Polarization-dependent X-ray Absorption Spectroscopy Research Articles

Overlap ofCu 3dandF 2porbitals and low-energy excitations inKCuF3studied by polarization-dependent x-ray absorption and emission spectroscopy

Site-, elemental-, and orbital-resolved partial density of states in ${\text{KCuF}}_{3}$ single crystal have been obtained by polarization-dependent x-ray absorption and emission spectroscopy measurements across the $\text{F}\text{ }K$-edge and ab initio full-multiple-scattering cluster calculations. The $\text{F}\text{ }K$ x-ray absorption spectroscopy spectra show well-defined polarization dependence. The measurements reveal strong overlap between $\text{Cu}\text{ }3d$ and $\text{F}\text{ }2p$ states and identify clearly the $\text{F}\text{ }2{p}_{z}$ and $\text{F}\text{ }2{p}_{x,y}$ empty states created at different F sites. The calculations, performed for the nonequivalent F sites in the structure, confirm the energy overlap, the charge transfer between $\text{Cu}\text{ }3d$ and $\text{F}\text{ }2p$ states, and the different $\text{F}\text{ }2p\text{-Cu}\text{ }3d$ overlap for the two nonequivalent F sites of ${\text{KCuF}}_{3}$. Across the $\text{F}\text{ }K$ edge, resonant x-ray emission spectroscopy detects a broad peak due to $d\text{\ensuremath{-}}d$ excitations centered at about 1.2--1.6 eV and an intense charge-transfer-like transition around 7 eV.

Electronic structure and magnetic anisotropies in orthorhombic multiferroic YMnO3 thin films

Thermodynamically YMnO3 (YMO) can only exist with hexagonal structure at ambient conditions. In this study, we demonstrate that the stain between film and substrate can play an important role in forming orthorhombic YMO (o-YMO). o-YMO films with nearly perfect crystallographic orientation alignment were obtained on LaAlO3(110) substrates. These films allow us to unambiguously disclose the intrinsic magnetic property and electronic structure anisotropies along different crystallographic orientations. The antiferromagnetic (AFM) phase transition was around 42 K. Moreover, we observed a signature of spin reordering at lower temperatures when the applied field was parallel to the a- or c-axis. On the other hand, only the AFM transition was observed when the field is parallel to the b-axis. By comparing the X-ray absorption spectroscopy (XAS) results to standard manganese oxides, YMnO3 exhibits the dominant Mn+3 characteristics as that obtained from the standard Mn2O3 powder. The polarization dependent XAS also revealed anisotropic bonding behavior along each crystalline axis.

Fe/NiO(100) and Fe/MgO(100) interfaces studied by X-ray absorption spectroscopy and non-linear Kerr effect

A structural characterization of the Fe/NiO(100) and Fe/MgO(100) interfaces is reported. Polarization-dependent X-ray absorption spectroscopy has been employed to explore the local atomic structure of the interfaces. A strong difference between the two systems is observed: the Fe/MgO(100) interface does not show a significant Fe oxidation or oxygen diffusion while a buckled Fe–O like layer is observed at Fe/NiO(001). Moreover, while on the MgO(001) substrate the tetragonal strain is released at 10 ML Fe thickness, a residual epitaxial strain can be observed on a film of the same thickness on NiO(001). A magnetic characterization of the Fe/NiO(001) system by linear and non-linear magneto-optical Kerr effect measurements is also reported and discussed.

Bonding anisotropy in multiferroic TbMnO3 probed by polarization dependent x-ray absorption spectroscopy

O K- and Mn L2,3-edges x-ray absorption spectra of orthorhombic TbMnO3 single crystals show strong polarization dependence, in contrast with results of previous experiments on orthorhombic LaMnO3 and DyMnO3 thin films that show nearly isotropic spectral structure. First-principles calculations reveal that TbMnO3 exhibits a zigzag eg orbital ordering ground state. The highly distorted MnO6 octahedron and orbital ordering in TbMnO3 produce highly anisotropic Mn–O bonding within the ab plane, a frustration of the magnetic ordering, and the formation of complicated incommensurate magnetic structures.

Low energy excitations in cuprate superconducting superlattices and in BaCuO2+x and CaCuO2 constituent blocks: disentangling the role of conduction planes and charge reservoirs

This is a copy of the slides presented at the meeting but not formally written up for the volume. We propose a novel approach to correlate electronic properties with superconductivity in high critical temperature cuprates. We have disassembled the superconductor in the constituent blocks and investigated separately the charge reservoir (CR) and the conduction planes or infinite layer (IL) blocks, which consist of BaCuO2+x (BCO) and CaCuO2 (CCO) respectively. Artificial BCO/CCO superconducting superlattices [1], obtained by stacking in a sequence the two blocks, have been also measured to study the effect of charge transfer (from the CR to the IL block). On such samples, in form of c-axis oriented thin films, we have performed polarization dependent X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS) measurements at the Cu L3 edge, to investigate the hole doping effect and the low energy excitations. XAS results clearly show a significant number of Cu d(z2) holes, both in the pure BCO samples and the BCO/CCO superlattices linked to the presence of apical oxygen. The high energy shoulder related to the existence of itinerant Cu 3d holes hybridised with O 2p (doping holes) is evident in the XAS spectra of the pure BCO and the BCO/CCO superconducting superlattices. The relative shoulder intensity decreases by increasing the CCO block thickness, in agreement with the expected decrease of the doping level. RIXS spectra show similar dd-excitations in the ab-planes for BCO, CCO and 2X2 superlattice, as a consequence of similar in-plane crystal field effects [2]. On the contrary, the charge transfer excitations [3] in ab-plane look similar only for the BCO film and the 2X2 superlattice. Namely, fluorescence activates at the absorption edge because of the metal character of both the CR block and the superconducting superlattice. On the contrary in CCO (insulator) the onset of the fluorescence signal is moved at higher photon energies, in agreement with the presence of a charge transfer band gap. In addition, we have detected an excitation at 0.5 eV in BCO and BCO/CCO superlattices, but not in CCO. This excitation could be interpreted as the stabilization energy of the Zhang-Rice (ZR) singlet [4] in doped compounds, or as the x2-y2 to z2 dd excitation. All the results are in agreement with the scenario of charge transfer from the BCO charge reservoir block to the CCO infinite layer block. [1] G. Balestrino et al., Phys. Rev. B 62, 9835 (2000) [2] P. Kuiper et al., Phys. Rev. Lett. 80, 5204 (1998) [3] G. Ghiringhelli et al. Phys. Rev. Lett. 92, 117406 (2004) [6] N.B.Brookes et al., Phys. Rev. Lett. 87, 237003 (2001).

Ferroelectricity Driven by Yd0-ness with Rehybridization inYMnO3

We investigated electronic structure of hexagonal multiferroic YMnO3 using the polarization dependent x-ray absorption spectroscopy (XAS) at O K and Mn L(2,3) edges. The spectra exhibit strong polarization dependence at both edges, reflecting anisotropic Mn 3d orbital occupation. Moreover, the O K edge spectra show that Y 4d states are strongly hybridized with O 2p ones, resulting in large anomalies in Born effective charges on off-centering Y and O ions. These results manifest that the Y d(0)-ness with rehybridization is the driving force for the ferroelectricity, and suggest a new approach to understand the multiferroicity in the hexagonal manganites.

Spectroscopic investigations on layered sodium cobaltates

We have performed spectroscopic experiments and theoretical calculations on layered NaxCoO2. As an experimental tool, we used polarization dependent X-ray absorption spectroscopy in a wide doping range at various temperatures. In order to interpret these experiments, we calculated the local electronic structure around a Co atom by solving exactly a CoO6 cluster. From the results relevant aspects of the electronic structure of NaxCoO2 are disclosed.

Electronic structure of single-crystalline thermoelectricBi2−xPbxSr2Co2Oy(x=0,0.6)from photoemission and x-ray absorption

The electronic structures of ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{y}$ $(x=0,0.6)$ thermoelectric single crystals have been investigated by employing photoemission spectroscopy (PES) and polarization-dependent soft x-ray absorption spectroscopy. Co ions are found to be in the ${\mathrm{Co}}^{3+}\text{\ensuremath{-}}{\mathrm{Co}}^{4+}$ mixed-valent states with the low-spin configuration. The valence-band PES study reveals that the Co $3d$ states are located at the top of the valence bands, corresponding to the occupied electron configuration of ${e}_{g}^{\ensuremath{\pi}4}{a}_{1g}^{1+x}$ $(x<1)$. Both the topmost occupied states and the lowest unoccupied states have the primarily out-of-plane Co ${a}_{1g}$ character, suggesting that carriers have the Co ${a}_{1g}$ hole character. Angle resolved photoemission spectroscopy measurements show that the lowest binding-energy states are shifted toward ${E}_{F}$ with increasing $x$ in ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{y}$, indicating that the carrier states are shifted toward ${E}_{F}$. This study suggests that the thermoelectric property of ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{y}$ is improved for $x>0$ mainly by the reduced resistivity.

Higher Order Liquid Crystalline Structure in Low-Polydispersity DEH-PPV

Monodisperse, low molecular weight poly(2,5-di(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (DEH-PPV) demonstrates significantly better structural order than polydisperse PPVs. Since optical and electrical properties of polymer electronics are closely related to the structure and morphology of the active layer, morphological control is important for the fabrication of PPV-based devices. Soluble, monodisperse DEH-PPVs with a range of molecular weights showed a sequence of transitions through crystalline, smectic, nematic, and isotropic phases upon heating, and the transition temperatures increased sharply with increasing molecular weight. The layer spacing of the smectic phase is in good agreement with the length of a PPV molecule. The Maier−Saupe parameter for this polymeric system is estimated, and both energetic and entropic contributions are found to be important in describing the liquid crystalline interaction. Thin films revealed that the PPV molecules ordered into thermally stable layers a single molecule thick. The layers arranged into grains that were aligned by shearing. The orientation of molecules within the aligned layers was characterized by polarization-dependent X-ray absorption spectroscopy, and the PPV molecular axis was found to align parallel to the shear direction while the layers aligned perpendicular to the shear direction. Low polydispersity is critical to forming these types of highly ordered structures, and an analogous PPV sample with polydispersity greater than 5 does not form smectic layers.

Ti 3dOrbital Change Across Metal–Insulator Transition in Ti2O3: Polarization-Dependent Soft X-ray Absorption Spectroscopy at Ti 2pEdge

The Ti 3 d electronic structure of Ti 2 O 3 across metal–insulator transition (MIT) has been investigated by means of polarization-dependent soft X-ray absorption spectroscopy (XAS) at the Ti 2 p edge. The XAS spectra exhibit a remarkable linear dichroism (LD) between the polarization vectors of the incident light parallel and perpendicular to the Ti ion pair along the c -axis. LD intensity is significantly reduced at 520 K in the metallic phase in comparison with that at 300 K in the insulating phase, indicating a large change in a 1 g and e g π orbital occupations. The results are qualitatively consistent with a recent theory, where the MIT is due to the change of the 3 d electronic configuration in the Ti pair along the c -axis driven by a change in the t 2 g level splitting relative to the on-site exchange interaction.

Studies on the La substitution site in Bi4−xLaxTi3O12 by polarization-dependent x-ray absorption spectroscopy

We performed polarization-dependent x-ray absorption spectroscopy studies on epitaxially grown Bi4−xLaxTi3O12 thin films at the 3p→6s edges to find the La substitution site. Our results show a clear difference between the absorption spectra taken with parallel and perpendicular polarizations, which is consistent with the view that La replaces Bi in the layer rather than at the perovskite site. Calculated absorption spectra also support the interpretation. We therefore conclude that some, if not all, La atoms replace the Bi atoms in the layer.

The Fe/NiO interface studied by polarization dependent X-ray absorption spectroscopy

In order to provide a structural basis for the modelling of the electronic and magnetic properties of the Fe/NiO(001) interface we have performed polarization dependent Fe K-edge X-ray absorption measurements. A multi-shell fit of the data is presented and discussed. We find that a 2ML Fe film exhibits a complete tetragonal distortion of the unit cell and demonstrate the formation of a planar FeO layer with expanded Fe–O distances perpendicular to the growth plane. We discuss a model for the interface with the FeO layer at the oxide–metal interface. At 10ML thickness the tetragonal strain of the Fe film is partially released.

Electronic structure and surface structure of Cu 2S nanorods from polarization dependent X-ray absorption spectroscopy

Highly aligned Cu 2S nanorods have been studied by polarization dependent X-ray absorption spectroscopy. In contrast to bulk Cu 2S, strong s, p, and d hybridization is found in the nanorods. The polarization dependence shows a predominant d z 2 character of Cu 3d states. Ab initio multiple-scattering calculations confirm the strong hybridization, and reveal that Cu 2S nanorods are grown along the z-axis of chalcocite structure with Cu 7 and Cu 10 sites being the main building blocks. The hybridized absorption peak in the nanorods is shifted towards lower energies for smaller diameter of nanorods, which is attributed to surface reconstruction due to strong Cu–Cu interactions on the Cu-rich surface of the nanorods.

Controlling Orbital Moment and Spin Orientation in CoO Layers by Strain

We have observed that CoO films grown on different substrates show dramatic differences in their magnetic properties. Using polarization dependent x-ray absorption spectroscopy at the Co L2,3 edges, we revealed that the magnitude and orientation of the magnetic moments strongly depend on the strain in the films induced by the substrate. We presented a quantitative model to explain how strain together with the spin-orbit interaction determine the 3d orbital occupation, the magnetic anisotropy, as well as the spin and orbital contributions to the magnetic moments. Control over the sign and direction of the strain may, therefore, open new opportunities for applications in the field of exchange bias in multilayered magnetic films.

Highly ordered phthalocyanine thin films on a technically relevant polymer substrate

We have studied the molecular orientation of well-known representatives of organic semiconductors from the family of the phthalocyanines [copper phthalocyanine (CuPc) and its perfluorinated relative (CuPcF16)] on a conducting polymer thin film using polarization-dependent x-ray absorption spectroscopy. As a polymer substrate PEDOT:PSS [a mixture of poly-3,4-ethylenedioxy-thiophene (PEDOT) and polystyrenesulfonate (PSS), which is often applied as an electrode material in (all-)organic semiconductor devices] was spin coated onto indium-tin-oxide substrates. Even if the interfaces themselves are relatively ill defined (we found recently a mixing of the two organic materials and charge-transfer processes), a very high degree of molecular ordering is observed in the 20–50nm thick phthalocyanine films.

NiO and MgO ultrathin films by polarization dependent XAS

We present a comparative study of MgO and NiO ultrathin films on Ag(0 0 1) by polarization dependent X-ray absorption spectroscopy on both the metal and oxygen K-edges. The data have been analyzed using ab initio multiple-scattering calculations (FEFF code). We show that the films have the rock-salt structure with a negligible atomic interdiffusion with the substrate in all cases. Both MgO and NiO films are tetragonally strained to match the Ag substrate at low thickness and gradually relax their structure to the bulk one. The local in-plane and out-of-plane interatomic distances in the oxides have been accurately determined as a function of thickness and the elastic constants of the films have been found to be compatible with the bulk values. The oxygen absorption site has been found to be on top of Ag atoms, while the metal occupies the interstitial site in both systems. A significant increase of the interplanar distance at the interface between both MgO and NiO and the Ag substrate has been measured.

Electronic properties of interfaces between model organic semiconductors and metals

We present a summary of recent studies of the interface properties of interfaces that are formed between well-established molecular organic semiconductors and various metals. The energy level alignment, the formation of interface dipoles, a possible band bending and interactions at the interfaces have been investigated using photoemission spectroscopy, while polarization-dependent X-ray absorption spectroscopy provided information on the molecular orientation at the interfaces. The observed interface characteristics reach from physisorption to a reaction depending upon both the substrate and the organic molecule under investigation. Different contributions to the often observed interface dipoles are discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray absorption study at the Mg and O K edges of ultrathin MgO epilayers on Ag(001)

We determined the local atomic structure of MgO epilayers on Ag(001) by means of polarization-dependent x-ray absorption spectroscopy measurements at the Mg and O $K$ edges. A quantitative analysis of the data in the extended energy range has been performed using multiple scattering simulations. We found that, even in the ultrathin limit, the local structure of the films is rocksalt and we obtained a quantitative evaluation of the average in-plane and out-of-plane film strain at the different thicknesses investigated. An in-plane compressive strain, due to lattice mismatch with the Ag substrate, is clearly present for the 3 ML film. The out-of-plane lattice constant is found to be expanded, in agreement with the expected behavior for a tetragonal distortion of the unit cell. This growth-induced strain is gradually released with increasing thickness and it is almost completely relaxed at 20 ML. Any significant intermixing with the Ag substrate can be ruled out. An expansion of the interplanar distance at the MgO-Ag interface is detected and its sign and magnitude are found to be in agreement with recent ab initio simulations. This work provides previously unavailable input for modeling the physical properties of the system and supports the hypothesis that the different electronic properties of MgO films on Ag(001) are not related to structural or compositional differences at the ultrathin limit.

Growth of NiO on Ag(001): Atomic Environment, Strain, and Interface Relaxations Studied by Polarization Dependent Extended X-ray Absorption Fine Structure

This paper reports a structural study of 3 and 10 monolayer (ML) thick NiO films grown on Ag(001). Polarization-dependent X-ray absorption spectroscopy at the Ni K edge allowed us to obtain an accurate description of the local atomic environment of the Ni atoms up to the seventh coordination shell, including the determination of the in-plane and out-of-plane strains for the NiO films (determined with an accuracy better than 1%, resulting from the determination of Ni-O bond distances with a statistical error lower than 0.01 A). Multishell multiple scattering calculations have been used to fit the data. Within the experimental errors, the 3 ML film has both in and out-of-plane Ni-O distances compatible with those expected in the case of perfect pseudomorphism on Ag(001). A rough film morphology, possibly including also the presence of NiO islands, is also suggested by the optimized coordination numbers. Conversely the 10 ML film is partially relaxed without significant deviation from ideal bidimensional film. The present EXAFS study allows us to rule out any significant atomic interdiffusion process between the substrate and the NiO film. Comparison with previous literature data allows us to make the hypothesis of a two step growth mechanism for NiO films on Ag(001). The NiO-Ag(001) interface distance determined in this EXAFS study (d i n t e r f a c e = 2.36 ′ 0.05 A) is in remarkable agreement with theoretical calculations (d i n t e r f a c e = 2.38 A).

Unusual electronic structure of the pseudoladder compoundCaCu2O3

Experimental and theoretical studies of the unoccupied electronic structure of ${\mathrm{CaCu}}_{2}{\mathrm{O}}_{3}$ single crystals have been performed using polarization-dependent x-ray absorption spectroscopy and band-structure calculations. The measured hole distribution shows an unusual large number of holes in orbitals parallel to the interlayer direction which is in agreement with the theoretical analysis. ${\mathrm{CaCu}}_{2}{\mathrm{O}}_{3}$ deviates significantly from the standard $\mathrm{pd}\ensuremath{\sigma}$ cuprate picture. The corresponding strong interlayer exchange is responsible for the missing spin gap which is generic for other two-leg ladder cuprates.