Electronic Magnetic Circular Dichroism Research Articles

Electronic structure and x-ray magnetic circular dichroism in cr-doped topological insulator bi2se3

We have studied the structural, electronic, and magnetic properties of the Cr-doped topological insulator Bi2Se3 within the density functional theory using the generalized gradient approximation in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The X-ray absorption spectra and X-ray magnetic circular dichroism at the Cr K and L2,3 edges have been investigated theoretically from the first principles. The calculated results are in good agreement with the experimental data. The complex fine structure of the Cr L2,3 X-ray absorption spectra in Cr-doped Bi2Se3 has been found to be not compatible with a Cr2+ valency state. Its interpretation demands a mixed valent state.

Electronic structure and X-ray magnetic circular dichroism in the MAX phases T2AlC (T = Ti and Cr) from first principles

We study the electronic and magnetic properties of T2AlC (T = Ti and Cr) compounds in the density-functional theory using the generalized gradient approximation (GGA) with consideration of strong Coulomb correlations (GGA+U) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital (LMTO) band-structure method. The X-ray absorption spectra and X-ray magnetic circular dichroism (XMCD) at the Cr L2,3 and Cr, Ti, and C K edges were investigated theoretically. The calculated results are in good agreement with experimental data. The effect of the electric quadrupole E2 and magnetic dipole M1 transitions at the Cr K edge has been investigated.

Induced circular dichroism from helicoidal nano substrates to porphyrins: the role of chiral self-assembly.

Because the combination of chiral and magnetic properties is becoming more and more attractive for magneto-chiral phenomena, we here aim at exploring the induction of chirality to achiral magnetic molecules as a strategy for the preparation of magneto-chiral objects. To this end, we have associated free base- and metallo-porphyrins with silica nano helices, using a variety of elaboration methods, and have studied them mainly by electronic natural circular dichroism (NCD) and magnetic circular dichroism (MCD) spectroscopies. While electrostatic or covalent surface grafting uniformly yielded very low induced CD (ICD) for the four assayed porphyrins, a moderate response was observed when the porphyrins were incorporated into the interior of the double-walled helices, likely due to the association of the molecules with the chirally-organized gemini surfactant. A generally stronger, but more variable, ICD was observed when the molecules were drop casted onto the helices immobilised on a quartz plate, likely due to the different capacities of the porphyrins to aggregate into chiral assemblies. Electronic spectroscopy, electron microscopy and IR spectroscopy were used to interpret the patterns of aggregation and their influence on ICD and MCD. No enhancement of MCD was observed as a result of association with the nanohelices except in the case of the free base, 5,10,15,20-tetra-(4-sulfonatophenyl)porphyrin (TPPS). This nanocomposite demonstrated a large ICD in the Soret region and a large MCD in the Q-region due to J-aggregation. However, no induced MChD was observed, possibly due to the spectral mismatch between the ICD and MCD peaks.

Fluorinated Porphycenes: Synthesis, Spectroscopy, Photophysics, and Tautomerism.

Six porphycenes have been synthesized, bearing one, two, or three fluorine atoms attached directly to the 18-π-electron system at the meso positions. These novel compounds have been characterized by structural, electrochemical, and spectral techniques, combined with quantum chemical calculations. In three fluoroporphycenes, the unsymmetric substitution pattern leads to the presence of two nonequivalent trans tautomeric forms. They have been identified using electronic absorption, emission, and magnetic circular dichroism spectroscopies. Their relative energies have been estimated for the ground and lowest excited electronic states. Tautomerization potential is quasi-symmetric in S0 , but becomes strongly nonsymmetric in S1 . Femtosecond transient absorption studies allowed determination of tautomerization rates, larger and similar for both directions of the double hydrogen transfer in S0 , lower and disparate in S1 . Fluoroporphycenes emerge as good candidates for detailed studies of mechanisms of double hydrogen transfer, as well as processes responsible for rapid radiationless excited state depopulation.

Triangular Expanded Hemiporphyrazines: Electronic Structures and Nanoscale Characterization of Their Adlayers on Au(111)

Abstract Electronic structures and direct observation of adlayers on Au(111) of triangular expanded hemiporphyrazines that can be classified into azaporphyrinoids and which can trap three transition metal ions are reported. The electronic structure was examined by electronic absorption and magnetic circular dichroism (MCD) spectroscopies and interpreted in conjunction with molecular orbital (MO) calculations. Both metal-free and metallated compounds were 30 π-electron systems with n = 7 in 4n + 2 aromaticity. In addition, calculations of the anisotropy of the induced current density (ACID) and nucleus-independent chemical shift (NICS) were performed. The inner core region of the metal-free compound was weakly anti-aromatic or nonaromatic, but after metal insertion, this region increased aromatic character. The direct observation of adlayers of one of their cobalt and nickel complexes on Au(111) by scanning tunneling microscopy (STM) revealed that these three ions are arranged specifically in the shape of a triangle.

First principles calculations of intersite disorder influence on the electronic structure and X-ray magnetic circular dichroism in CoFeMnSi Heusler alloy

Electronic structure, X-ray absorption, and magnetic circular dichroism (XMCD) spectra in the CoFeMnSi Heusler alloy were studied from first principles. Fully relativistic Dirac linear muffin-tin orbital band structure method was implemented with various exchange–correlation functionals tested. The supercell approach was used to study the influence of intersite disorder, at the levels of 6.25%, 12.5%, and 25% within transition metal sites, on the XMCD spectra at [Formula: see text] edges and spin polarization (SP) at the Fermi level. It is found that most sensitive to Fe–Mn and Co–Fe disorder are XMCD spectra at [Formula: see text] edges of Fe, while the sensitivity decreases from Mn to Co. It is shown that magnetic moments estimated with the use of magneto-optical (MO) sum rules agree with the ab initio calculated ones to within [Formula: see text], [Formula: see text], and [Formula: see text], for Co, Fe, and Mn, respectively. The calculated SP decreases from 99% for ordered CoFeMnSi alloy, to 96% upon 25% Co–Fe disorder, to 83% for Fe–Mn disorder, and to 42% in the case of Co–Mn disorder. The calculated spectra agree well with the available experimental data. The rich XMCD spectral structures are predicted from first principles at Fe, Co, Mn and Si [Formula: see text] edges.

Synthesis and electronic properties of meso-tetra(3,4,5-trimethoxy)phenyl-porphyrin and -tetrabenzoporphyrin and their phosphorus complexes

meso-Tetra(3,4,5-trimethoxy)phenyl-porphyrin and -tetrabenzoporphyrin and their phosphorus complexes were synthesized and their electronic absorption and magnetic circular dichroism (MCD) spectra discussed based on TD-DFT molecular orbital (MO) calculations. Nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations were also carried out for metal-free (and phosphorus) tetrabenzoporphyrins in order to discuss the aromaticity and flow of electrons in the molecule.

Electronic structure, Fermi surface and x-ray magnetic circular dichroism in the CeAgSb2

The electronic structure, Fermi surface, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface as well as x-ray magnetic circular dichroism (XMCD) in the CeAgSb2 compound were investigated from first principles using the fully relativistic Dirac linear muffin-tin orbital method. In our calculations Ce 4f states have been considered as: 1) itinerant using the generalized gradient approximation (GGA), 2) fully localized, treating them as core states, and 3) partly localized using the GGA + U approximation. The effect of the spin-orbit (SO) interaction and Coulomb repulsion U in a frame of the GGA + U method on the Fermi surface, orbital dependence of the cyclotron masses, and extremal cross sections of the Fermi surface are examined in details. We show that the conventional GGA band calculations fail to describe the Fermi surface of the CeAgSb2 due to wrong position of Ce 4f states (too close to the EF). On the other hand, fully localized (4f states in core) and the GGA + U approach produce similar Fermi surfaces and dHvA frequencies in the CeAgSb2. A good agreement with the experimental data of XMCD spectra at the Ce M4.5 edges was achieved using the GGA + U approximation. The origin of the XMCD spectra in the compound is examined. The core hole effect in the final states has been investigated using a supercell approximation. It improves the agreement be-tween the theory and the experiment of the XAS and the XMCD spectra at the Ce M4.5 edges.

Electronic structure and x-ray magnetic circular dichroism in the multiferroic oxide h−YbFeO3

Hexagonal ferrites ($h\text{\ensuremath{-}}R{\mathrm{FeO}}_{3}$, $R$ = Sc, Y, Ho-Lu) have recently been identified as a new family of multiferroic complex oxides. We study electronic and magnetic properties of $h\text{\ensuremath{-}}{\mathrm{YbFeO}}_{3}$ ferrite within the density-functional theory using the generalized gradient approximation (GGA) with consideration of strong Coulomb correlations $(\mathrm{GGA}+U)$ in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The $4f$ electrons of Yb are explicitly treated as valence electrons. The x-ray absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Yb ${M}_{4,5}$, Fe ${L}_{2,3}$, and O $K$ edges were investigated theoretically. The calculated results are in good agreement with experimental data. We found that the $\mathrm{GGA}+U$ approach with Hubbard ${U}_{\mathrm{eff}}=6.1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ and 3.3 eV for Yb and Fe, respectively, well describes the XMCD spectra at the Yb ${M}_{4,5}$, Fe ${L}_{2,3}$, and O $K$ edges.

An Estimation Method of Metal‐Ligand Orbital Mixing in Lanthanide(III) Complexes Using Magnetic Circular Dichroism

AbstractThe photophysical properties of lanthanide(III) complexes depend on the magnitude of orbital mixing between the lanthanide(III) and ligands. Herein, an estimation method of orbital mixing in lanthanide(III) complexes is reported using LnCl3⋅nH2O (Ln=Pr, Gd, Tb) and Ln(tfa)3(H2O)2 (tfa: trifluoroacetylacetonate, Ln=Pr, Gd, and Tb) for the first time. The electronic state properties were assessed from the electronic absorption and magnetic circular dichroism (MCD) spectra. The MCD spectra revealed the energy levels of the spin‐allowed and spin‐forbidden 4 f–5d transitions of the Tb(III) complex that is hidden by the intense π–π* transition of the ligands. This energy gap corresponds to the exchange splitting energy between the 5d and 4fn−1 electrons, which provides information regarding the orbital mixing between the Tb(III) ion and the ligands.

Electronic Structure and X-Ray Magnetic Circular Dichroism in Sm-Doped Bi2Se3

Electronic Structure and X-Ray Magnetic Circular Dichroism in Sm-Doped Bi₂Se₃

Electronic structure and x-ray magnetic circular dichroism in Mn-doped topological insulators Bi2Se3 and Bi2Te3

We studied the structural, electronic, and magnetic properties of Mn-doped topological insulators ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ within the density-functional theory (DFT) using the generalized gradient approximation (GGA) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism at the Mn $K$ and ${L}_{2,3}$ edges were investigated theoretically from first principles. The calculated results are in good agreement with experimental data. The complex fine structure of the Mn ${L}_{2,3}$ XAS in Mn-doped ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ was found to be not compatible with a pure ${\mathrm{Mn}}^{3+}$ valency state. Its interpretation demands mixed valent states. The theoretically calculated x-ray emission spectra at the Mn $K$ and ${L}_{2,3}$ edges are also presented and analyzed.

The influence of the Cys46/Cys55 disulfide bond on the redox and spectroscopic properties of human neuroglobin

Neuroglobin is a monomeric globin containing a six-coordinate heme b, expressed in the nervous system, which exerts an important neuroprotective role. In the human protein (hNgb), Cys46 and Cys55 form an intramolecular disulfide bond under oxidizing conditions, whose cleavage induces a helix-to-strand rearrangement of the CD loop that strengthens the bond between the heme iron and the distal histidine. Hence, it is conceivable that the intramolecular disulfide bridge modulates the functionality of human neuroglobin by controlling exogenous ligand binding. In this work, we investigated the influence of the Cys46/Cys55 disulfide bond on the redox properties and on the pH-dependent conformational equilibria of hNgb, using UV–vis spectroelectrochemistry, cyclic voltammetry, electronic absorption spectroscopy and magnetic circular dichroism (MCD). We found that the SS bridge significantly affects the heme Fe(III) to Fe(II) reduction enthalpy (ΔH°′rc) and entropy (ΔS°′rc), mostly as a consequence of changes in the reduction-induced solvent reorganization effects, without affecting the axial ligand-binding interactions and the polarity and electrostatics of the heme environment. Between pH3 and 12, the electronic properties of the heme of ferric hNgb are sensitive to five acid-base equilibria, which are scarcely affected by the Cys46/Cys55 disulfide bridge. The equilibria occurring at extreme pH values induce heme release, while those occurring between pH5 and 10 alter the electronic properties of the heme without modifying its axial coordination and low spin state. They involve the sidechains of non-coordinating aminoacids close to the heme and at least one heme propionate.

Electronic structure and x-ray magnetic circular dichroism in A2CrB′O6 (A = Ca, Sr; B′ = W, Re, and Os) oxides

A systematic electronic structure study of A2CrB′O6 (A = Ca, Sr; B′ = W, Re, and Os) has been performed by employing the local spin-density approximation (LSDA) as well as the GGA and LSDA + U methods using the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. We investigated the effects of the subtle interplay among the spin-orbit coupling, electron correlations, and lattice distortion on the electronic structure of the double perovskites. First principles calculations predict that Sr2CrOsO6 is (before considering spin-orbit coupling) actually a ferrimagnetic semimetal with precisely compensating spin moments, or spin-asymmetric compensated semimetallic ferrimagnet in which the electrons and holes are each fully polarized and have opposite spin directions, in spite of a zero net moment and hence no macroscopic magnetic field. Spin-orbit coupling degrades this by giving a nonzero total moment, but the band structure is little changed. Therefore, the observed saturation moment of ferrimagnetic Sr2CrOsO6 is entirely due to spin-orbit coupling. The x-ray absorption spectra and x-ray magnetic circular dichroism at the W, Re, Os, and Cr L2,3, and Cr and O K edges were investigated theoretically from first principles. A qualitative explanation of the XMCD spectra shape is provided by the analysis of the corresponding selection rules, orbital character and occupation numbers of individual orbitals. The calculated results are in good agreement with experimental data. The complex fine structure of the Cr L2,3 XAS in Sr2CrWO6 and Sr2CrReO6 was found to be not compatible with a pure Cr3+ valency state. The interpretation demands mixed valent states. We found that possible oxygen vacancies lead to a mixed valency at the Cr site, double peak structure at the Cr L2,3 edges and reduce the saturation magnetization in Sr2CrWO6 and Sr2CrReO6.

Electronic and magnetic properties of the ferromagnetic superconductor UCoGe

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the ferromagnetic superconductor UCoGe at the U N4,5, Ge and Co K and Co L2,3 edges were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with a generalization of the LSDA+U method which takes into account the non-diagonal occupation matrix (in spin indexes) of localized electrons. A stable ferromagnetic ground state was found. The uranium total magnetic moment is quite small (about −0.171μB) in the LSDA approximation as a result of almost complete cancellation between the spin magnetic moment of 0.657 μB and the opposite orbital magnetic moment of −0.828 μB, resulting from strong spin-orbit coupling at the uranium site. Valency of U ion in UCoGe is close to 3+. The ratio orbital and spin magnetic moments Ml/Ms ranged from 1.163 in the GGA approa...

Flexible Metal-Porphyrin Dimers (M=MnIII Cl, CoII , NiII , CuII ): Synthesis, Spectroscopy, Electrochemistry, Spectroelectrochemistry, and Theoretical Calculations.

Four metalloporphyrin dimers linked by bridging amide-bonded xanthene moieties and that contain either MnIII , CoII , NiII , or CuII metal centers were synthesized. Various spectroscopic, electrochemical, and spectroelectrochemical methods were used to study trends in their properties. Their electronic structure and optical properties were analyzed through a comparison of the electronic absorption and magnetic circular dichroism (MCD) spectral data with the results of time-dependent (TD)-DFT calculations.

Electronic structure and X-ray magnetic circular dichroism in the Ni-Mn-Ga Heusler alloys

The electronic structure and X-ray magnetic circular dichroism (XMCD) spectra of the Ni2MnGa and Cu doped Ni2MnGa Heusler alloys were investigated theoretically from first principles, using the fully relativistic Dirac linear MT-orbital (LMTO) band structure method. Densities of valence states, orbital and spin magnetic moments are analyzed and discussed. The electronic and magnetic structure of Ni-Mn-Ga Heusler alloys were investigated for the cubic austenitic and modulated 7M-like incommensurate martensitic phases. The X-ray absorption spectra (XAS) and XMCD at the Mn, Ni, Ga, and Cu L2,3, and Mn, Ni, and Ga K edges were investigated theoretically from first principles. The origin of the XMCD spectra in the Ni2MnGa compound is examined. The ab initio calculations reproduce well experimental XAS and XMCD spectra. The XAS at the Mn L2,3 edges remains mostly unchanged through martensitic phase transition. A fingerprint of the martensitic phase transition has been found in the Ni L2,3 XAS spectra. The experimental Ni L3 XAS has a pronounced shoulder at the L3 peak at around 853 eV. This peak is nearly suppressed in the martensitic state in comparison with the austenitic phase due to the lifting of the degeneracy in the Ni 3d related unoccupied electronic states. The XMCD of the martensitic phase is increased compared to the XMCD of the austenite phase at the Ni and Mn L2,3 edges.

Electronic Spectra of the Tetraphenylcyclobutadienecyclopentadienylnickel(II) Cation and Radical

Properties of the tetraphenylcyclobutadienecyclopentadienylnickel(II) cation 1 and its tetra-o-fluoro derivative 1a have been measured and calculated. The B3LYP/TZP optimized geometry of the free cation 1 agrees with a single-crystal X-ray diffraction structure except that in the crystal one of the phenyl substituents is strongly twisted to permit a close-packing interaction of two of its hydrogens with a nearby BF4(-) anion. The low-energy parts of the solution electronic absorption and magnetic circular dichroism (MCD) spectra of 1 and 1a have been interpreted by comparison with TD-DFT (B3LYP/TZP) results. Reduction or pulse radiolysis lead to a neutral 19-electron radical, whose visible absorption and MCD spectra have been recorded and interpreted as well. The reduction is facilitated by ∼0.1 V upon going from 1 to 1a. Unsuccessful attempts to prepare several other aryl substituted derivatives of 1 by the classical synthetic route are described in the Supporting Information .

Electronic Structure of a Cu(II)-Alkoxide Complex Modeling Intermediates in Copper-Catalyzed Alcohol Oxidations.

In the copper-catalyzed oxidation of alcohols to aldehydes, a Cu(II)-alkoxide (Cu(II)-OR) intermediate is believed to modulate the αC-H bond strength of the deprotonated substrate to facilitate the oxidation. As a structural model for these intermediates, we characterized the electronic structure of the stable compound Tp(tBu)Cu(II)(OCH2CF3) (Tp(tBu) = hydro-tris(3-tert-butyl-pyrazolyl)borate) and investigated the influence of the trifluoroethoxide ligand on the electronic structure of the complex. The compound exhibits an electron paramagnetic resonance (EPR) spectrum with an unusually large gzz value of 2.44 and a small copper hyperfine coupling Azz of 40 × 10(-4) cm(-1) (120 MHz). Single-crystal electron nuclear double resonance (ENDOR) spectra show that the unpaired spin population is highly localized on the copper ion (≈68%), with no more than 15% on the ethoxide oxygen. Electronic absorption and magnetic circular dichroism (MCD) spectra show weak ligand-field transitions between 5000 and 12,000 cm(-1) and an intense ethoxide-to-copper charge transfer (LMCT) transition at 24,000 cm(-1), resulting in the red color of this complex. Resonance Raman (rR) spectroscopy reveals a Cu-O stretch mode at 592 cm(-1). Quantum chemical calculations support the interpretation and assignment of the experimental data. Compared to known Cu(II)-thiolate and Cu(II)-alkylperoxo complexes from the literature, we found an increased σ interaction in the Cu(II)-OR bond that results in the spectroscopic features. These insights lay the basis for further elucidating the mechanism of copper-catalyzed alcohol oxidations.

Electronic structure and x-ray magnetic circular dichroism of Mn-doped TiO2

The electronic structure of (Ti,Mn)O2 diluted magnetic semiconductors was investigated theoretically from first principles using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure was obtained with the local spin-density approximation taking into account strong Coulomb correlations in the frame of the LSDA + U approximation. The x-ray absorption spectra and x-ray magnetic circular dichroism spectra at the Mn and Ti L2,3 and O K edges were investigated theoretically from first principles. The origin of the XMCD spectra in these compounds was examined. The calculated results are compared with available experimental data.