \cdot {\bf S} \right\rangle$. Up to the high-pressure structural transition at 53 GPa, this behavior can be explained within a single-ion model, where pressure increases the strength of the cubic crystal field, which suppresses the spin-orbit induced hybridization of $J_{\text{eff}} = 3/2$ and $e_g$ levels. We observe a further reduction of the branching ratio above the structural transition, which cannot be explained within a single-ion model of spin-orbit coupling and cubic crystal fields. This change in $\left\langle {\bf L} \cdot {\bf S} \right\rangle$ in the high-pressure, metallic phase of Sr$_3$Ir$_2$O$_7$ could arise from non-cubic crystal fields or a bandwidth-driven hybridization of $J_{\text{eff}}=1/2,\,3/2$ states, and suggests that the electronic ground state significantly deviates from the $J_{\text{eff}}=1/2$ limit.

Spin reorientation and giant low-temperature magnetostriction of polycrystalline NdFe1.9 compound

The spin reorientation and magnetostriction of polycrystalline NdFe1.9 cubic Laves phase compound were investigated. A prominent transition from tetragonal symmetry to orthorhombic symmetry in NdFe1.9 compound was determined by X-ray crystallographic study. Meanwhile, a large spontaneous magnetostriction λ111 of ∼3100 ppm was detected at 15 K, which is larger than the theoretical value of 2000 ppm predicted by single-ion model. NdFe1.9 exhibits larger low-field magnetostriction than PrFe1.9 and TbFe1.9 at 5 K in the magnetic field range of H ≤ 13 kOe, which makes it a promising material for low-temperature applications. The present work might be helpful to discover inexpensive Nd-based high-performance magnetostrictive and even magnetoelectric materials for low-temperature applications.

Influence of chemical composition on the X-ray photoemission, thermopower, specific heat, and magnetic properties of CeNi2(Si1-yGey)2

We report our studies of the intermediate compositions between CeNi2Si2 and CeNi2Ge2, i.e., the alloys CeNi2(Si1-yGey)2 by means of the thermopower, electrical resistivity, specific heat, magnetic susceptibility, and X-ray photoemission measurements. CeNi2Si2 is a fluctuating valence system and CeNi2Ge2 is known to show the heavy fermion behaviour. The change of the temperature dependence of the resistivity towards the typical metallic behaviour occurs below y ~ 0.25. The transition between CeNi2Si2 and CeNi2Ge2 is discussed in the frames of competition between the crystal electric field and Kondo interactions. It is found that valence stabilisation occurs for Ge content y > 0.25. The hybridization energy Δ determined from the XPS Ce 3d spectrum reflects well the behaviour of the parameter Eex obtained from the analysis of the magnetic susceptibility by the interconfiguration fluctuation model. It has been also shown that thermopower data can be successfully described employing the single ion model for 0.6 < y < 1.0 and two-band model including the crystal electric field splitting for y ≤ 0.25.

Hybrid excitations due to crystal field, spin-orbit coupling, and spin waves in LiFePO4

We report on the spin waves and crystal field excitations in single crystal LiFePO$_4$ by inelastic neutron scattering over a wide range of temperatures, below and above the antiferromagnetic transition of this system. In particular, we find extra excitations below $T_N=50$ K that are nearly dispersionless and are most intense around magnetic zone centers. We show that these excitations correspond to transitions between thermally occupied excited states of Fe$^{2+}$ due to splitting of the $S=2$ levels that arise from crystal field and spin-orbit interaction. These excitations are further amplified by the highly distorted nature of the oxygen octahedron surrounding the iron atoms. Above $T_N$, magnetic fluctuations are observed up to at least 720~K, with additional excitation around 4 meV, likely caused by single-ion splittings through spin-orbit and crystal field interactions. The latter weakens slightly at 720~K compared to 100~K, which is consistent with calculated cross-sections using a single-ion model. Our theoretical analysis, using the MF-RPA model, provides both detailed spectra of the Fe $d-$ shell and estimates of the average ordered magnetic moment and $T_N$. By applying the MF-RPA model to a number of existing spin-wave results from other Li$M$PO$_4$ ($M=$ Mn, Co, and Ni), we are able to obtain reasonable predictions for the moment sizes and transition temperatures.

Two-Coordinate Co(II) Imido Complexes as Outstanding Single-Molecule Magnets

The pursuit of single-molecule magnets (SMMs) with better performance urges new molecular design that can endow SMMs larger magnetic anisotropy. Here we report that two-coordinate cobalt imido complexes featuring highly covalent Co═N cores exhibit slow relaxation of magnetization under zero direct-current field with a high effective relaxation barrier up to 413 cm-1, a new record for transition metal based SMMs. Two theoretical models were carried out to investigate the anisotropy of these complexes: single-ion model and Co-N coupling model. The former indicates that the pseudo linear ligand field helps to preserve the first-order orbital momentum, while the latter suggests that the strong ferromagnetic interaction between Co and N makes the [CoN]+ fragment a pseudo single paramagnetic ion, and that the excellent performance of these cobalt imido SMMs is attributed to the inherent large magnetic anisotropy of the [CoN]+ core with |MJ = ± 7/2⟩ ground Kramers doublet.

Weak ferromagnetism along the triad axis of FeBO3 crystals

Weak ferromagnetic moment along the triad axis of FeBO3 crystals has been calculated on the basis of the single-ion model taking into account the cubic invariant of the crystal field in the spin Hamiltonian.

Magnetic-field-induced spin crossover of Y-dopedPr0.7Ca0.3CoO3

The family of hole-doped Pr-based perovskite cobaltites, Pr$_{0.5}$Ca$_{0.5}$CoO$_{3}$ and (Pr$_{1-y}$RE$_{y}$)$_{0.3}$Ca$_{0.7}$CoO$_{3}$ (where RE is rare earth) has recently been found to exhibit simultaneous metal-insulator, spin-state, and valence transitions. We have investigated magnetic-field-induced phase transitions of (Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_{3}$ by means of magnetization measurements at 4.2$-$100 K up to an ultrahigh magnetic field of 140 T with the chemical pressure varied by $y$ = 0.0625, 0.075, 0.1. The observed magnetic-field-induced transitions were found to occur simultaneously with the metal-insulator transitions up to 100 T. The obtained magnetic field-temperature ($B$-$T$) phase diagram and magnetization curves are well analyzed by a spin-crossover model of a single ion with interion interactions. On the other hand, the chemical pressure dependence of the experimentally obtained magnetization change during the phase transition disagrees with the single ion model when approaching low temperatures. The significant $y$ dependence of the magnetization change at low temperatures may arise from the itinerant magnetism of Co$^{3+}$ in the paramagnetic metallic phase, where the chemical pressure enhances the exchange splitting by promoting the double-exchange interaction. The observed $B$-$T$ phase diagrams of (Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_{3}$ are quite contrary to that of LaCoO$_{3}$, indicating that in (Pr$_{1-y}$Y$_{y}$)$_{0.7}$Ca$_{0.3}$CoO$_{3}$ the high-field phase possesses higher entropy than the low-field phase, whereas it is the other way around in LaCoO$_{3}$.

Structural and magnetic properties of morphotropic phase boundary involved Tb1−xGdxFe2 compounds

In the present paper, structural, magnetic and magnetostrictive properties of Tb1−xGdxFe2 (0 ≤ x ≤ 1.0) were studied. Synchrotron x-ray diffraction (XRD) results show the non-cubic symmetry of Tb1−xGdxFe2 at room temperature and composition-induced crystallographic phase transition from rhombohedral phase to tetragonal phase. The Gd concentration dependent lattice parameters, lattice distortion and change of easy magnetic direction were detected by synchrotron XRD. With the Gd concentration increases, Curie temperature Tc increases while room temperature magnetization and magnetostriction coefficient λ111 and the anisotropy of TbFe2 decrease. The decrease in spontaneous magnetostriction coefficient λ111 with increasing Gd substitution can be understood on the basis of the single-ion model; the corresponding decrease of magnetostriction for Tb1−xGdxFe2, and the large magnetostriction value occurs on the Tb-rich side, are ascribed to decrease of λ111.

Temperature Dependence of Magnetization, Anisotropy, and Hyperfine Fields of NiFe2–xYbxO4($x = 0$ , 0.05, 0.075)

Nickel ferrite (NiFe 2 O 4 ), an inverse spinel in which the tetrahedral (A) sites are occupied by Fe3+ ions and the octahedral (B) sites are occupied by Fe3+ and Ni2+ ions, can be represented as Fe A 3+ Ni B 2+ Fe B 3+ O 4 2− [1-3]. The cation distribution and the resulting magnetic properties are reported to be quite interesting in rare-earth doped nickel ferrite due to the absence of center of symmetry [2, 3]. Kamala Bharathi et al. have reported that, substitution of Fe3+ by Dy3+ ion in NiFe 2 O 4 causes development of magnetocapacitance, leads to ferroelectricity and affects several other properties [3]. These materials play an important role in applications (spintronic memory cell, high frequency devices etc) [4]. There are no reports on thermal behavior of magnetic anisotropy in the rare earth substituted Ni ferrites. Understanding these behaviors will be useful in novel applications such as heat-assisted magnetic recording [4, 5]. In the present work, the temperature dependence of magnetic anisotropy and hyperfine fields of NiFe 2−x Yb x O 4 (x = 0, 0.05, 0.075) are reported. All the samples were prepared by solid state reaction. The materials are found to have formed in inverse spinel structure, from the powder XRD patterns. Rietveld refinement carried out on the XRD patterns revealed that Yb3+ ions occupy the B site and the lattice is found to be expanded with the introduction of Yb3+. The lattice constants are 8.3415(1) A (reported value is 8.34 A [1]), 8.3436(0) A and 8.3463(5) A for x = 0, 0.05 and 0.075 compounds respectively. Magnetization was measured at 5 K, 100 K, 200 K, and 300 K and the magnetization was observed to decrease with x. This is attributed to substitution of Fe3+ by Yb3+ in the B-site. The saturation magnetization values, at all the measured temperatures obtained through Honda plots are given in Table 1. The moments calculated using Hund's rule (assuming that Yb3+ and Fe3+ moments are parallel) are 2.82 μ B /f.u., 2.76 μ B /f.u. and 2.71 μ B /f.u. for x = 0, 0.05 and 0.075 compounds respectively. The experimental values are less in comparison with the calculated moments. In NiFe 2 O 4 , this difference is in accordance with the reported literature [1-3]. At each temperature, M-H data were fitted to the law of approach to saturation (LAS) [1, 6] and K1(T), the first order magnetocrystalline anisotropy constant at a temperature T, was obtained from LAS. At any given temperature, the observed increase in K1 of x = 0.05 and 0.075 compared with x = 0, is explained on the basis of single-ion model i.e. localized divalent ions in the octahedral site. The temperature dependence of K1 of cubic systems was investigated by plotting ln K1(T) vs ln M(T). The slopes of the straight line obtained are 7.5, 12.5 and 11.44 for x = 0, 0.05 and 0.075 respectively. This can be compared with the value of 10 proposed for magnetic anisotropy in cubic systems [1, 6]. The deviation of the exponent is explained on the extent of localization of the divalent ion.

Alternating magnetic anisotropy ofLi2(Li1−xTx)N(T=Mn,Fe,Co,andNi)

Substantial amounts of the transition metals Mn, Fe, Co, and Ni can be substituted for Li in single crystalline Li$_2$(Li$_{1-x}T_x$)N. Isothermal and temperature-dependent magnetization measurements reveal local magnetic moments with magnitudes significantly exceeding the spin-only value. The additional contributions stem from unquenched orbital moments that lead to rare-earth-like behavior of the magnetic properties. Accordingly, extremely large magnetic anisotropies have been found. Most notably, the magnetic anisotropy alternates as easy-plane $\rightarrow$ easy-axis $\rightarrow$ easy-plane $\rightarrow$ easy-axis when progressing from $T$ = Mn $\rightarrow$ Fe $\rightarrow$ Co $\rightarrow$ Ni. This behavior can be understood based on a perturbation approach in an analytical, single-ion model. The calculated magnetic anisotropies show a surprisingly good agreement with the experiment and capture the basic features observed for the different transition metals.

Modeling the reactive ion dynamics in the rhizosphere of tree roots growing in acid soils. I. Rhizospheric distribution patterns and root uptake of Mb cations as affected by root-induced pH and Al dynamics

Trees growing in acid soils often suffer from nutrient imbalances and inadequate supply of base cations (Mb) but correlations between soil chemical conditions and nutritional status of forest trees are often inconclusive. Therefore, there is a need of studies that assess the Mb acquisition potential of the absorbing fine roots from the rhizosphere soil. Previous rhizosphere models, mostly implemented as a single-ion model (SIM), calculate the actual root nutrient uptake rates. But SIM often fail to reproduce measurements which is interpreted as being caused by root-induced ionic interactions. Hence, a multi-ion model (MIM) is presented which simultaneously describes the rhizospheric dynamics of H+, Al3+, Ca2+, Mg2+, K+, Na+, NH4+, NO3−, SO42−, H2PO4− and Cl− which takes into account interactions among the ions involved. In MIM the ion diffusion transport is modeled via the Nernst–Planck equation. A root-induced constant or daily-patterned water flux is assumed. The cation sorption is defined according to the cation selectivity approach. Al-solution complexes and a kinetic expression of the dissolution or precipitation of Al(OH)3(s) (gibbsite) are included in MIM. The selective nutrient root uptake is balanced by the excretion of H+ (cation uptake excess) or OH− (anion uptake excess) ions. These model features guarantee electro neutrality in the rhizosphere system but lead to ionic interactions. The objectives of this study are to calculate the rhizospheric gradients of protons, Al3+ ions and base cations (Mb), their concentration changes at the root surface (RS) and in rhizospheric sub-volumes termed as soil–root interface (SRI) and inner rhizosphere (Rh). It is hypothesized that root-induced changes of pH and the pH-dependent dissolution or precipitation of Al(OH)3(s) affect the rhizospheric concentration gradients and the actual root uptake rates (UMb) of Mb cations. In various scenarios the hypothesis is tested on the basis of different ion concentrations in the bulk soil and root uptake capacities of nitrogen and Mb ions. The simulations demonstrate that the rates of root excretions as H+ or OH− ions are determined by the preferential nitrogen root uptake as NH4+ or NO3−, respectively. A high NH4+ root uptake leads to a decrease of rhizospheric pH and a dissolution of Al(OH)3(s). An accumulation of Al3+ cations in solution and exchanger mostly on RS and in SRI is calculated due to water flux and Al(OH)3(s)-dissolution. Accumulation of exchangeable Al3+ cations cause an enhanced desorption of Mb cations in SRI if compared with SIM-results and lead to a Mb concentration increase in Rh-solution and a RS-depletion for Ca2+ and K+. MIM-calculated UMb are slightly higher compared with SIM-calculated UMb. A high NO3− root uptake leads to a rhizospheric pH increase, a depletion of Al3+ in rhizospheric solution and exchanger also at water flux caused by an Al(OH)3(s)-formation, an accumulation of exchangeable Mb cations mainly in SRI, a Mb-depletion in rhizospheric soil solution and to significantly lower UMb if compared with SIM-results. Al(OH)3(s)-induced differences in rhizospheric Mb gradients and UMb-values are determined by the magnitude of the H+/OH− root excretion rates, are highest at low Mb solution concentrations, and also occur in extremely low Al3+ bulk soil solution concentrations. An Al(OH)3(s)-formation may be inhibited at high Al3+ bulk soil solution concentrations and high H+-concentrations in solution and exchanger of the bulk soil. The range of calculated Mb depletions and accumulations in SRI and Rh correspond to the measurement results reported in the literature. It is concluded that, in contrast to SIM, MIM-simulations present asynchronous ion concentration gradients in soil solution and exchanger which include opposite concentration gradients. At NO3− surplus a high NO3− root uptake and a low availability of Mb cations may lead to wide NO3−:Mb root uptake ratios and tree nutrient imbalances.

High-Resolution Solid-State Oxygen-17 NMR of Actinide-Bearing Compounds: An Insight into the 5f Chemistry

A massive interest has been generated lately by the improvement of solid-state magic-angle spinning (MAS) NMR methods for the study of a broad range of paramagnetic organic and inorganic materials. The open-shell cations at the origin of this paramagnetism can be metals, transition metals, or rare-earth elements. Actinide-bearing compounds and their 5f unpaired electrons remain elusive in this intensive research area due to their well-known high radiotoxicity. A dedicated effort enabling the handling of these highly radioactive materials now allows their analysis using high-resolution MAS NMR (>55 kHz). Here, the study of the local structure of a series of actinide dioxides, namely, ThO2, UO2, NpO2, PuO2, and AmO2, using solid-state (17)O MAS NMR is reported. An important increase of the spectral resolution is found due to the removal of the dipolar broadening proving the efficiency of this technique for structural analysis. The NMR parameters in these systems with numerous and unpaired 5f electrons were interpreted using an empirical approach. Single-ion model calculations were performed for the first time to determine the z component of electron spin on each of the actinide atoms, which is proportional to the shifts. A similar variation thereof was observed only for the heavier actinides of this study.

Resonant X-Ray Scattering and thejeff=1/2Electronic Ground State in Iridate Perovskites

The resonant x-ray scattering (magnetic elastic, RXMS, and inelastic, RIXS) of Ir4+ at the L2,3 edges relevant to spin-orbit Mott insulators A(n+1)Ir(n)O(3n+1) (A = Sr, Ba, etc.) are calculated using a single-ion model which treats the spin-orbit and tetragonal crystal-field terms on an equal footing. Both RXMS and RIXS in the spin-flip channel are found to display a nontrivial dependence on the direction of the magnetic moment, μ. Crucially, we show that for μ in the ab plane, RXMS in the cross-polarized channel at the L2 edge is zero irrespective of the tetragonal crystal field; spin-flip RIXS, relevant to measurements of magnons, behaves reciprocally, being zero at L2 when μ is perpendicular to the ab plane. Our results have important implications for the assignment of a j(eff) = 1/2 ground state on the basis of resonant x-ray experiments.