Edge Spectra Research Articles

Systematic investigation of relativistic effects in EXAFS data analysis

Modern XAFS (x-ray absorption fine structure) data analysis is based on accurate multiple-scattering (MS) calculations of the x-ray absorption cross section, usually carried out solving the nonrelativistic Schr\"odinger equation for complex effective optical muffin-tin potentials describing the scattering of the atoms. The introduction of relativistic effects in extended XAFS (EXAFS) multiple-scattering calculations has been described in several papers and shown to be important for heavy atoms. However, few examples of applications and detailed studies of relativistic effects were given so far. In this work, we have performed a systematic investigation of relativistic corrections in systems of increasing atomic number, using a reliable simulation scheme recently developed and based on the incorporation of a pseudo-Schr\"odinger equation effectively replacing the Dirac relativistic form. Calculations have been put to a test in 12 different pure-element condensed-state systems, with the atomic number ranging from $Z=10$ for crystalline Ne to $Z=90$ for crystalline Th. The importance of accounting for relativistic effects has been highlighted for elements with $Z\ensuremath{\gtrsim}60$, as ones for which relativistic corrections for amplitudes of calculated XAFS MS signals exceed $10%$. The size of relativistic effects for calculated higher-order XAFS signal (with respect to the dominant single-scattering first-neighbor signal) has been shown, taking as example the ${L}_{3}$-edge spectra of crystalline Au and Pb. The size of relativistic effects for the $K$ and ${L}_{3}$ edges has been also evaluated, showing a slight increase of relativistic corrections for the ${L}_{3}$ edge. The improvement in the accuracy of XAFS simulations has been demonstrated comparing the results obtained for structural refinements of the ${L}_{3}$ edge of crystalline Au at 300 K.

Iridium pair sites anchored to Zr6O8 nodes of the metal–organic framework UiO-66 catalyze ethylene hydrogenation

Isolated metal pair sites on metal oxide and zeolite supports are drawing attention as catalysts, because—in contrast to single atomically dispersed metals—they provide neighboring metal centers that can act cooperatively. We now report pairs of iridium atoms anchored to the Zr6O8 nodes of the metal–organic framework (MOF) UiO-66, synthesized by chemisorption of Ir2(μ-OCH3)2(COD)2 (COD is cyclooctadienyl) followed by removal of the COD ligands. The supported species were characterized with infrared spectra of adsorbed CO combined with iridium LIII-edge extended X-ray absorption fine structure and high-energy-resolution fluorescence detection X-ray absorption near edge (HERFD XANES) spectra. The HERFD XANES spectra were recorded with the sample in a variety of atmospheres in which the iridium pair sites were stable, including CO, H2, and C2H4 + H2 at temperatures in the range of 35–80 °C. The data provide sensitive measures of the electronic structure of the iridium in the pair sites. The samples were evaluated as catalysts for ethylene hydrogenation, with the pair-sites being more selective for hydrogenation than analogous isolated atomically dispersed iridium, which catalyzes both hydrogenation and isomerization. Metal pair sites on MOFs offer uncharted opportunities for catalysts having reactivities associated with neighboring metal centers.

Insight into the Gd–Pt Bond: Slow Magnetic Relaxation of a Heterometallic Gd–Pt Complex

Abstract Lanthanide (Ln) compounds are common research targets in the field of magnetism and optics. Their properties arise from the electrons localized in the f-orbital. Moreover, the effect of the covalency between lanthanide and ligands on magnetism has attracted significant attention. We have provided insight into the Gd–Pt bond (of the heterometallic Ln-Pt complexes: {[Pt(PhSAc)4]Ln[(PhSAc)4Pt]} NEt4·2DMF (Ln = Y(0), La(1), Gd(2); PhSAc = thiobenzoate, NEt4 = tetraethylammonium)); single-crystal polarized X-ray absorption near edge structure (XANES) reveal the electronic states around metal ion, where spectra of Gd-LIII edges show the Gd–Pt direction has the highest covalency (less ionic) around Gd ion in 2. In addition, calculating natural bonding (NBO) analysis, natural population analysis (NPA), LOL, and atoms in molecules (AIM), ab initio calculations reveal the role of metallic and organic ligands in the electronic and magnetic properties of Ln complexes. The slow magnetization relaxation of the Gd complex, which has not been reported previously in the Pt–Gd–Pt system, was observed up to 45 K, the highest temperature reported to date among isolated Gd-complexes (frequency range: 0.1–10000 Hz).

Theoretical realization of rich magnon topology by symmetry-breaking in honeycomb bilayer ferromagnets

We reveal the rich magnon topology in honeycomb bilayer ferromagnets (HBF) induced by the combined effect of interlayer exchange, Dzyaloshinskii-Moriya interaction (DMI), and electrostatic doping (ED). In particular, we present a systematic study of the Hamiltonian non-adiabatic evolution in the HBF parametric space, spanned by the symmetry-breaking terms (DMI and ED) and interlayer exchange. We determine the band closure manifolds which are found to divide the parametric space into six distinct regions, matched with five distinct topological phases and one topologically trivial phase. The characteristic Chern numbers and thermal Hall conductivities are calculated for the topological phases. Edge spectra, dictated by the bulk-edge correspondence, are also analyzed in the nanoribbon version of the model. Both bulk and edge spectra are found to be nonreciprocal as a consequence of ED and edge magnons are observed to counter propagate on opposite edges. The predicted results offer new insights on the manipulation of magnonic Chern numbers and magnon topological transport via experimentally tunable parameters.

Quantum de Sitter horizon entropy from quasicanonical bulk, edge, sphere and topological string partition functions

Motivated by the prospect of constraining microscopic models, we calculate the exact one-loop corrected de Sitter entropy (the logarithm of the sphere partition function) for every effective field theory of quantum gravity, with particles in arbitrary spin representations. In doing so, we universally relate the sphere partition function to the quotient of a quasi-canonical bulk and a Euclidean edge partition function, given by integrals of characters encoding the bulk and edge spectrum of the observable universe. Expanding the bulk character splits the bulk (entanglement) entropy into quasinormal mode (quasiqubit) contributions. For 3D higher-spin gravity formulated as an sl(n) Chern-Simons theory, we obtain all-loop exact results. Further to this, we show that the theory has an exponentially large landscape of de Sitter vacua with quantum entropy given by the absolute value squared of a topological string partition function. For generic higher-spin gravity, the formalism succinctly relates dS, AdS± and conformal results. Holography is exhibited in quasi-exact bulk-edge cancelation.

New bound for edge spectral radius and edge energy of graphs

Let $ X(V,E) $ be a simple graph with $ n $ vertices and $ m $ edges without isolated vertices. Denote by $ B = (b_{ij})_{mtimes m} $ the edge adjacency matrix of $ X $. Eigenvalues of the matrix $ B $, $mu_1, mu_2, cdots, mu_m $, are the edge spectrum of the graph $ X $. An important edge spectrum-based invariant is the graph energy, defined as $ E_e(X) =sum_{i=1}^{m} vert mu_i vert $. Suppose $ B^{'} $ be an edge subset of $ E(X) $ (set of edges of $ X $). For any $ e in B^{'} $ the degree of the edge $ e_i $ with respect to the subset $ B^{'} $ is defined as the number of edges in $ B^{'} $ that are adjacent to $ e_i $. We call it as $ varepsilon $-degree and is denoted by $ varepsilon_i $. Denote $ mu_1(X) $ as the largest eigenvalue of the graph $ X $ and $ s_i $ as the sum of $ varepsilon $-degree of edges that are adjacent to $ e_i $. In this paper, we give lower bounds of $ mu_1(X) $ and $ mu_1^{D^{'}}(X) $ in terms of $ varepsilon $-degree. Consequently, some existing bounds on the graph invariants $ E_e(X) $ are improved.

Electronic structure and electrochemical properties of La-doped BiFeO3 nanoparticles

We investigated the effect of La substitutional ions on the structural, optical, and electrochemical properties of bismuth ferrite (BiFeO3) and used them as electrode materials for high performance supercapacitors. The compositions of 1.05Bi1-xLaxFeO3 (0.0 < x <0.10) nanoparticles were prepared through sol-gel conventional route and denoted as 1.05BiFeO3: BF0; 1.05Bi0.95La0.05FeO3: BF5; 1.05Bi0.90La0.10FeO3: BF10. The Rietveld refinement results of XRD patterns suggested the formation of rhombohedrally distorted perovskite structure (space group: R3c) with negligible secondary phase. The crystallite sizes determined using Scherrer’s formula were found to be in the 59−48 nm range and observed to decrease with an increase in La content. The bandgap measured using UV absorption spectroscopy was found to decrease with an increase in La content in BiFeO3. The SEM micrographs revealed that the prepared samples were composed of nanocrystalline grains with particle sizes ranging from 25 to 40 nm. The electronic structural modification has been confirmed by the XAS analysis of Fe L3,2 and O K edge spectra as a result of La doping. Electrochemical measurements showed that 5% La-doped BFO (BF5) as an electrode demonstrated excellent performance for supercapacitors with a specific capacitance of 328 F g−1 measured with a scan rate of 10 mV s−1. It also exhibited tremendous cyclic stability with capacitance retention of >97 % for 1000 cycles measured at 1A g−1.

Spectral properties of periodic systems cut at an angle

We consider a semi-periodic two-dimensional Schr\"odinger operator which is cut at an angle. When the cut is commensurate with the periodic lattice, the spectrum of the operator has the band-gap Bloch structure. We prove that in the incommensurable case, there are no gaps: the gaps of the bulk operator are filled with edge spectrum.

Correlating X-ray absorption spectra and ultraviolet photoelectron spectra to understand magnetic and transport properties of charge-ordered perovskite manganites

X-ray absorption spectra and ultraviolet photoelectron spectra have been used to compare the electronic structures of (La0.6Pr0.4)0.65Ca0.35MnO3 and (La0.4Pr0.6)0.65Ca0.35MnO3 systems to understand their magnetic and transport properties. Structural analysis showed that increased Pr-substitution triggers expansion in apical Mn-O bond lengths, which eventually causes narrowing of the eg band width. (La0.6Pr0.4)0.65Ca0.35MnO3 exhibits ferromagnetic ordering below 215 K and shows semiconductor to metal transition around 212 K, while (La0.4Pr0.6)0.65Ca0.35MnO3 exhibit complex magnetic behaviour and also show local semiconductor to metal transition around 95 K before re-entering into semiconducting region again below 80 K. A slight change in the slope of resistivity around 220 K and four orders increase in resistivity indicates presence of charge-ordering in (La0.4Pr0.6)0.65Ca0.35MnO3. Moreover, (La0.4Pr0.6)0.65Ca0.35MnO3 exhibits ~ 100% magneto-resistance. X-ray absorption studies of Mn-L2,3 edge and O-K edge reveal the onset of Mn3+-Mn4+ charge-ordering in (La0.4Pr0.6)0.65Ca0.35MnO3 at room temperature. O-K edge spectra exhibits an increased spectral intensity of O2p-Mn3d(eg) states near Fermi-level for (La0.4Pr0.6)0.65Ca0.35MnO3, which reflects the number of unoccupied eg states. Valence band ultraviolet photoemission spectroscopy study revealed that Mn3d(eg) states of (La0.6Pr0.4)0.65Ca0.35MnO3 extends into conduction band by crossing-over the Fermi-level. On the other hand, (La0.4Pr0.6)0.65Ca0.35MnO3 exhibits no electronic state at Fermi-level explaining stronger semiconducting nature of x = 0.6 system than x = 0.4 near room temperature.

Absolutely Continuous Edge Spectrum of Hall Insulators on the Lattice

The presence of chiral modes on the edges of quantum Hall samples is essential to our understanding of the quantum Hall effect. In particular, these edge modes should support ballistic transport and therefore, in a single particle picture, be supported in the absolutely continuous spectrum of the single-particle Hamiltonian. We show in this note that if a free fermion system on the two-dimensional lattice is gapped in the bulk, and has a nonvanishing Hall conductance, then the same system put on a half-space geometry supports edge modes whose spectrum fills the entire bulk gap and is absolutely continuous.

The Study of Magnetic Properties for Non-Magnetic Ions Doped BiFeO3.

BiFeO3 is considered as a single phase multiferroic. However, its magnetism is very weak. We study the magnetic properties of BiFeO3 by Cu and (Cu, Zn). Polycrystalline samples Bi(Fe0.95Cu0.05)O3 and BiFe0.95(Zn0.025Cu0.025)O3 are prepared by the sol-gel method. The magnetic properties of BiFe0.95(Zn0.025Cu0.025)O3 are greater than that of BiFeO3 and Bi(Fe0.95Cu0.05)O3. The analyses of X-ray absorption fine structure data show that the doped Cu atoms well occupy the sites of the Fe atoms. X-ray absorption near edge spectra data confirm that the valence state of Fe ions does not change. Cu and Zn metal ion co-doping has no impact on the local structure of the Fe and Bi atoms. The modification of magnetism by doping Zn can be understood by the view of the occupation site of non-magnetically active Zn2+.

Thermoelectric Hall conductivity of fractional quantum Hall systems on a disk

For the fractional quantum Hall states on a finite disc, we study the thermoelectric transport properties under the influence of an edge and its reconstruction. In a recent study on a torus [Phys. Rev. B 101, 241101 (2020)], Sheng and Fu found a universal non-Fermi liquid power-law scaling of the thermoelectric conductivity $\alpha_{xy} \propto T^{\eta}$ for the gapless composite Fermi-liquid state. The exponent $\eta \sim 0.5$ appears an independence of the filling factors and the details of the interactions. In the presence of an edge, we find the properties of the edge spectrum dominants the low-temperature behaviors and breaks the universal scaling law of the thermoelectric conductivity. In order to consider individually the effects of the edge states, the entanglement spectrum in real space is employed and tuned by varying the area of subsystem. In non-Abelian Moore-Read state, the Majorana neutral edge mode is found to have more significant effect than that of the charge mode in the low temperature.

Core electron spectroscopic studies for 11 iron-based superconductors

Using density functional theory (DFT)-based first principal calculations we have investigated the electron energy loss near edge spectra (ELNES) of two different iron-based superconducting material from 11 family (FeSe and FeTe). The origin of spectral features in Fe K-edge absorption spectra are thoroughly described in light of Fe/Se/Te atom projected partial density of states (PDOS). When include the “core–hole effect”, we found drastic change in absorption spectra. Our theoretically calculated results including core–hole effect agrees well with experimentally observed X-ray Absorption Near Edge Structure (XANES) data available in literature.

Early Identification of Root Rot Disease by Using Hyperspectral Reflectance: The Case of Pathosystem Grapevine/Armillaria

Armillaria genus represents one of the most common causes of chronic root rot disease in woody plants. Prompt recognition of diseased plants is crucial to control the pathogen. However, the current disease detection methods are limited at a field scale. Therefore, an alternative approach is needed. In this study, we investigated the potential of hyperspectral techniques to identify fungi-infected vs. healthy plants of Vitis vinifera. We used the hyperspectral imaging sensor Specim-IQ to acquire leaves’ reflectance data of the Teroldego Rotaliano grapevine cultivar. We analyzed three different groups of plants: healthy, asymptomatic, and diseased. Highly significant differences were found in the near-infrared (NIR) spectral region with a decreasing pattern from healthy to diseased plants attributable to the leaf mesophyll changes. Asymptomatic plants emerged from the other groups due to a lower reflectance in the red edge spectrum (around 705 nm), ascribable to an accumulation of secondary metabolites involved in plant defense strategies. Further significant differences were observed in the wavelengths close to 550 nm in diseased vs. asymptomatic plants. We evaluated several machine learning paradigms to differentiate the plant groups. The Naïve Bayes (NB) algorithm, combined with the most discriminant variables among vegetation indices and spectral narrow bands, provided the best results with an overall accuracy of 90% and 75% in healthy vs. diseased and healthy vs. asymptomatic plants, respectively. To our knowledge, this study represents the first report on the possibility of using hyperspectral data for root rot disease diagnosis in woody plants. Although further validation studies are required, it appears that the spectral reflectance technique, possibly implemented on unmanned aerial vehicles (UAVs), could be a promising tool for a cost-effective, non-invasive method of Armillaria disease diagnosis and mapping in-field, contributing to a significant step forward in precision viticulture.

Local electronic and atomic structures of the mixed B-site ions in SrFe1−xMnxO3−δ studied with X-ray absorption spectroscopy

Local electronic and atomic structures of the B-site ions in perovskite SrFe[Formula: see text]Mn[Formula: see text]O[Formula: see text] were evaluated with X-ray absorption spectroscopy (XAS) at Mn, Fe [Formula: see text]- and [Formula: see text]-edges and O [Formula: see text]-edge. The energy of [Formula: see text]- and [Formula: see text]-edge peaks for Mn and Fe ions stayed unchanged against the change in Mn content [Formula: see text]. The analysis of Mn [Formula: see text]-edge extended X-ray absorption fine structure (EXAFS) revealed that Mn−O polyhedra was kept as an octahedron with Mn[Formula: see text] irrespective of the [Formula: see text] value, agreeing with the results of [Formula: see text]-edge spectra. The Fe [Formula: see text]-edge EXAFS analysis showed that the Fe–O distance decreased with decrease of the Mn contents, suggesting that the ratio of FeO5 polyhedra to FeO6 octahedra is increased due to smaller oxygen amount. By analyzing both the [Formula: see text]-edge and [Formula: see text]-edge spectra, we clarified the local electronic and atomic structural changes particularly occurred in the B-site mixed perovskite oxides.

Tuning the Electrical Properties of NiO Thin Films by Stoichiometry and Microstructure

Here, the electrical properties of NiO thin films grown on glass and Al2O3 (0001) substrates have been investigated. It was found that the resistivity of NiO thin films strongly depends on oxygen stoichiometry. Nearly perfect stoichiometry yields extremely high resistivity. In contrast, off-stoichiometric thin films possess much lower resistivity, especially for oxygen-rich composition. A side-by-side comparison of energy loss near the edge structure spectra of Ni L3 edges between our NiO thin films and other theoretical spectra rules out the existence of Ni3+ in NiO thin films, which contradicts the traditional hypothesis. In addition, epitaxial NiO thin films grown on Al2O3 (0001) single crystal substrates exhibit much higher resistivity than those on glass substrates, even if they are deposited simultaneously. This feature indicates the microstructure dependence of electrical properties.

Electronic structure and K-edge X-ray absorption of iron monosilicide

In this paper, we present theoretical modelling of the band structure of iron monosilicide FeSi. Special attention is paid to the total and partial densities of electronic states along with K-edge X-ray absorption near edge spectra of iron. A good agreement was found between known experimental data and our theoretical calculations.

The Charge State of Pt in Binary Compounds and Synthetic Minerals Determined by X-ray Absorption Spectroscopy and Quantum Chemical Calculations

The binary synthetic compounds of Pt with chalcogens (O, S, Se, Te), pnictogens (As, Sb, Bi), and intermetallic compounds with Ga, In, and Sn of various stoichiometry were studied via X-ray absorption spectroscopy (XAS). The partial atomic charges of Pt in the compounds were computed using quantum chemical density functional theory (DFT) based methods: the Bader (QTAIM) method, and the density-derived electrostatic and chemical (DDEC6) approach. Strong positive correlations were established between the calculated partial atomic charges of Pt and the electronegativity (χ) of ligands. The partial charge of Pt in PtL2 compounds increases much sharply when the ligand electronegativity increases than the Pt partial charge in PtL compounds. The effect of the ligand-to-Pt atomic ratio on the calculated Pt partial charge depended on ligand electronegativity. The DDEC6 charge of Pt increases sharply with the growth of the number of ligands in PtSn (n = 1, 2; electronegativity χ(S) &gt;&gt; χ(Pt)), weakly depends on the phase composition in PtTen (n = 1, 2; χ(Te) is slightly lower than χ(Pt)), and decreases (becomes more negative) with increase of the ligand-to-Pt ratio in intermetallic compounds with electron donors (χ(L) &lt; χ(Pt), L = Ga, In, Sn). According to XANES spectroscopy, the number of 5d (L2,3 absorption edges) and 6p (L1-edge) electrons at the Pt site decreased when ligand electronegativity increased in chalcogenides and pnictides groups. An increase of the ligand-to-Pt ratio resulted in the increase of the Pt L3-edge white line intensity and area in all studied compounds. In the case of chalcogenides and pnictides, this behavior was consistent with the electronegativity rule as it indicated a loss of Pt 5d electrons caused by the increase of the number of ligands, i.e., acceptors of electrons. However, in the case of ligands–electron donors (Te, Sn, Ga, In) this observation is in apparent contradiction with the electronegativity arguments as it indicates the increase of the number of Pt 5d-shell vacancies (holes) with the increase of the number of the ligands, for which the opposite trend is expected. This behavior can be explained in the framework of the charge compensation model. The loss of the Pt d-electrons in compounds with low ligand electronegativity (χ(Pt) &gt; χ(L)) was overcompensated by the gain of the hybridized s-p electron density, which was confirmed by Pt L1 - edge spectra analysis. As a result, the total electron density at the Pt site followed the electronegativity rule, i.e., it increased with the growth of the number of the ligands-electron donors. The empirical correlations between the Pt partial atomic charges and parameters of XANES spectral features were used to identify the state of Pt in pyrite, and can be applied to determine the state of Pt in other ore minerals.

Origin of intense blue-green emission in SrTiO3 thin films with implanted nitrogen ions: An investigation by synchrotron-based experimental techniques

The present study utilizes synchrotron-based x-ray diffraction (XRD), photoluminescence (PL), and x-ray absorption near edge structure (XANES) spectroscopic techniques to comprehend the evolution of optical intense blue-green emission in 100 keV nitrogen (N) ion implanted $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ (STO) thin films deposited by RF magnetron sputtering technique. The XRD pattern shows a shift in reflections at lower N ion fluences and the amorphization of the films at higher fluences. A disordered phase induced by implantation in the STO films leads to an intense blue-green emission due to oxygen (O) vacancies and N ($2p$) bound states. A schematic diagram of energy levels has been proposed to explain the origin of PL emission. The XANES spectra at Ti $K$ edge reflect a change in the valency of Ti ions and the local atomic structure of ordered and disordered phases of STO with an increase in N ion fluence. The splitting of peak assigned to ${e}_{g}$ orbitals, and discrepancy in ratio ${d}_{{z}^{2}}/{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ observed in the Ti $L$- and O $K$-edge spectra, confirm a distortion in $\mathrm{Ti}{\mathrm{O}}_{6}$ octahedral structure and modifications in O $2p$-Ti $3d$ hybridization states. The synchrotron-based techniques reveal that N ion implanted STO can be a good photoluminescent material exhibiting a variety of emissions through bound states of O vacancies and implanted N ions.

4D Imaging of ZnO-Coated Nanoporous Al2O3 Aerogels by Chemically Sensitive Ptychographic Tomography: Implications for Designer Catalysts

The 3D chemical structure (4D spectromicroscopy) of nanoporous Al2O3 aerogels coated with ZnO by atomic layer deposition (ALD) was studied by multienergy scanning transmission X-ray microscopy. These materials are representative of a class of designer catalysts in which the nanoporous support is prepared separately from the active catalyst material, which is subsequently introduced by ALD, thereby allowing independent optimization of the morphology, chemistry, and spatial distribution of the support and catalyst. The samples studied were prepared by Ga ion and Xe plasma focused ion beam (FIB) milling as well as drop casting from water suspension. Zn L and Al K edge spectra of six samples with three different ZnO loadings were measured to investigate how loading and different sample preparation methods affect the 3D distribution of the ZnO and Al2O3. Scanning transmission X-ray microscopy (STXM) and ptychographic imaging at two energies each at the Zn L3 and Al K edge were measured. The ptychography data were analyzed by using the SHARP reconstruction code to generate quantitative 2D chemical maps of the Al2O3 and the ZnO. The STXM and ptychography maps were then measured at a sequence of tilt angles, covering up to 160° of rotation. The 3D structure of the ZnO and Al2O3 was derived from the tilt series data by tomographic reconstruction using a compressed sensing algorithm. A two-dimensional spatial resolution (half-period) of 6 nm, measured by Fourier ring correlation, and a 3D spatial resolution (half-period) of 9 nm, measured by Fourier shell correlation, were achieved when using the COSMIC beamline at the Advanced Light Source (ALS). The results show that for all of the ZnO loadings studied there is nonuniform coverage of the ZnO on the Al2O3 aerogel framework. In addition, we found that both FIB methods create sample artifacts, although the distortion was less with Xe plasma than Ga ion FIB.