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The identification of electronic states and the analysis of their evolution with $n$ is key to understanding $n$-layered ruthenates. To this end, we combine polarization-dependent O 1$s$ x-ray absorption spectroscopy, high-purity Sr${}_{n+1}$Ru${}_{n}$O${}_{3n+1}$ ($n=1,2,3$) single crystals, and ab initio and many-body calculations. We find that the energy splitting between the empty ${x}^{2}\ensuremath{-}{y}^{2}$ and $3{z}^{2}\ensuremath{-}1$ state is considerably smaller than previously suggested and that, remarkably, Sr bands are essential to understanding the spectra. At low energy, we identify the main difference among the materials with a substantial rearrangement of ${t}_{2g}$ orbital occupations with increasing $n$. This rearrangement is controlled by the interplay of Coulomb repulsion, dimensionality, and changes in the ${t}_{2g}$ crystal field.

Here we report extensive ultrafast time-resolved reflectivity experiments on overdoped Bi${}_{2}$Sr${}_{2}$Ca${}_{1\ensuremath{-}x}$Y${}_{x}$Cu${}_{2}$O${}_{8+\ensuremath{\delta}}$ single crystals ($T$${}_{c}=78$ K) aimed to clarify the nature of the superconducting-to-normal-state photoinduced phase transition. The data show a lack of the quasiparticle decay time divergence at the fluence required to induce this phase transition, in contrast to the thermally driven phase transition observed at $T$${}_{c}$ and at variance with recently reported photoinduced transitions from charge-density and spin-density waves to a metal. Our data demonstrate the nonthermal character of the superconducting-to-normal-state photoinduced phase transition. The data are analyzed using an ad-hoc time-dependent Rothwarf-Taylor model, opening the question of the order of this nonequilibrium phase transition.

This communication presents and discusses an experimental proof of the correlation among local structure and second harmonic generation (SHG) in bulk nanostructured potassium niobiosilicate (KNS) glasses. In particular, SHG shows a maximum in correspondence of the early stages of nanostructuring, that are characterized by the segregation within the amorphous matrix of nanosized inhomogeneities. EXAFS experiments indicate that these inhomogeneities are determined by the maximum size reached by the niobium second coordination shell combined with the sudden inclusion of potassium atoms in it. Such rearrangement at the local level determines the maximum fluctuation of the bulk glass refractive index and in turn its maximum SHG activity.

Here, we report on a novel experimental apparatus for performing time-resolved soft x-ray absorption spectroscopy in the sub-ns time scale using non-hybrid multi-bunch mode synchrotron radiation. The present setup is based on a variable repetition rate Ti:sapphire laser (pump pulse) synchronized with the ~500 MHz x-ray synchrotron radiation bunches and on a detection system that discriminates and singles out the significant x-ray photon pulses by means of a custom made photon counting unit. The whole setup has been validated by measuring the time evolution of the L(3) absorption edge during the melting and the solidification of a Ge single crystal irradiated by an intense ultrafast laser pulse. These results pave the way for performing synchrotron time-resolved experiments in the sub-ns time domain with variable repetition rate exploiting the full flux of the synchrotron radiation.

In strongly-correlated systems the electronic properties at the Fermi energy (EF) are intertwined with those at high energy scales. One of the pivotal challenges in the field of high-temperature superconductivity (HTSC) is to understand whether and how the high energy scale physics associated with Mott-like excitations (|E-E$_{F}$|>1 eV) is involved in the condensate formation. Here we show the interplay between the many-body high-energy CuO2 excitations at 1.5 and 2 eV and the onset of HTSC. This is revealed by a novel optical pump supercontinuum-probe technique, which provides access to the dynamics of the dielectric function in Bi$_2$Sr$_2$Ca$_{0.92}$Y$_{0.08}$Cu$_2$O$_{8+{\delta}}$ over an extended energy range, after the photoinduced suppression of the superconducting pairing. These results unveil an unconventional mechanism at the base of HTSC both below and above the optimal hole concentration required to attain the maximum critical temperature (T$_c$)., Comment: Text and Supplementary Information

The Italian Free Electron Laser (FEL) facility FERMI@Elettra has started to produce photon radiation at the end of 2010. The photon beam is presently delivered by the first undulator chain (FEL1) that is supposed to produce photons in the 100-20 nm wavelength range. A second undulator chain (FEL2) will be commissioned at the end of 2011, and it will produce radiation in the 20-4nm range. The Photon Analysis Delivery and Reduction System (PADReS) was designed to collect the radiation coming from both the undulator chains (FEL1 and FEL2), to characterize and control it, and to redirect it towards the following beamlines. The first parameters that are checked are the pulse-resolved intensity and beam position. For each of these parameters two dedicated monitors are installed along PADReS on each FEL line. In this way it possible to determine the intensity reduction that is realized by the gas reduction system, which is capable of cutting the intensity by up to four orders of magnitude. The energy distribution of each single pulse is characterized by an online spectrometer installed in the experimental hall. Taking advantage of a variable line-spacing grating it can direct the almost-full beam to the beamlines, while it uses a small fraction of the beam itself to determine the spectral distribution of each pulse delivered by the FEL. The first light of FERMI@Elettra, delivered to the PADReS section in late 2010, is used for the first commissioning runs and some preliminary experiments whose results are reported and discussed in detail.

X-ray absorption spectroscopy performed on a CuFeO${}_{2}$ single crystal reveals the pivotal role of the Cu ions in the discussion of the anisotropic properties of the compound. In particular, it is shown that there is a $3d$ hole density on the nominal $3{d}^{10}$ Cu${}^{+}$ ions which may lie at the origin of the intriguing low temperature magnetic properties of CuFeO${}_{2}$. In addition, the finding has important consequences regarding the field-induced ferroelectricity of the system.

High-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces. Here we study this phenomenon in the hypersonic frequency limit. By modeling the thermomechanics from first-principles, we calculate the system's initial heat-driven impulsive response and follow its time evolution. A scheme is introduced to quantitatively access frequencies and lifetimes of the composite system's excited eigenmodes. A spectral decomposition of the calculated response on the eigemodes of the system reveals asymmetric resonances that result from the coupling between surface and bulk acoustic modes. This finding allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes and expands our understanding of the scattering of surface waves in mesoscale metamaterials. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on one-dimensional and two-dimensional surface phononic crystals, probed using light at extreme ultraviolet and near-infrared wavelengths.

Interpretations are given for the core-level binding energies, BE's, of the surface and bulk atoms of metals; in particular, the origin of possible differences, or shifts, between these BE's is considered. The mechanisms which are responsible for the BE shifts are identified and characterized through theoretical analyses of the surface electronic structure. The dominant mechanisms for the BE shifts are those which lead to the initial-state changes in the orbital energies and, hence, the Koopmans' Theorem BE's. This is particularly important because it means that the shifts can be directly related to the chemistry and physics of the systems. The surface electronic structure is described with molecular-orbital wavefunctions for cluster models of the surface; this theoretical approach is particularly well suited for the study of local chemical interactions. The different mechanisms act in a cancelling fashion which leads to rather small net shifts between the BE's of bulk and surface atoms. The mechanisms which explain the surface and bulk core-level BE shifts are rather general and may provide a basis for understanding other cases of core-level BE shifts.

The electronic structure of Al 70 Ni 15 Co 15 decagonal quasi-crystal has been studied by X-ray photoelectron spectroscopy (XPS). The disappearance of the Ni2p(3d 9 ) satellite and the decreased intensity of the 3 F state in the NiL 3 M 45 M 45 Auger spectra indicate a strong d—sp hybridization accompanied by the filling of the Ni3d bands. The filling of the Ni3d bands produces closed-shell type XPS spectra of the Ni core lines, while Co core line XPS spectra show that a significant amount of d holes are still localized on Co. This picture suggests that the magnetic properties are mainly determined by Co. The dispersion of the 3d holes is also responsible for the drop of the 3d states near E F but the angle integrated density of states does not exhibit a pseudo-gap. Finally, the correlation energy, U A , has been calculated from the kinetic energy of the 1 G term of the NiL 3 M 45 M 45 Auger emission.

A new impurity-cluster configuration-interaction model that treats charge transfer and exchange correlations on the same footing is applied to interpret the 3s core-level X-ray photoelectron spectra of NiO, CoO, K 2 NiF 4 and K 2 CoF 4 . The interplay between charge transfer and exchange correlations in determing the final state satellite structures is clarified and an estimate of the exchange integrals between the 3s hole and the 3d electrons is given. The results are consistent with the electronegativity scale, as regards charge transfer energy, and with the trend suggested by the Van Vleck rule, as concerns the exchange integrals.

From a combination of NQR, NMR and AFNMR spectra in Cu1-xLixO information on the electric and magnetic electron-nucleus interactions is obtained. In particular, it is argued that Li+ enters the CuO lattice up to x approximately=4%, with a relatively small effect on the Neel temperature TN. 63Cu NQR and AFNMR, and 7Li NMR spin-lattice relaxation measurements are used to study the spin dynamics in the paramagnetic (PA) and in the antiferromagnetic (AF) phase and the effects associated with the creation of holes upon lithium doping. The Li doping modifies in a dramatic way the spin fluctuations in the AF phase. These effects are related to the fluctuations in the local Cu2+ spin configuration due to the diffusional motion of the extra holes in the electronic bands. A satisfactory explanation of the temperature and frequency dependences of Li relaxation rates is given in terms of a correlation time tau h for the hole motion controlled by an energy gap E(x) and quantitative information on tau h and E(x) are thus derived.

An X-ray photoelectron spectroscopy study of yttria-stabilised zirconia (YSZ) single crystals is reported. It is shown that the shape and the binding energy positions of zirconium and yttrium core levels are independent of the yttria stoichiometry when the content of this oxide ranges from 12 to 24 mol.% of Y2O3. These results could indicate that the local crystallographic order around the Zr4+ and Y3+ ions does not depend on the Zr/Y ratio.

This paper reports an X-ray photoemission study (XPS) of the core levels and the valence band of vanadia-titania catalysts and their precursor oxides. The XPS spectra of the vanadium core levels indicate that surface vanadium atoms are present with different oxidation states. Furthermore, it is demonstrated, for the first time, that the electronic states observed in the XPS valence-band spectra between the Fermi edge and the O 2p bands have mainly a V3d character.

In order to provide experimental support for quantum confinement models describing electronic effects in porous silicon (p-Si), the Si 2s and Si 2p plasmon losses have been studied by x-ray photoelectron spectroscopy. The p-Si plasmon energy was found at a value 0.8÷1.6 eV higher than that of bulk Si (17.4 eV), as measured on the cleaned Si substrate as a reference. The magnitude of these shifts suggests possible quantum confinement effects ascribed to the p-Si nanostructures.

We report an extensive study on the intrinsic bulk electronic structure of the high-temperature superconductor ${\text{CeFeAsO}}_{0.89}{\text{F}}_{0.11}$ and its parent compound CeFeAsO by soft and hard x-ray photoemissions, x-ray absorption, and soft x-ray emission spectroscopies. The complementary surface/bulk probing depth, and the elemental and chemical sensitivity of these techniques allow resolving the intrinsic electronic structure of each element and correlating it with the local structure, which has been probed by extended x-ray absorption fine-structure spectroscopy. The measurements indicate a predominant $4{f}^{1}$ (i.e., ${\text{Ce}}^{3+}$) initial-state configuration for cerium and an effective valence-band-to-$4f$ charge-transfer screening of the core hole. The spectra also reveal the presence of a small $\text{Ce}\text{ }{f}^{0}$ initial-state configuration, which we assign to the occurrence of an intermediate-valence state. The data reveal a reasonably good agreement with the partial density of states as obtained in standard density-functional calculations over a large energy range. Implications for the electronic structure of these materials are discussed.

In this work we report on the measurement of the Young modulus of the external surface of Clostridium tyrobutyricum spores in air with an atomic force microscope. The Young modulus can be reliably measured despite the strong tip-spore adhesion forces and the need to immobilize the spores due to their slipping on most substrates. Moreover, we investigate the disturbing factors and consider some practical aspects that influence the measurements of elastic properties of biological objects with the atomic force microscopy indentation techniques.

In this letter we demonstrate the possibility to determine the temporal and spectral structure (spectrogram) of a complex light pulse exploiting the ultrafast switching character of a non-thermal photo-induced phase transition. As a proof, we use a VO2 multi-film, undergoing an ultrafast insulator-to-metal phase transition when excited by femtosecond near-infrared laser pulses. The abrupt variation of the multi-film optical properties, over a broad infrared/visible frequency range, is exploited to determine, in-situ and in a simple way, the spectrogram of a supercontinuum pulse produced by a photonic crystal fiber. The determination of the structure of the pulse is mandatory to develop new pump-probe experiments with frequency resolution over a broad spectral range (700-1100 nm)., Comment: 4 pages, 3 figures, letter

X-ray photoemission spectroscopy (XPS) and ab initio molecular orbital cluster model wavefunctions are used in a combined study of the electronic structure of CuO. High-resolution XPS spectra for single-crystal CuO identify and clarify several features of the core and valence level spectra. The molecular orbital cluster wavefunctions show that there is a significant covalent contribution to the ionic CuO bond. For the copper core hole final states, the states where the core hole is screened by charge transfer from O2p to Cud lie at lower binding energy than the states where charge transfer does not screen the core hole. The multiplet splitting of the higher binding energy 2p-hole states is large (ca. 3 eV).

Cluster model wavefunctions for the alkaline-earth oxides are analyzed to determine the origin of the trend of the chemical shifts of the binding energies, BEs, of the metal core levels. Two cancelling initial states mechanisms in the oxides are responsible for the trend. One is the increase of the BEs for the dication over the neutral atom. The second is the decrease of the free dication BEs due to the Madelung potential of the ionic oxides. The trend given by the sum of these two contributions to the chemical shifts is in full agreement with experiment. These mechanisms are electrostatic and do not involve chemical bonding effects.

This paper shows that in copper oxide based compounds where Cu ions are in a square planar geometry with four coordination oxygen ions, i.e. CuO and Nd2CuO4, the Cu2p 3 2 (3d)10L) X-ray photoelectron core lines are narrower than in similar compounds with Cu ions in a pyramidal or octahedral geometry with five or six neighboring oxygens, i.e. Bi2Sr2CaCu2O8 and La2CuO4. To explain this difference two different charge transfer mechanisms from the oxygen p orbitals to the Cu3d orbitals are proposed. For square planar geometry (CuO4) only the px,y → dx2 - y2 charge transfer is allowed, whereas for octahedral or pyramidal geometry the additional pz → dz2 transition could take place. These two different final state configurations will produce different ionization potentials for the Cu2p 3 2 core levels.

We report the experimental evidence of an abrupt transition of the ultrafast electronic response of underdoped superconducting ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$, under the impulsive photoinjection of a high density of excitations, using ultrashort laser pulses and avoiding significant laser heating. The direct proof of this process is the discontinuity of the transient optical electronic response, observed at a critical fluence of ${\ensuremath{\Phi}}_{\text{th}}\ensuremath{\simeq}70\text{ }\ensuremath{\mu}\text{J}/{\text{cm}}^{2}$. Below this threshold, the recovery dynamics is described by the Rothwarf-Taylor equations, whereas, above the critical intensity, a fast electronic response is superimposed to a slower dynamics related to the superconductivity recovery. We discuss our experimental findings within the frame of the available models for nonequilibrium superconductivity, i.e., the ${T}_{\text{eff}}$ and ${\ensuremath{\mu}}_{\text{eff}}$ models. The measured critical fluence is compatible with a first-order photoinduced phase transition triggered by the impulsive shift of the chemical potential. The measured value, significantly in excess of the condensation energy, indicates that, close to the threshold, the largest amount of energy is delivered to phonons or to other gap-energy excitations strongly coupled to Cooper pairs.

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.

The high-order harmonic generation (HHG) of coherent light pulses in gases is strictly dependent on the ionization dynamics that delocalizes the electron wave function in the laser field continuum. Here we report on the investigation of such dynamics by varying the energy of a 400 nm, 60 fs driving laser pulse. The experimental data provide the unambiguous evidence of a hybrid dynamical region, not described by the full-tunneling approximation models, where the HHG photon number exponentially decays when the Keldysh parameter is varied from 1 to 3. Finally, a phenomenological model, suitable to account for this hybrid regime, is reported.

The mechanics of two pendulums coupled by a stressed spring is discussed, and the behavior for small oscillations is described. When the system is in its highest symmetry configuration, the pendulums are independent and the normal frequencies are degenerate.

We report polarization-dependent on-resonance and off-resonance photoemission and x-ray absorption measurements to probe the character and the symmetry of the occupied and empty valence-band (VB) states of the four-layered misfit cobaltate ${[{\text{Bi}}_{2}{\text{Ca}}_{2}{\text{O}}_{4}]}^{\text{RS}}\ensuremath{\cdot}{[{\text{CoO}}_{2}]}_{1.67}$. These experiments bring clear evidence that the one-electron removal VB photoemission spectra, between the Fermi energy $({E}_{F})$ and $\ensuremath{\sim}4.5\text{ }\text{eV}$, contain $\text{Co}\text{ }3d$ states, which are strongly hybridized with $\text{O}\text{ }2p$ states in the binding energy region of $\ensuremath{\sim}1.8--\ensuremath{\sim}4.5\text{ }\text{eV}$ below ${E}_{F}$. The states at the Fermi energy have ${a}_{1g}$ out-of-plane symmetry. In addition, by tuning the incident photon energy across the $\text{Co}\text{ }2{p}_{3/2}\ensuremath{\rightarrow}\text{Co}\text{ }3d$ $({a}_{1g})$ threshold and the $\text{O}\text{ }1s\ensuremath{\rightarrow}\text{O}\text{ }2p--\text{Co}\text{ }3d$ $({a}_{1g})$ hybridized threshold, both found at $\ensuremath{\sim}2\text{ }\text{eV}$ below the absorption edge maximum, it is possible to observe a transition from a Raman regime to an Auger regime. From this transition the charge transfer dynamics from the $\text{Co}\text{ }3d$ to the $\text{O}\text{ }2p$ states and from the $\text{O}\text{ }2p$ to the $\text{Co}\text{ }3d$ states have been estimated to be $\ensuremath{\sim}3$ and $\ensuremath{\sim}3.3\text{ }\text{fs}$, respectively. This combined information suggests an electron hopping process between neighboring $\text{Co}\text{ }{a}_{1g}$ states across ${E}_{F}$ and mediated by $\text{O}\text{ }2p$ states with suitable symmetry.

In this work an optical pump‐probe set‐up is used to study the photo‐induced non‐equilibrium dynamics of a superconducting underdoped Bi2212 single crystal in a strong excitation regime (10

Time-and-frequency resolved pump-probe optical spectroscopy is used to investigate the effect of the impulsive injection of delocalized excitations through a charge-transfer process in insulating CuGeO3. A large broadening of the charge-transfer edge is observed on the sub-ps timescale. The modification of this spectral feature can not be attributed to the local increase of the effective temperature, as a consequence of the energy absorbed by the pump pulse. The measured modifications of the optical properties of the system are consistent with the creation of a non-thermal state, metastable on the ps timescale, after the pump-induced impulsive modification of the electron interactions., Comment: 8 pages, 5 figures. Published on Physical Review B.

The thermal dynamics induced by ultrashort laser pulses in nanoscale systems, i.e. all-optical time-resolved nanocalorimetry is theoretically investigated from 300 to 1.5 K. We report ab-initio calculations describing the temperature dependence of the electron-phonon interactions for Cu nanodisks supported on Si. The electrons and phonons temperatures are found to decouple on the ns time scale at 10 K, which is two orders of magnitude in excess with respect to that found for standard low-temperature transport experiments. By accounting for the physics behind our results we suggest an alternative route for overhauling the present knowledge of the electron-phonon decoupling mechanism in nanoscale systems by replacing the mK temperature requirements of conventional experiments with experiments in the time-domain., Comment: 5 pages, 3 figures. Accepted on Physical Review B.

We present a theoretical framework allowing to properly address the nature of surface-like eigenmodes in a hypersonic surface phononic crystal, a composite structure made of periodic metal stripes of nanometer size and periodicity of 1 micron, deposited over a semi-infinite silicon substrate. In surface-based phononic crystals there is no distinction between the eigenmodes of the periodically nanostructured overlayer and the surface acoustic modes of the semi-infinite substrate, the solution of the elastic equation being a pseudo-surface acoustic wave partially localized on the nanostructures and radiating energy into the bulk. This problem is particularly severe in the hypersonic frequency range, where semi-infinite substrate's surface acoustic modes strongly couple to the periodic overlayer, thus preventing any perturbative approach. We solve the problem introducing a surface-likeness coefficient as a tool allowing to find pseudo-surface acoustic waves and to calculate their line shapes. Having accessed the pseudo-surface modes of the composite structure, the same theoretical frame allows reporting on the gap opening in the now well-defined pseudo-SAW frequency spectrum. We show how the filling fraction, mass loading and geometric factors affect both the frequency gap, and how the mechanical energy is scattered out of the surface waveguiding modes., 8 pages, 7 figures, 2 tables. Published on Phys. Rev. B

We have performed angle-resolved-photoemission experiments and local-density-functional (LDA) band calculations on NiO to study correlation and band effects of this conceptually important compound. Our experimental result suggests a dual nature of the electronic structure of NiO. On the one hand, the LDA band calculation has some relevance to the electronic structure of NiO, and the inclusion of the antiferromagnetic order is essential. For the lower O 2p bands, the LDA calculation agrees almost perfectly with experimental energy positions and dispersion relations. On the other hand, discrepancies between the experiment and the LDA calculation do exist, especially for the Ni 3d bands and the O 2p bands that are heavily mixed with the Ni 3d bands. It appears that the main discrepancies between the experimental results and the LDA calculation are concentrated in the regions of the insulating gap and the valence-band satellite. In addition to these results, we also report the interesting angle and photon-energy dependence of the satellite emission. The above results show that the angle-resolved-photoemission studies can provide much additional information about the electronic structure of correlated materials like NiO.

We report off-normal angle-resolved photoemission data from NiO(001), which is complementary to the earlier normal-emission result. On the one hand, the energy position of the bands along this direction and the dispersion of the lowest oxygen band agree with the band calculation very well. On the other hand, the existence of the valence band satellite and the absence of the energy band in the gap region do not agree with the band calculation. For the first time, interesting movement of the Ni d7 satellite has been observed as a function of the emission angle and the photon energy, which is most likely a result of the satellite dispersion.

Sintered bulk samples of the orthorhombic, superconducting, phase of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ exhibit a \ensuremath{\simeq}1.5 eV Ba core-level decrease compared to the tetragonal, nonsuperconducting phase. This is also observed for thin-film material in the two phases. Using ab initio cluster calculations, we show that shifts of this magnitude are explainable by isotropic contraction of the Ba-O bond length in the 0.01- to 0.02-\AA{} range, values compatible with the known lattice-parameter changes between the phases. Finally, even in situ cleaved good-quality single crystals of the orthorhombic phase show a chemically shifted Ba component for some regions of the surface, which may be due to Ba-O bond-length contractions at the surface-vacuum interface.

High-quality ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ superconducting single crystals, annealed at different oxygen partial pressures, have been studied using angular-resolved x-ray photoelectron spectroscopy with a resolution higher than that used in any previous study. Two states of the oxygen, separated by \ensuremath{\approxeq}0.7 eV, are unambiguously observed. Examining these components at different angles makes it possible to distinguish bulk from surface components. Using this capability we discover that annealing under lower oxygen partial pressure (1 atm) results in oxygen intercalation beneath the Bi-O surface layer of the crystal, whereas for higher-pressure anneals (12 atm) additional oxygen is found on the Bi-O surfaces. This steplike intercalation mechanism is also confirmed by the changes observed in the Cu and Bi core lines as a function of the annealing oxygen partial pressure.

The electronic structure in the normal state of ${\mathrm{CeFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ oxypnictide superconductors has been investigated with x-ray absorption and photoemission spectroscopy. All of the data exhibit signatures of Fe $d$-electron itinerancy. Exchange multiplets appearing in the Fe $3s$ core level indicate the presence of itinerant spin fluctuations. These findings suggest that the underlying physics and the origin of superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors. These materials provide opportunities for elucidating the role of magnetic fluctuations in high-temperature superconductivity.

We present the experimental evidence of the interplay between the variation of the linewidth and effective mass of electrons in image potential states on a Cu(1 1 1) surface induced by a selective alteration of the hot-electron gas density generated by ultra-short laser pulse excitations. (C) 2008 Elsevier B.V. All rights reserved.

We present results of resonant-photoemission and high-resolution x-ray-photoemission-spectroscopy (XPS) studies on single-crystalline CuO, which confirms most of the earlier data obtained from polycrystalline samples. However, some minor differences with earlier results were also observed, which are concentrated on a mainly oxygen-related feature. The important first ionization state is much better resolved in our high-resolution XPS data, and is found to have both copper and oxygen orbital character. This is consistent (though not a proof) with the theoretical prediction that the first ionization state is a {sup 1}{ital A}{sub 1{ital g}} singlet with holes on both oxygen and copper sites. The 16-eV satellite is found to show a Cu antiresonance, and is assigned to a {sup 1}{ital A}{sub 1{ital g}} singlet with both holes on the copper sites.

The modification of the morphological and adhesive binding of PAFC electrodes has been studied by electron spectroscopy and electron microscopy with regard to their endurance and preparative treatments. The corrosion of the electrodes has been found to dramatically affect the electrical and adhesive contact between PTFE (polytetrafluoroethylene) and active carbon. This has been detected in X-ray photoelectron spectroscopy (XPS) by the ‘charging’ shift seen for F 1 s due to the loss in electronic contact between these components. In addition, the adhesive character of the contact between the SiC matrix and the gas diffusion electrodes has been revealed by the XPS spectral characteristics. Here a major difference between the anode and the cathode has been observed with the cathode having a greater deadhesion. Extensive physical characterization by pore volume and size distribution, wetting, and weight loss has been performed. These results have been correlated with the XPS results to give a complete model of the deterioration of this technologically important system. A possible explanation for the observed loss of PTFE from the surface of the cathode based on an electrostatic rejection model of the delaminated PTFE particles is also suggested.

Li/Ni ratios in LixNi1 − xO films were determined by simultaneous use of the PIXE (particle-induced X-ray emission) and PIGE (proton-induced gamma-ray emission) techniques using the 3.5 MeV proton beam provided by the CISE tandem Van de Graaff accelerator. The general features of the experimental setup are presented and discussed along with the results on the dependence of Li/Ni ratios on the e-beam evaporation and thermal treatments. Preliminary results obtained by XRD (X-ray diffraction) are also discussed. The combined use of PIXE and PIGE proves to be a unique opportunity for nondestructive measurement in thin films and, through the correlation with structural data, it allows a careful optimization of the deposition process and thermal treatment parameters to be obtained.

This paper reports the experimental evidence for morphological changes that occur at the electrodes of phosphoric acid fuel cells (PAFC) under very severe (OCV) conditions. In addition, the dissolution of Pt-supported crystallites in H3PO4 is demonstrated by Transmission Electron Microscopy (TEM) confirming previous results already reported in the literature. Finally, the experimental data are discussed on the basis of an analysis of the reaction rates and models for the corrosion mechanism is postulated.

In this paper an X-ray photoelectron Spectroscopy (XPS) study of the interaction between oxygen and Pt clusters on Teflon and on SiO 2 supports is presented. Unambiguous experimental data give direct evidence of the oxidation of Pt clusters. PtO x appears to form spontaneously in conditions where, in contrast, a continuous Pt surface does not oxidize. It is also shown that the oxidation rate depends on the coverage and on the nature of the substrate. These results are interpreted in terms of the relative occupations of the Pt s and d orbitals, the effective coordination number of the Pt cluster atoms and the different shape of the clusters on Teflon or on SiO 2 supports.

The absence of a clear signature of the Zhang-Rice singlet in the photoelectron valence band spectra of copper germanate raised the question of the stability of this state, induced by one-electron removal processes, in large-gap systems. Following Okada and Kotani [Phys. Rev. B 63, 045103 (2001)], we have performed a study of the O $1s$ resonant x-ray emission for $\mathrm{Cu}{\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}{\mathrm{O}}_{3}$ ($x=0$, 0.05, and 0.1) single crystals as a function of temperature and polarization. Our experiment shows that the temperature, angle, and polarization dependence of the inelastic scattering emission detected at a $3.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ Raman shift in the O $1s$ resonant x-ray spectra can be unambiguously identified with the Zhang-Rice-singlet excitation.

The electronic structure of the magnetic refrigerant Gd(5)Ge(2)Si(2) has been experimentally investigated by photoemission and x-ray absorption spectroscopy. The resonant photoemission and x-ray absorption measurements performed across the Gd N(4,5) and Gd M(4,5) edges identify the position of Gd 4f multiplet lines, and assess the 4f occupancy (4f(7)) and the character of the states close to the Fermi edge. The presence of Gd 5d states in the valence band suggests that an indirect 5d exchange mechanism underlies the magnetic interactions between Gd 4f moments in Gd(5)Ge(2)Si(2). From 175 to 300 K the first 4 eV of the valence band and the Gd partial density of states do not display clear variations. A significant change is instead detected in the photoemission spectra at higher binding energy, around 5.5 eV, likely associated to the variation of the bonding and antibonding Ge(Si) s bands across the phase transition.

The ultrafast thermal and mechanical dynamics of a two-dimensional lattice of metallic nano-disks has been studied by near infrared pump-probe diffraction measurements, over a temporal range spanning from 100 fs to several nanoseconds. The experiments demonstrate that, in these systems, a two-dimensional surface acoustic wave (2DSAW), with a wavevector given by the reciprocal periodicity of the array, can be excited by ~120 fs Ti:sapphire laser pulses. In order to clarify the interaction between the nanodisks and the substrate, numerical calculations of the elastic eigenmodes and simulations of the thermodynamics of the system are developed through finite-element analysis. At this light, we unambiguously show that the observed 2DSAW velocity shift originates from the mechanical interaction between the 2DSAWs and the nano-disks, while the correlated 2DSAW damping is due to the energy radiation into the substrate., 13 pages, 10 figures

The charge transfer dynamics of an underdoped manganite is probed by resonant x-ray emission and Auger electron spectroscopies at the $\mathrm{Mn}\phantom{\rule{0.3em}{0ex}}{L}_{3}$ edge. The dispersion of peak positions, as the incident photon energy is tuned through the absorption threshold, gives direct information on the relaxation rate in terms of the core-hole lifetime and the localization behavior of the states involved. The charge transfer time is found to be around $0.45\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, with a dependency on the final state $d$-band occupancy.

Recent advances in both the experimental resolution and in the computational capabilities motivate new studies of surface properties. In particular, a detailed comparison between theoretical and experimental data is expected to provide a better insight into surface and image states. In this work we present a joint effort analyzing such features of the Cu(111) and Cu(001) surfaces. The experiments are performed by using both linear and non-linear angle-resolved photoemission. From the theoretical point of view, we make use of the Green function embedding technique within density functional theory. We are able to account for the image states by suitably modifying the effective potential in the Kohn-Sham equation and the generalized boundary conditions on the Green function. Comprehensive theoretical and experimental results on the effective mass and the binding energy of the Shockley state and the first image states are reported. (c) 2006 Elsevier B.V. All rights reserved.

We have investigated for the first time the orbital ordering in a three-dimensional colossal magnetoresistance manganite, namely La(7/8)Sr(1/8)MnO3, by applying soft X-ray linear dichroism (XLD) to the Mn L edge. We found that the cooperative Jahn-Teller distorted orthorhombic phase, which is present at a temperature of 240 K, is probably accompanied by a predominantly cross type (x2 - z2)/(y2 - z2) orbital ordering. This result is discussed in the light of different exchange interaction models.