Polarization-dependent X-ray Absorption Spectroscopy Research Articles

Unraveling dynamic Jahn-Teller effect and magnetism in FeTiF6×6H2O single crystal

Hydrated iron fluoridotitanate (FeTiF6 × 6 H2O) single crystals are fascinating magnetic materials with unique properties. To understand the underlying mechanisms, this study combines X-ray absorption near-edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) techniques, complemented by density functional theory (DFT) calculations. Polarization-dependent X-ray absorption spectroscopy, encompassing XANES and XMCD, is a powerful technique for probing the local structures and magnetic properties of materials. It is element-selective, bulk-sensitive, and compatible with a wide range of experimental conditions. In this study, we used XANES and XMCD spectroscopies to investigate the local structures and magnetic properties of Fe and Ti in FeTiF6 × 6 H2O single crystals. XANES analysis revealed distinct local environments around Fe and Ti, providing insights into their coordination environments. Element-selective magnetization measurement at the Fe K-edge demonstrated that iron sites in the oxidation state Fe2+ have an unambiguous paramagnetic contribution to the magnetization along the b-axis. Notably, the absence of an XMCD signal at the Ti K-edge confirmed the absence of a magnetic moment in Ti atoms within the crystal. DFT calculations corroborate the experimental findings and provide insights into the electronic structure and magnetic interactions. The combined results provide a comprehensive understanding of the dynamic Jahn-Teller effect in FeTiF6 × 6 H2O single crystals, highlighting the significance of polarization-dependent X-ray absorption spectroscopy in unraveling the intricate magnetic behavior of such materials. This study contributes to the fundamental understanding of magnetism in these materials and paves the way for the development of novel magnetic materials with tailored properties.

Effect of annealing on the molecular ordering and crystal structure of iron phthalocyanine thin films on Si (111) surface

The Effect of annealing on molecular ordering in Iron Phthalocyanine (FePc) thin films deposited on Si (111) substrate surface was investigated using lab source (Cu Kα) based x-ray diffraction, synchrotron based 2-dimensional grazing incidence x-ray diffraction, polarization dependent x-ray absorption spectroscopy and hard x-ray photoelectron spectroscopy. Room temperature deposition of FePc on Si substrate resulted in amorphous thin films with randomly oriented FePc molecules. However, annealing of these amorphous films at 200°C led to crystallization with highly ordered FePc molecules having monoclinic crystal lattice structure. The ordered FePc molecules followed edge-on stacking on Si substrate surface with an azimuthally averaged tilt angle of 75±5° between molecular plane and substrate surface. Also, the b-c plane of the unit cell was lying almost parallel to the substrate surface. The edge-on stacking ordering of FePc molecules in annealed thin films led to higher charge conduction as compared to as-deposited films.

Anisotropic hybridization probed by polarization dependent x-ray absorption spectroscopy in VI3 van der Waals Mott ferromagnet

Polarization dependent x-ray absorption spectroscopy was used to study the magnetic ground state and the orbital occupation in bulk-phase VI3 van der Waals crystals below and above the ferromagnetic and structural transitions. X-ray natural linear dichroism and x-ray magnetic circular dichroism spectra acquired at the V edges are compared against multiplet cluster calculations within the frame of the ligand field theory to quantify the intra-atomic electronic interactions at play and evaluate the effects of symmetry reduction occurring in a trigonally distorted VI6 unit. We observed a non zero linear dichroism proving the presence of an anisotropic charge density distribution around the V3+ ion due to the unbalanced hybridization between the vanadium and the ligand states. Such hybridization acts as an effective trigonal crystal field, slightly lifting the degeneracy of the ground state. However, the energy splitting associated to the distortion underestimates the experimental band gap, suggesting that the insulating ground state is stabilized by Mott correlation effects rather than via a Jahn–Teller mechanism. Our results clarify the role of the distortion in VI3 and establish a benchmark for the study of the spectroscopic properties of other van der Waals halides, including emerging 2D materials with mono and few-layers thickness, whose fundamental properties might be altered by reduced dimensions and interface proximity.

Unusual anisotropic magnetic orbital moment obtained from x-ray magnetic circular dichroism in a multiferroic oxide system

The electric-field control of $d$-electron magnetism in multiferroic transition metal oxides is attracting widespread interest for the underlying fundamental physics and for next generation spintronic devices. Here, we report an extensive study of the $3d$ magnetism in magnetoelectric Ga$_{0.6}$Fe$_{1.4}$O$_3$ (GFO) epitaxial films by polarization dependent x-ray absorption spectroscopy. We found a non-zero integral of the x-ray magnetic circular dichroism, with a sign depending upon the relative orientation between the external magnetic field and the crystallographic axes. %By reliably enlarging the limit of the spin and orbital sum rules, which usually holds for materials where the magnetic ions exhibit a unique crystal field symmetry This finding translates in a sign-reversal between the average Fe magnetic orbital and spin moments. Large Fe-displacements, among some of the octahedral sites, lower the symmetry of the system producing anisotropic paths for the Fe-O bondings giving rise to a large orbital-lattice interaction akin to a preferential crystallographic direction for the magnetic orbital moment. The latter may lead to a partial re-orientation of the magnetic orbital moment under an external magnetic field that, combined to the ferrimagnetic nature of the GFO, can qualitatively explain the observed sign-reversal of the XMCD integral. The results suggest that a control over the local symmetry of the oxygen octahedra in transition metal oxides can offer a suitable leverage over the manipulation of the effective orbital and spin moments in magnetoelectric systems.

Giant orbital polarization of Ni2+ in a square planar environment

Finding large orbital polarization in Ni-based oxides has become a topic of paramount interest in recent years due to the prospect of finding superconductivity. In this work, we investigate the electronic structure of single-crystalline samples of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ and Ni-doped ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ containing $\mathrm{Cu}/{\mathrm{NiO}}_{4}$ square planar units. Our polarization-dependent x-ray absorption spectroscopy experiments reveal extremely large orbital polarization $(\ensuremath{\sim}63%)$ for ${\mathrm{Ni}}^{2+}$. Cluster calculations and ab initio calculations find that the giant orbital polarization arises due to the low-spin $(S=0)$ configuration with two holes in the Ni $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital, contrary to the expected high-spin $(S=1)$ state from Hund's first rule.

Strain control of phase transition and magnetic property in multiferroic BiFeO3 thin films

BiFeO3 (BFO), a room-temperature antiferromagnetic-ferroelectric multiferroic, is widely researched due to its potential applications for electric-field control of the magnetism. In this work, the strain control of the phase transition and magnetic properties in the BFO/LaAlO3 heterostructures were investigated. The O K edge polarization-dependent X-ray absorption spectroscopy (XAS) spectra show that the Fe 3d level splits into five levels, which proves that the FeO5 pyramid is asymmetric in the highly strained tetragonal-like BFO. The spin canting induced by the asymmetric structure leads to the magnetic moment. Thus, an obvious magnetic signal in the 17-nm-thick BFO thin films was observed by the Quantum Design magnetic property measurement system. With the increase of the BFO film thickness, the clamping effect induced by the substrate becomes weak, further leading to the BFO phase transition. The O K edge polarization-dependent XAS spectra demonstrate that the orbital reconstruction exists at the mixed BFO phase boundaries. Since the orbital reconstructions can induce the strong magnetic coupling, the magnetic order of the different BFO phases will be coupled with each other. It causes a variation of the magnetic property at the phase boundaries or in the BFO phases.

In-plane orientation-dependent metal-insulator transition in vanadium dioxide induced by sublattice strain engineering

Selectively modulating the sublattices in 3D transition metal oxides via strains could tailor the electronic configurations with emerging anomalous properties, which provides new platforms for fundamental researches as well as designs of devices. Here, we report tailoring the oxygen octahedral sublattices in vanadium dioxide (VO2) thin films by anisotropic in-plane strains, and the observation of in-plane orientation-dependent metal–insulator transition. Through multimodal characterizations based on high-resolution X-ray diffraction, electrical transport measurements, and polarization-dependent X-ray absorption spectroscopy at different temperatures, we demonstrate that nonequal strains were successfully induced along A and B oxygen octahedral chains in VO2 films via a special design of epitaxial growth on vicinal substrates. The V 3d1 orbital configurations are modulated in the two oxygen octahedral chains, resulting in in-plane orientation-dependent metal–insulator transition behaviors such as reduced hysteresis width and anisotropic phase transition temperature. This work provides new fundamental insights on metal–insulator transitions, and more importantly, opens up new opportunities for material and device developments

Anomalous orbital structure in two-dimensional titanium dichalcogenides

Generally, lattice distortions play a key role in determining the electronic ground states of materials. Although it is well known that trigonal distortions are generic to most two dimensional transition metal dichalcogenides, the impact of this structural distortion on the electronic structure and topological properties has not been understood conclusively. Here, by using a combination of polarization dependent X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and atomic multiplet cluster calculations, we have investigated the electronic structure of titanium dichalcogenides TiX2 (X = S, Se, Te), where the magnitude of the trigonal distortion increase monotonically from S to Se and Te. Our results reveal the presence of an anomalously large crystal field splitting. This unusual kind of crystal field splitting is likely responsible for the unconventional electronic structure of TiX2 compounds and ultimately controls the degree of the electronic phase protection. Our findings also indicate the drawback of the distorted crystal field picture in explaining the observed electronic ground state and emphasize the key importance of trigonal symmetry, metal-ligand hybridization and electron-electron correlations in defining the electronic structures at the Fermi energy.

Observation of the coherent quasiparticle states in SrRu1–xIrxO3 films via polarization-dependent soft X-ray absorption spectroscopy

The electronic structures of SrRu1−xIrxO3 films (0≤x≤1) have been investigated by employing polarization-dependent soft X-ray absorption spectroscopy. In SrIrO3, the coherent quasi-particle peak at the Fermi level (EF) is clearly observed, reflecting the spin-orbit-coupled correlated metallic nature of SrIrO3. X-ray linear dichroism signals for x=0 and x=1 are negligibly weak, demonstrating the pseudo-cubic structure for SrRuO3 and the strong spin-orbit coupling in SrIrO3. In contrast, finite linear dichroism is observed for the intermediate x regime, indicating that the narrow dxz↓ and dyz↓ bands close to EF are occupied less than the wide dxy↓ band due to the enhanced in-plane hopping.

Single antiferromagnetic axis of Fe in orthorhombic YMn0.5Fe0.5O3 films observed by x-ray magnetic linear dichroism

The electronic and magnetic structure of orthorhombic (o-) YMn0.5Fe0.5O3 (YMFO) epitaxial film (space group Pbnm) deposited on the YAlO3 (010) substrate have been investigated using linear polarization-dependent x-ray absorption spectroscopy and magnetization measurements. The magnetic-ordering temperature of around 280 K is observed in o-YMFO film. The x-ray absorption spectra at the Fe-K and the Mn-K edges indicate the existence of anisotropic crystal field in o-YMFO film, with the longest and shortest Fe–O and Mn–O bonds tend to align with the crystallographic b- and a-axis, respectively, whereas the medium Fe(Mn)–O bond is aligned with the c-axis. The experimental x-ray magnetic linear dichroism at the Fe-L2 edges demonstrate an unusual single antiferromagnetic axis of Fe3+ ions below magnetic-ordering temperature, while a multi antiferromagnetic axis is generally observed in o-RFeO3 (R = rare earth) thin films. Our configuration-interaction cluster calculations also reveal that the single antiferromagnetic axis of the Fe sublattice is aligned with the b-axis in o-YMFO film, whereas it is directed along the a-axis in the bulk o-YFeO3.

Direct observation of anisotropic small-hole polarons in an orthorhombic structure of BiVO4 films

Here, we report an anisotropic small-hole polaron in an orthorhombic structure of $\mathrm{BiV}{\mathrm{O}}_{4}$ films grown by pulsed-laser deposition on yttrium-doped zirconium oxide substrate. The polaronic state and electronic structure of $\mathrm{BiV}{\mathrm{O}}_{4}$ films are revealed using a combination of polarization-dependent x-ray absorption spectroscopy at $\mathrm{V}\phantom{\rule{0.16em}{0ex}}{L}_{3,2}$ edges, spectroscopic ellipsometry, x-ray photoemission spectroscopies, and high-resolution x-ray diffraction with the support of first-principles calculations. We find that in the orthorhombic phase, which is slightly different from the conventional pucherite structure, the unoccupied V 3d orbitals and charge inhomogeneities lead to an anisotropic small-hole polaron state. Our result shows the importance of the interplay of charge and lattice for the formation of a hole polaronic state, which has a significant impact in the electrical conductivity of $\mathrm{BiV}{\mathrm{O}}_{4}$, hence its potential use as a photoanode for water splitting.

Evolution of ground-state wave function in CeCoIn5 upon Cd or Sn doping

We present linear polarization-dependent soft x-ray absorption spectroscopy data at the Ce $M_{4,5}$ edges of Cd and Sn doped CeCoIn$_5$. The 4$f$ ground state wave functions have been determined for their superconducting, antiferromagnetic and paramagnetic ground states. The absence of changes in the wave functions in CeCo(In$_{1-x}$Cd$_x$)$_5$ suggests the 4$f$\,--\,conduction electron ($cf$) hybridization is not affected by globally Cd doping, thus supporting the interpretation of magnetic droplets nucleating long range magnetic order. This is contrasted by changes in the wave function due to Sn substitution. Increasing Sn in CeCo(In$_{1-y}$Sn$_y$)$_5$ compresses the 4$f$ orbitals into the tetragonal plane of these materials, suggesting enhanced $cf$ hybridization with the in-plane In(1) atoms and a homogeneous altering of the electronic structure. As these experiments show, the 4$f$ wave functions are a very sensitive probe of small changes in the hybridization of 4$f$ and conduction electrons, even conveying information about direction dependencies.

Electronically highly cubic conditions for Ru in α−RuCl3

We studied the local Ru 4d electronic structure of alpha-RuCl3 by means of polarization dependent x-ray absorption spectroscopy at the Ru-L2,3 edges. We observed a vanishingly small linear dichroism indicating that electronically the Ru 4d local symmetry is highly cubic. Using full multiplet cluster calculations we were able to reproduce the spectra excellently and to extract that the trigonal splitting of the t2g orbitals is -12 $\pm10$ meV, i.e. negligible as compared to the Ru 4d spin-orbit coupling constant. Consistent with our magnetic circular dichroism measurements, we found that the ratio of the orbital and spin moments is 2.0, the value expected for a Jeff = 1/2 ground state. We have thus shown that as far as the Ru 4d local properties are concerned, alpha-RuCl3 is an ideal candidate for the realization of Kitaev physics.

Orientation and Ordering of Organic and Hybrid Inorganic–Organic Polyurea Films Using Molecular Layer Deposition

The molecular orientation and ordering of films produced by a vapor-phase deposition technique known as molecular layer deposition (MLD) is investigated. Specifically, the influence of incorporating an inorganic component into MLD films is determined. Film growth properties and surface saturating conditions of a purely organic polyurea film and a mixed inorganic–organic polyurea film are investigated by in situ quartz crystal microbalance. Polarization-dependent X-ray absorption spectroscopy was employed to determine the orientation and ordering of these films. Both films demonstrate preferential orientation parallel to the substrate surface. However, the degree of ordering between the films is found to be starkly different. In-depth analysis revealed that the hybrid inorganic–organic films display a high degree of ordering compared to purely organic polyurea films. This study sheds light on the ability of MLD to finely tune oligomer packing density, growth rate, molecular orientation, and film ordering.

Individual Titanate Nanoribbons Studied by 3D-Resolved Polarization Dependent X-ray Absorption Spectra Measured with Scanning Transmission X-ray Microscopy

Polarization dependent X-ray absorption spectroscopy (XAS) is a powerful probe of the anisotropic electronic structure of bulk single crystals, but its application to nanostructured samples is challenging. Here we describe a method for obtaining linearly polarized XAS spectra along three orthogonal axes of an individual nano-object using scanning transmission X-ray microscopy (STXM). The technique is applied to a single sodium titanate nanoribbon [(Na,H)Ti NR]. Significant linear dichroism is observed at both the Ti 2p and O 1s edges. The experimental results are compared with first-principles calculations; good agreement is achieved. The spectral changes among the three axes are attributed to the anisotropic Ti–O bonding of the various Ti and O sites in the monoclinic crystal structure of the nanoribbon. The methodology for 3D dichroic STXM measurements developed in this study is a powerful way to investigate the anisotropic geometric and electronic structure of nanomaterials.

Investigation of the Electronic Structure of BiFeO<sub>3</sub> Epitaxial Films by Polarized X-Ray Absorption Spectroscopy

To investigate the epitaxial strain effect on local electronic structure of FeO6octahedron of BiFeO3epitaxial film, polarization-dependent FeL23-edge x-ray absorption spectroscopy studies were performed on both tetragonal (T)-like BFO/LAO and rhombohedral (R)-like BFO/STO epitaxial films. Charge transfer multiplet theory based fittings were also performed to reveal the local electronic structure difference. Due to dramatic structural difference caused by epitaxial strain between these two samples, significant electronic structure differences were observed between these two specimens. For BFO/LAO, anisotropic electronic structure appears in vertically-elongated FeO6octahedron and an additional shift of Fe ion off the central position is suggested. For BFO/STO, electronic structure is almost isotropic.

Helical versus all-trans conformations of oligo(ethylene glycol)-terminated alkanethiol self-assembled monolayers.

The complex mixture of conformational states exhibited by oligo(ethylene glycol)-terminated alkanethiols on Ag and Au surfaces is explored by polarization-dependent X-ray absorption spectroscopy. Three self-assembled monolayers (SAMs) with known helical or all-trans conformations are used as references to characterize a SAM with unknown conformations. This case study is used as a prototype for developing a systematic framework to extract the conformations of SAMs from the polarization dependence of several orbitals. In the case at hand, these are associated with the C-H/Rydberg bonds of the alkane, the C-H/Rydberg bonds of ethylene glycol, and the C-C bonds of the backbone. The C-H/Rydberg orbitals of the alkane and ethylene glycol are distinguished via the chemical shift of the corresponding C 1s core levels.

Anisotropic orbital occupation and Jahn-Teller distortion of orthorhombic YMnO3 epitaxial films: A combined experimental and theoretical study on polarization-dependent x-ray absorption spectroscopy

The b-axis oriented orthorhombic YMnO3 (o-YMnO3) epitaxial films on a YAlO3 (010) substrate were fabricated with pulsed-laser deposition. The anisotropic orbital occupation and Jahn-Teller (JT) distortion of an o-YMnO3 film were investigated with polarization-dependent x-ray absorption spectra and configuration-interaction multiplet-cluster calculations. A significant energy difference, ∼3.8 eV, for the main white line along E//b and E//a in polarization-dependent Mn K-edge spectra of o-YMnO3 indicates an extraordinary JT distortion and significant anisotropic Mn–O bonding within the ab plane in the o-YMnO3 film. Most importantly, although the orbital occupation of 3d electrons in o-YMnO3 films is almost the same as that in single crystalline o-DyMnO3, the JT distortion of o-YMnO3 films is larger than that of single crystalline o-DyMnO3, deduced from the multiplet calculations. We speculate that this JT distortion predominantly contributes to the origin of the cycloidal spin deformation in bulk o-YMnO3, because of a suppressed nearest-neighbor superexchange interaction and an enhanced next-nearest-neighbor superexchange interaction. These complementary results provide insight into the origin of the E-type magnetic configuration of o-YMnO3.

Crystal fields of porphyrins and phthalocyanines from polarization-dependent 2p-to-3d multiplets

Polarization-dependent X-ray absorption spectroscopy is combined with density functional calculations and atomic multiplet calculations to determine the crystal field parameters 10Dq, Ds, and Dt of transition metal phthalocyanines and octaethylporphyrins (Mn, Fe, Co, Ni). The polarization dependence facilitates the assignment of the multiplets in terms of in-plane and out-of-plane orbitals and avoids ambiguities. Crystal field values from density functional calculations provide starting values close to the optimum fit of the data. The resulting systematics of the crystal field can be used for optimizing electron-hole separation in dye-sensitized solar cells.

Orientation of a Monolayer of Dipolar Molecules on Graphene from X-ray Absorption Spectroscopy

Recently, single-walled carbon nanotubes as well as graphene functionalized with azobenzene chromophores have drawn attention for applications in optoelectronics due to their ability to undergo cis-trans isomerization when exposed to light. The electronic properties of the nanocarbon materials at these unconventional interfaces can be tailored by gaining structural insight into the organic monolayers at the molecular level. In this work, we use polarization-dependent X-ray absorption spectroscopy to probe the orientation of three chromophores on graphene, all identical except for their terminal groups. All three terminal groups (methyl, nitro, and nitrile) are well-oriented, with a tilt angle of about 30° from the substrate for the shared azobenzene group. Density functional theory calculations are in good agreement with experimental results and give two similar, stable configurations for the orientation of these molecules on graphene.