Electronic structure of warm dense copper studied by ultrafast x-ray absorption spectroscopy

We use time-resolved x-ray absorption spectroscopy to investigate the unoccupied electronic density of states of warm dense copper that is produced isochorically through the absorption of an ultrafast optical pulse. The temperature of the superheated electron-hole plasma, which ranges from 4000 to 10?000 K, was determined by comparing the measured x-ray absorption spectrum with a simulation. The electronic structure of warm dense copper is adequately described with the high temperature electronic density of state calculated by the density functional theory. The dynamics of the electron temperature is consistent with a two-temperature model, while a temperature-dependent electron-phonon coupling parameter is necessary.     

Time resolved spectroscopy with femtosecond soft-x-ray pulses

The most challenging application of time resolved spectroscopy is to directly observe the structural and electronic dynamics. Here we present the combination of x-ray absorption spectroscopy with laser driven x-ray sources, offering atomic spatial and temporal resolution. Our new approaches for optimization of laser driven x-ray sources resulted in the demonstration of spatially coherent sub-20 fs x-ray pulses in a range up to several keV. We excited polycrystalline silicon with an ultrashort laser pulse and characterized the collective motion of atoms with time resolved x-ray absorption spectroscopy at a temporal resolution of less than 20 fs. Finally, we have shown the feasibility of probing the dynamics of the electronic structure of silicon and carbon with near edge x-ray absorption spectroscopy.     

Electronic structure of metal nanoclusters

The electronic structure and the shape of the K absorption edge for small-sized clusters formed by transition metal atoms (titanium, nickel, and copper) are investigated using the quantum-mechanical multiple scattering method (FEFF8 code) and the molecular mechanics technique. It is shown that the x-ray photoelectron spectra and K x-ray absorption spectra of clusters containing 55 and more atoms are similar to the corresponding experimental spectra of macroscopic samples. The computer simulation of the electronic structure and the shape of the K absorption edge is performed for nanoclusters whose equilibrium geometric shape is determined by the molecular dynamics method.     

Cooperative Jahn-Teller phase transition in LaMnO3 studied by X-ray absorption spectroscopy

The local structure of LaMnO3 across the Jahn-Teller (JT) transition at T(JT)=750 K was studied by means of x-ray absorption near edge structure and extended x-ray absorption fine structure at the Mn K-edge. Our results indicate a similar electronic local structure for Mn atoms above and below T(JT) and a dynamical tetragonal JT distortion of MnO6 octahedra above T(JT). The structural transition is originated by the ordering of tetragonally distorted octahedra. The entropy content of the transition is analyzed within the framework of the three-state Potts model with nearest neighbor antiferrodistortive coupling.     

Change of electronic structure in Ca2RuO4 induced by orbital ordering

Optical conductivity spectra sigma(omega) were used to investigate the effect of orbital ordering on the electronic structure of Ca2RuO4. Our LDA+U calculation predicts Ru 4d(xy) ferro-orbital ordering at the ground state, and well explains the present sigma(omega) as well as the reported O 1s x-ray absorption spectra. Variation of temperature (T) causes a large change of spectral weight over several eV as well as collapse of a charge gap accompanied by elongation of the c-axis Ru-O bond length. These results clearly indicate that the d(xy) orbital ordering plays a crucial role in the metal-insulator transition and the T-dependent electronic structure on a large energy scale.     

Understanding the nature of the kinetic process in a VO2 metal-insulator transition

Understanding the kinetics during the metal-insulator transition process is crucial to sort out the underlying physical nature of electron-lattice interactions in correlated materials. Here, based on the temperature-dependent in situ x-ray absorption fine structure measurement and density-functional theory calculations, we have revealed that the monoclinic-to-tetragonal phase transition of VO2 near the critical temperature is characterized by a sharp decrease of the twisting angle δ of the nearest V-V coordination. The VO2 metallization occurs in the intermediate monoclinic like structure with a large twist of V-V pairs when the δ angle is smaller than 1.4°. The correlation between structural kinetics and electronic structure points out that the structural rearrangement is a key factor to narrow the insulating band gap.     

Properties of liquid silicon observed by time-resolved x-ray absorption spectroscopy

Time-resolved x-ray spectroscopy at the Si L edges is used to probe the electronic structure of an amorphous Si foil as it melts following absorption of an ultrafast laser pulse. Picosecond temporal resolution allows observation of the transient liquid phase before vaporization and before the liquid breaks up into droplets. The melting causes changes in the spectrum that match predictions of molecular dynamics and ab initio x-ray absorption codes.     

Structure and bonding of water on Pt(111)

We address the adsorption of water on Pt(111) using x-ray absorption, x-ray emission, and x-ray photoelectron spectroscopy along with calculations in the framework of density functional theory. Using the direct relationship between the electronic structure and adsorbate geometry, we show that in the first layer all the molecules bind directly to the surface and to each other through the in-layer H bonds without dissociation, creating a nearly flat overlayer. The water molecules are adsorbed through alternating metal-oxygen (M-O) and metal-hydrogen (M-HO) bonds.     

X-ray studies of the structure and electronic behavior of alkanethiolate-capped gold nanoparticles: the interplay of size and surface effects

We report a study of the structure and electronic properties of a series of thiol-capped Au nanoparticles (NP) of nominal sizes of 1.6, 2.4, and 4.0 nm. Transmission electron microscopy, x-ray powder diffraction, x-ray absorption fine structure, and x-ray photoemission spectroscopy have been used to investigate the size-dependent systematics of lattice contraction and charge redistribution of these NPs. It is found that the lattice contracts and the d charge at the Au atom site depletes relative to bulk Au as the size of the NP decreases. The implication of these observations is discussed in terms of the interplay of quantum-size and surface effect.     

X-ray spectra and electronic structure of aluminum in AlN and B x Al1 − x N crystals with a wurtzite structure

The electronic band structure and x-ray spectra of aluminum in wurtzite-type crystals of B_xAl_{1 ? x} N compounds are investigated and compared with those of the binary aluminum nitride AlN. The main features and the features revealed for the first time in the L _{II, III} x-ray emission and absorption spectra are interpreted.     

Band-selective measurements of electron dynamics in VO2 using femtosecond near-edge x-ray absorption

We report on the first demonstration of femtosecond x-ray absorption spectroscopy, made uniquely possible by the use of broadly tunable bending-magnet radiation from "laser-sliced" electron bunches within a synchrotron storage ring. We measure the femtosecond electronic rearrangements that occur during the photoinduced insulator-metal phase transition in VO2. Symmetry- and element-specific x-ray absorption from V2p and O1s core levels (near 500 eV) separately measures the filling dynamics of differently hybridized V3d-O2p electronic bands near the Fermi level.     

Metal-induced gap states at well defined alkali-halide/metal interfaces

In order to search for states specific to insulator/metal interfaces, we have studied epitaxially grown interfaces with element-selective near edge x-ray absorption fine structure. An extra peak is observed below the bulk edge onset for LiCl films on Cu and Ag substrates. The nature of chemical bonds as probed by x-ray photoemission spectroscopy and Auger electron spectroscopy remains unchanged, so we regard this as evidence for metal-induced gap states (MIGS) formed by the proximity to a metal, rather than local bonds at the interface. The dependence on the film thickness shows that the MIGS are as thin as one monolayer. An ab initio electronic structure calculation supports the existence of the MIGS that are strongly localized at the interface.     

Orbital symmetry and electron correlation in NaxCoO2

Measurements of polarization-dependent soft x-ray absorption reveal that the electronic states determining the low-energy excitations of NaxCoO2 have predominantly a(1g) symmetry with significant O 2p character. In contrast to the prediction of band theory, doping-dependent O 1s x-ray absorption shows a large transfer of spectral weight, providing spectral evidence for strong electron correlations of the layered cobaltates. We also found that NaxCoO2 exhibits a charge-transfer electronic character rather than a Mott-Hubbard character.     

X射线吸收近边结构理论

在X射线吸收谱中,阈值之上60eV以内的低能区的谱出现强的吸收特性,称之为近边吸收结构(XANES)。它是由于激发光电子经受周围原子的多重散射造成的。它不仅反映吸收原子周围环境中原子几何配置,而且反映凝聚态物质费米能级附近低能位的电子态的结构。因此成为研究凝聚态物质的有用工具。本文较详细地介绍了XANES理论中的多重散射模型与计算方法。同时,也揭示多重散射理论与广延系统中通常采用的能带结构理论以及有限系统中的单电子的分子轨道方法之间的关系。并给出若干实例以说明XANES的多重散射理论的应用。     

Phase transition of solid bismuth under high pressure

As a widely used pressure calibrator, the structural phase transitions of bismuth from phase I, to phase II, to phase III,and then to phase V with increasing pressure at 300 K have been widely confirmed. However, there are different structural versions for phase III, most of which are determined by x-ray diffraction(XRD) technology. Using x-ray absorption fine structure(XAFS) measurements combined with ab initio calculations, we show that the proposed incommensurate composite structure of bismuth of the three configurations is the best option. An abnormal continuous increase of the nearest-neighbor distance of phase III with elevated pressure is also observed. The electronic structure transformation from semimetal to metal is responsible for the complex behavior of structure transformation.     

Evidence for localizing properties of XPS applied to valence bands

X-ray photoelectron spectroscopy (XPS)-measurements of the valence bands of Cu, Ag and Au have been performed and their integrated intensities have been compared. A model calculation which considers the influence of the x-ray absorption length as well as the electronic escape depth on the obtained intensities was established and applied to the measurements. An additional electron microscopic study on the surface structure of the evaporated samples has been performed. Its influence on the obtained photoelectron yield is discussed. The results support a marked dependence of the integrated valence band intensity on the atomic number. The implications for the interpretation of XPS valence band spectra of alloys are discussed and compared with corresponding x-ray isochromat spectroscopy (XIS)-measurements. Furthermore we deal with the important question whether XPS-as well as XIS-spectra can be described mainly in terms of the electronic band structure or whether atomic features are also present, though both methods are free from participation of core levels. The results are discussed in terms of localizing properties of XPS.     

Temperature dependence of bismuth structures under high pressure

It is unclear whether there is a liquid-liquid phase transition or not in the bismuth melt at high temperature and high pressure. If so, it will be necessary to confirm the boundary of the liquid-liquid phase transition and clarify whether it is a first-order phase transition. Here, based on x-ray absorption spectra and simulations, the temperature dependence of bismuth structures is investigated under different pressures. According to the similarity of characteristic peaks of x-ray absorption near edge structure (XANES) spectra, we estimate the possible temperature ranges of liquid-liquid phase transition to be 779-799 K at 2.74 GPa and 859-879 K at 2.78 GPa, 809-819 K at 3.38 GPa and 829-839 K at 3.39 GPa and 729-739 K at 4.78 GPa. Using ab initio molecular dynamics (AIMD) simulations, we obtain the stable structures of the bismuth melt at different temperatures and pressures, and calculated their electronic structures. Meanwhile, two stable phases (phase III-like and phase IV-like) of bismuth melts are obtained from different initial phases of bismuth solids (phase III and phase IV) under the same condition (3.20 GPa and 800 K). Assuming that the bismuth melt undergoes a phase transition from IV-like to III-like between 809 K and 819 K at 3.38 GPa, the calculated electronic structures are consistent with the XANES spectra, which provides a possible explanation for the first-order liquid-liquid phase transition.     

Total-reflection inelastic X-ray scattering from a 10-nm thick La0.6Sr0.4CoO3 thin film

To study equilibrium changes in composition, valence, and electronic structure near the surface and into the bulk, we demonstrate the use of a new approach, total-reflection inelastic x-ray scattering, as a sub-keV spectroscopy capable of depth profiling chemical changes in thin films with nanometer resolution. By comparing data acquired under total x-ray reflection and penetrating conditions, we are able to separate the O K-edge spectra from a 10 nm La0.6Sr0.4CoO3 thin film from that of the underlying SrTiO3 substrate. With a smaller wavelength probe than comparable soft x-ray absorption measurements, we also describe the ability to easily access dipole-forbidden final states, using the dramatic evolution of the La N4,5 edge with momentum transfer as an example.     

Effect of pressure on magnetoelastic coupling in 3d metal alloys studied with x-ray absorption spectroscopy

Using x-ray absorption spectroscopy, we have studied the effect of pressure on femtometer-scale bond strain due to anisotropic magnetostriction in a thin FeCo film. At 7 GPa local magnetostrictive strain is found to be larger than at ambient, in agreement with spin-polarized ab initio electronic structure calculations, but contrary to the expected effect of compression on bond stiffness. The availability of high pressure data on local magnetostrictive strain opens new capabilities for validating theoretical predictions and can lead to the development of materials with the desired properties.