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Momentum resolved inverse photoemission spectroscopy measurements show that the dispersion of the unoccupied bands of Ni2MnGa is significant in the austenite phase. Furthermore, in the martensite phase, it is markedly reduced, which is possibly related to the structural transition to an incommensurate modulated state in the martensite phase. Finally, based on the first principle calculations of the electronic structure of Ni–Mn–Ga, we show that the modification of the spectral shape with surface composition is related to change in the hybridization between the Mn 3d and Ni 3d-like states that dominate the unoccupied conduction band.

The electronic structure of nearly stoichiometric Ni–Mn–Sn and Ni–Mn–In surface is investigated by inverse photoemission and photoemission spectroscopy. Comparison of the experimental and calculated inverse photoemission spectra shows that the dominant feature is related to Mn 3 d -like states. The overall shape and peak position of the theoretically obtained spectra show good agreement with the experimental ultraviolet photoemission valence band spectra. The changes in the composition dependent ultraviolet photoemission spectra reveal the change in degree of Ni 3 d and Mn 3 d band hybridization. Both inverse photoemission and ultraviolet photoemission study show a rigid band shift between Ni 2 MnIn and Ni 2 MnSn because of band filling, due to increase in the number of 5 p electrons from In to Sn. Mn 2 p and 3 s core-level reveal unambiguous existence of exchange splitting in both the materials.

This article presents a brief overview of our recent work on the nature of long period modulation in the premartensite and martensite phases of Ni2MnGa ferromagnetic shape memory alloy using high resolution synchrotron x-ray powder diffraction patterns. The commensurate structure model using the Pnnm space group is unable to account for the peak positions of the satellite reflections that appear due to modulations correctly. LeBail and Rietveld refinements using the (3+1)-D super space group $Immm(00\gamma)s00$ show that the peak positions of all the reflections, including the satellites, can be explained satisfactorily using incommensurate modulations for both the premartensite and martensite phases. The incommensurate modulation vectors are found to be q= 0.33761(5)c* = (1/3+$\delta$1)c* and 0.43160(3)c*= (3/7+$\delta$2)c*, where $\delta$1 and $\delta$2 are the degrees of incommensuration for the premartensite and martensite phases, respectively. The periodicity of the closest rational approximant of the premartensite and martensite phases are confirmed to be 3M and 7M, respectively, in agreement with single crystal diffraction results., Comment: arXiv admin note: text overlap with arXiv:1405.3478

Mn 0.3 Ni 0.3 Zn 0.4 Fe 2 O 4 ferrite nanoparticles were synthesized by thermally initiated autocatalytic decomposition of metal fumarato-hydrazinate complex. The chemical phase analysis has been carried out by X-ray powder diffraction method. The formation of complete ferrite phase occurred when the sample was annealed at 1100 °C. The physical properties such as lattice parameter, bulk density and porosity were studied. The infra red spectra displayed two bands characteristics of spinel ferrite due to M–O stretching vibrations in tetrahedral and octahedral sites. The X-ray photoelectron and Mossbauer spectroscopy were used to study the valence and distribution of cations in tetrahedral (A) and octahedral [B] sites of the spinel. The frequency dependence of the dielectric properties such as dielectric constant, e ′, complex dielectric constant, e ″ and dielectric loss factor, tan δ , were studied at variable temperatures and reported in this paper.

ZnO epitaxial thin films were grown on p-type Si(100) substrates by dual ion beam sputtering deposition system. The crystalline quality, surface morphology, optical and electrical properties of as-deposited ZnO thin films at different growth temperatures were studied. Substrate temperature was varied from 100 to 600 °C at constant oxygen percentage O2/(O2 + Ar) % of 66.67 % in a mixed gas of Ar and O2 with constant chamber pressure of 2.75 × 10−4 mBar. X-Ray diffraction analyses revealed that all the films had (002) preferred orientation. The minimum value of stress was reported to be −0.32 × 1010 dyne/cm2 from ZnO film grown at 200 °C. Photoluminescence measurements demonstrated sharp near-band-edge emission (NBE) was observed at ~375 nm along with deep level emission (DLE) in the visible spectral range at room temperature. The DLE Peak was found to have decrement as ZnO growth temperature was increased from 200 to 600 °C. The minimum FWHM of the NBE peak of 16.76 nm was achieved at 600 °C growth temperature. X-Ray photoelectron spectroscopy study revealed presence of oxygen interstitials and vacancies point defects in ZnO film grown at 400 °C. The ZnO thin film was found to be highly resistive when grown at 100 °C. The ZnO films were found to be n-type conducting with decreasing resistivity on increasing substrate temperature from 200 to 500 °C and again increased for film grown at 600 °C. Based on these studies a correlation between native point defects, optical and electrical properties has been established.

From the first principles calculation based on the density functional theory, we find that consideration of electron–electron correlation drives Fe2CrSn from a barely half-metallic state to a magnetic semiconductor. However, in both cases, the magnetic state remains ferrimagnetic with a total spin moment of 2 μ B , with Cr carrying a large local moment that is oppositely aligned to that of Fe.

We present a comparative study of the near fermi-level electronic structure of single crystal and highly oriented pyrolytic graphite (HOPG). Angle resolved photoelectron spectroscopy and angle resolved inverse photoelectron spectroscopy have been used to probe the occupied and unoccupied electronic states, respectively. The band dispersions showed by single crystal graphite along its Γ K and Γ M symmetry directions were found to be in agreement with calculated band structure of graphite. The π bands of single crystal graphite were found to have a splitting of ∼ 0.5 eV at the K-point. We also observe the presence of a quasiparticle peak below EF at the K point at low temperature which indicates a strong electron–phonon coupling in graphite. In HOPG, the M and K points like features were found to be present in the same radial direction due to the superposition of the Γ M and Γ K directions. Results from our angle resolved inverse photoemission spectroscopy present the dispersion of the conduction band states, particularly the lower π ⁎ band. We have also found the presence of some non-dispersive features in both the valence and the conduction bands.

Ni2MnGa(100) single crystal studied using low energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) exhibits interesting modification of the surface properties that are mainly influenced by surface composition as well as intrinsic effects. In the martensite phase, the LEED spot profiles show presence of an incommensurate modulation for the stoichiometric surface. In contrast, a commensurate modulation is observed for Mn-excess Ni–Mn–Ga surface. A pre-martensite phase is identified at the surface. Both the surface martensitic and pre-martensitic transition temperatures decrease as the Mn content increases. The UPS spectra in the austenite phase exhibit systematic change in shape as a function of surface composition that can be related to changes in the hybridization between Ni and Mn 3d states. The spectra in the martensite phase exhibit interesting modifications near the Fermi level, which has been compared to density of states calculated for a modulated structure by ab-initio density functional theory. Intrinsic surface properties dissimilar from the bulk are enhanced hysteresis width of the martensite transition and increased pre-martensitic transition temperature.

Nanocrystalline zinc oxide (ZnO) thin films have attracted great attention due to their possible applications in optoelectronic devices. The dependence of size and catalyst (indium and fluorine) in ZnO thin films synthesized by spray pyrolysis technique was studied. To understand the chemical bonding structure and electronic states of the catalyst based ZnO thin films, XPS analysis was carried out. A comprehensive micro-Raman scattering study of the phonons in catalyst based ZnO thin films grown in ambient air was made. Low temperature dependent Raman spectroscopy studies were carried out for the detailed structures, vibrations, and in situ size variation of nanostructured thin films. The size and catalyst dependence of the coupling strength between electron and LO phonon was experimentally estimated. The coupling strength determined by the ratio of second to first-order Raman scattering cross sections was found to diminish with decreasing crystallite size, and the Frohlich interaction plays the vital role in electron–phonon-coupling in ZnO. Influence of catalyst on the luminescence properties was investigated.

The authors report a comparative study on ex situ passivation of a near-surface GaAs0.86P0.14/Al0.6Ga0.4As quantum well using various sulfide solutions and nitrogen plasma treatments. The built-in surface electric field is changed via band bending by applying various surface passivation conditions. The band bending is measured using x-ray photoelectron spectroscopy. Reduction in surface electric field in the range of 10–35 kV/cm is observed depending on different passivation conditions. The photoreflectance spectra show enhancement in intensity and blueshift of ∼3 meV accompanied by significant reduction in the broadening parameter of the observed e1-lh1 transitions. Among all the methods studied here, passivation by Na2S⋅xH2O is found to be most effective as it removes the native oxide layer completely leading to almost flat band condition.

Zinc oxide (ZnO) thin films have been deposited with various substrate temperatures by spray pyrolysis technique onto glass substrates. X-ray diffraction (XRD) results showed the random growth orientation of the crystallites and the presence of the wurtzite phase of ZnO. The x-ray photoelectron spectroscopy (XPS) measurements reveal the presence of Zn 2+ and chemisorbed oxygen in ZnO thin films. Atomic force micrograms (AFM) revealed a granular, polycrystalline morphology for the films. The grain size is found to increase as the substrate temperature increases. All films exhibit a transmittance of about 85% in the visible region. The photoluminescence (PL) measurements indicated that the intensity of emission peaks significantly varying with substrate temperature. Electrical resistivity has been found to decrease; while the carrier concentration increases with substrate temperature.

We have studied the occupied and unoccupied electron states of the Pr 1 - x Ca x MnO 3 ( x = 0.2 , 0.33 and 0.4) near the Fermi level across their ferromagnetic–antiferromagnetic phase boundary using ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The photoemission data show that the pseudogap formation in these compounds occur over an energy scale of 0.48 ± 0.02 eV . From a combined analysis of the photoemission and inverse photoemission results, we have estimated the charge transfer energy in these compounds to be ∼ 2.8 ± 0.2 eV . The results have been explained on the basis of the theoretical models that describe the electronic phase separation.

The martensitic transition and the ferro- to paramagnetic transition have been studied in a series of Ga excess Ni-Mn-Ga specimens [Ni2-xMnGa1+x (0.4≤ x≤ 0.9)] by differential scanning calorimetry and magnetization measurements. The martensitic transition exhibits a hysteresis whose width is similar to Ni2MnGa, indicating that the transition is thermoelastic. The latent heat of transformation is comparable with other Ni-Mn-Ga alloys. A substantial increase in the martensitic transition temperature is observed due to Ga doping. Interestingly, the x-ray diffraction pattern of all the compositions studied show a modulated martensitic structure in the martensitic phase.

Evolution of domain structures across the martensitic transition (Tm) in the ferromagnetic shape memory alloy system Ni-Mn-Ga is studied using an optical microscope with a temperature variation. Compositions chosen have Tm < Tc, Tm = Tc and Tm > Tc, (Tc=Curie temperature) so that one can compare the nature of martensitic domains. There are no appreciable domain structures when Tm < Tc as compared to the one with Tm > Tc. However, giant morphological changes in the form of appearance of well-developed domains that are propagating with different directions are seen for the composition in which Tm=Tc. The results are discussed in the light of Magnetic Force Microscopy observations as well as giant entropy changes known to occur on samples with co-occurrence of Tm and Tc.

Nanocrystalline cerium oxide (CeO 2 ) thin films were deposited onto the fluorine doped tin oxide coated glass substrates using methanolic solution of cerium nitrate hexahydrate precursor by a simple spray pyrolysis technique. Thermal analysis of the precursor salt showed the onset of crystallization of CeO 2 at 300 °C. Therefore, cerium dioxide thin films were prepared at different deposition temperatures from 300 to 450 °C. Films were transparent ( T ~ 80%), polycrystalline with cubic fluorite crystal structure and having band gap energy (Eg) in the range of 3.04–3.6 eV. The different morphological features of the film obtained at various deposition temperatures had pronounced effect on the ion storage capacity (ISC) and electrochemical stability. The larger film thickness coupled with adequate degree of porosity of CeO 2 films prepared at 400 °C showed higher ion storage capacity of 20.6 mC cm − 2 in 0.5 M LiClO 4 + PC electrolyte. Such films were also electrochemically more stable than the other studied samples. The Ce 4+ /Ce 3+ intervalancy charge transfer mechanism during the bleaching–lithiation of CeO 2 film was directly evidenced from X-ray photoelectron spectroscopy. The optically passive behavior of the CeO 2 film (prepared at 400 °C) is affirmed by its negligible transmission modulation upon Li + ion insertion/extraction, irrespective of the extent of Li + ion intercalation. The coloration efficiency of spray deposited tungsten oxide (WO 3 ) thin film is found to enhance from 47 to 53 cm 2 C − 1 when CeO 2 is coupled with WO 3 as a counter electrode in electrochromic device. Hence, CeO 2 can be a good candidate for optically passive counter electrode as an ion storage layer.

The irradiation effect in Ni 3 N/Si bilayers induced by 100 MeV Au ions at fluence 1.5 × 10 14 ions/cm 2 was investigated at room temperature. Grazing incidence X-ray diffraction determined the formation of Ni 2 Si and Si 3 N 4 phases at the interface. The roughness of the thin film was measured by atomic force microscopy. X-ray reflectivity was used to measure the thickness of thin films. X-ray photoelectron spectroscopy has provided the elemental binding energy of Ni 3 N thin films. It was observed that after irradiation (Ni 2p 3/2 ) peak shifted towards a lower binding energy. Optical properties of nickel nitride films, which were deposited onto Si (100) by ion beam sputtering at vacuum 1.2 × 10 −4 torr, were examined using Au ions. In-situ I – V measurements on Ni 3 N/Si samples were also undertaken at room temperature which showed that there is an increase in current after irradiation.

The electronic and structural properties of different members of the Ni-Mn-Ga family calculated by ab initio density functional theory are discussed. From total energy calculations, we show that the martensitic phase is the stable low temperature phase. Moreover, occurrence of ferromagnetic and paramagnetic martensitic phases for Ni2MnGa and Ni2.25Mn0.75Ga, respectively, are explained. Modifications in the density of states near the Fermi level EF are observed across the martensitic transition for Ni2MnGa, as well as in Mn2NiGa. While Ni2MnGa is ferromagnetic, we find Mn2NiGa to be ferrimagnetic. The calculated lattice constants and the magnetic moments are in good agreement with experiment.

The XANES studies at Mn, Ni and Ga K-edge of Ni2MnGa compound have been carried out at room and low temperatures. The Mn K-edge and Ni K-edge spectra shows modulation in the post edge features when the sample is cooled below martensitic transition temperature. It is strongly reflected in the XANES of Mn K-edge where the peak after the edge gets totally suppressed when the sample is in martensitic phase. This peak shows a hysteretic behaviour when thermal cycling was done across the martensitic transition temperature. This clearly shows that the peak height is a measure of austenitic phase present at a particular temperature. This demonstrates the strong correlations of electronic states and crystal structures in these compounds.

A detailed kinetics study of the first-order structural transition in virgin NiTi, Ni47Fe3Ti50, and Ni2+xMn1-xGa (x= 0 and 0.26) manifests the varying role of renucleation-driven austenitic growth with the doping-induced disorder and the magnetization state. The austenite transitions were investigated using differential scanning calorimeter (DSC) at heating rates spanning over a decade. They revealed the existence of two Arrhenius processes, with their relative presence, nucleationbarrier energies, and validity-timescales suggesting that both intra- and inter-domain texturalorderings undergo de-structuring. In the stoichiometric Ni2MnGa, a single low-energy barrier ( ) fast kinetics observed may be attributed wholly to the short-distance textural order-disorder (a near absence of bigger, inter-domain interactions). On the other hand, two distinct Arrhenicities are found in equal strength in NITINOL (NiTi) and Ni47Fe3Ti50, and in unequal proportion in Ni2.26Mn0.74Ga, over the full range of temperature scanning rates covered (q= 2.5 to 50°C/min). The relatively fast nucleation-driven growth dominates higher T-scanning rates, with lower barrier activation (qhi) (albeit > , due to a change in the twins’ character). Another kinetics with higher barrier energy (qlo) manifests at slow heatings. The crossover in Ni47Fe3Ti50 is interpreted as increase in the (disorder-induced) A-domain-size dispersion, which also causes a broadening of the transition. Parameters characterizing the kinetics of various specimens are examined; comparisons of the relative energy/time scales of inter- and intra-domain processes made, and their transition/crossover temperature discussed.

Influence of structural transition in the evolution of the magnetic domains in the ferromagnetic shape memory alloy system Ni2+xMn1-xGa is reported here using Magnetic Force Microscopy (MFM) studies. Studies reported are with two samples with their martensite transition temperature TM less than and greater than the Curie temperature Tc. Present results show an evolution of MFM across the Tc with a clear twin domains and sub domain structures inside the twins. The higher spatial resolution of MFM (~50nm) as compared to optical microscope (400nm) is useful in probing the domain walls. Force derivative of the MFM signal that may be used as an order parameter seems to scale the onset of magnetic order in the system. One can clearly see the vanishing of the MFM patterns for T>Tc. Results are discussed in the light of models available for tip-sample interactions that track the local magnetization.

We report the structural studies on Mn excess and Ga deficient Ni2Mn1+zGa1-z specimens with z= 0, 0.05, 0.1, 0.15, 0.2 and 0.25. The crystal structure at room temperature was determined by the x-ray diffraction (XRD). Rietveld analysis has been performed to obtain the lattice parameters. For z= 0, 0.05 and 0.1, a cubic austenitic phase is observed. For 0.15≤ z ≤0.25, a tetragonal martensitic phase is obtained, whose lattice constant c increases and a decreases linearly with increasing z following Vegard’s law. Phase coexistence is observed for z= 0.15, confirming the first-order nature of the martensitic transition.

We report a detailed investigation of the magneto-transport and magnetic properties of Mn excess Ni-Mn-Ga using the resistivity and magnetization measurements. Magnetoresistance (MR) has been measured in the ferromagnetic state for different compositions in the austenitic, premartensitic and martensitic phases. With Mn doping in Ni2-yMn1+yGa, a decrease in magnetization and MR has been found, since the doped Mn atoms in Ni position are in the antiferromagnetic configuration with the Mn atoms in Mn position. MR for the parent stoichiometric composition Ni2MnGa varies almost linearly with field in the austenitic and pre-martensitic phases, and shows a cusp-like shape in the martensitic phase. This has been explained by the changes in twin and domain structures in the martensitic phase. Hysteresis in the heating and cooling cycles is a characteristic of the first order nature of the martensitic phase transition.

Growth and electronic structure of a 3d transition metal Mn on a free-electron-like metal Al(1 1 1) have been studied by photoelectron spectroscopy and low energy electron diffraction (LEED). A ( 3 × 3 ) R 30 ° LEED is observed at 6 ML Mn coverage, that is related to the α phase of bulk Mn. From the intensity of the adlayer and substrate core-level peaks and angle dependent studies, evidence of primarily layer by layer growth is observed. The Mn 2p core-level for 0.1 ML Mn coverage appears at 0.3 eV lower binding energy with respect to bulk Mn. With increasing Mn coverage, the Mn 2p binding energy increases. An extra component is observed at the lower binding energy side of the Al 2p spectra, that is related to interface alloying. The asymmetric line-shape of bulk-like Mn becomes symmetric for lower Mn coverages.

The formation of ⊥c texture of WS 2 thin films by solid state reaction between the spray deposited WO 3 and gaseous sulfur vapours with Ni interfacial layer has been reported. X-ray diffraction technique has been used to measure the degree of preferred orientation and texture of WS 2 films. Scanning electron microscopy, transmission electron microscopy and atomic force microscopy have been used to characterize the microstructure and morphology. The electronic structure and chemical composition was studied using X-ray photoelectron spectroscopy. The WS 2 films comprise single crystalline quality hexagonal crystallites of 15 μm × 15 μm size with their basal planes parallel to the substrate. The film consists of turbostratic stacking sequence of 2H and 3R polytypes of WS 2 . The tungsten-to-sulfur ratio was estimated to be 1:1.8. The various qualitative models used to explain promotional effects are briefly outlined and the plausible underlying mechanism of formation of ⊥c texture with nickel, in this study, is given.

Highly textured tungsten disulfide (WS2) thin films have been obtained by sulfurization of tungsten trioxide. The properties of WS2 thin films prepared with bismuth interfacial layer as texture promoter has been studied. The WS2 thin films were found to have predominant type-II orientation. The stacking of 2H-WS2 crystallites observed with scanning electron microscopy was not reported hitherto. The films can be pictured as an assembly of WS2 hexagonal crystallites. The energy dispersive X-ray analysis and X-ray photoelectron spectroscopy (XPS) studies confirm that the films are stoichiometric. The XPS analysis described the local environment of the tungsten atoms and the formal oxidation states of the tungsten and sulfur atoms were + 4 and − 2. Together with the high degree of crystallinity and excellent texture of the film, a relatively smooth morphology, on submicron scale, is revealed through atomic force microscopy study. The conditions for the desired textured growth with the van der Waals planes parallel to the substrate surface are reported.

We present here results of temperature dependent high resolution synchrotron x-ray powder diffraction study of sequence of phase transitions in Ni2MnGa. Our results show that the incommensurate martensite phase results from the incommensurate premartensite phase, and not from the austenite phase assumed in the adaptive phase model. The premartensite phase transforms to the martensite phase through a first order phase transition with coexistence of the two phases in a broad temperature interval (~40K), discontinuous change in the unit cell volume as also in the modulation wave vector across the transition temperature and considerable thermal hysteresis in the characteristic transition temperatures. The temperature variation of the modulation wave vector q shows smooth analytic behaviour with no evidence for any devilish plateau corresponding to an intermediate or ground state commensurate lock-in phases. The existence of the incommensurate 7M like modulated structure down to 5K suggests that the incommensurate 7M like modulation is the ground state of Ni2MnGa and not the Bain distorted tetragonal L10 phase or any other lock-in phase with a commensurate modulation. These findings can be explained within the framework of the soft phonon model.

We use bulk sensitive hard x-ray photoelectron spectroscopy to provide unambiguous evidence for a pseudogap in the density of states around the Fermi level in icosahedral Zn-Mg-Y and Zn-Mg-Dy quasicrystals, in agreement with our density functional theory calculations. The pseudogap in these Zn-based quasicrystals is less pronounced compared to the Al-based ones such as Al-Pd-Mn and Al-Cu-Fe [J. Nayak et al., Phys. Rev. Lett. 109, 216403 (2012)]. This observation is in agreement with transport studies that indicate a larger charge carrier concentration in the Zn-based quasicrystals. Compared to Zn-Mg-Dy, the pseudogap is somewhat deeper in Zn-Mg-Y. The larger width of the Mg 1s and Zn 2p core-level spectra in Zn-Mg-Y is explained by different configurations of the local atomic surrounding compared to Zn-Mg-Dy.

The irreversibility of the martensite transition in magnetic shape memory alloys (MSMAs) with respect to the external magnetic field is one of the biggest challenges that limits their application as giant caloric materials. This transition is a magnetostructural transition that is accompanied with a steep drop in magnetization (i.e.,$\mathrm{\ensuremath{\Delta}}M$) around the martensite start temperature $({\mathrm{M}}_{\mathrm{s}})$ due to the lower magnetization of the martensite phase. In this Rapid Communication, we show that $\mathrm{\ensuremath{\Delta}}M$ around ${\mathrm{M}}_{\mathrm{s}}$ in Mn-rich Ni-Mn-based MSMAs gets suppressed by two orders of magnitude in crushed powders due to the stabilization of the martensite phase at temperatures well above ${\mathrm{M}}_{\mathrm{s}}$ and the austenite finish $({\mathrm{A}}_{\mathrm{f}})$ temperatures due to residual stresses. Analysis of the intensities and the FWHM of the x-ray powder-diffraction patterns reveals stabilized martensite phase fractions as $97%, 75%$, and $90%$ with corresponding residual microstrains as $5.4%, 5.6%$, and $3%$ in crushed powders of the three different Mn-rich Ni-Mn alloys, namely, $\mathrm{M}{\mathrm{n}}_{1.8}\mathrm{N}{\mathrm{i}}_{1.8}\mathrm{I}{\mathrm{n}}_{0.4}, \mathrm{M}{\mathrm{n}}_{1.75}\mathrm{N}{\mathrm{i}}_{1.25}\mathrm{Ga}$, and $\mathrm{M}{\mathrm{n}}_{1.9}\mathrm{N}{\mathrm{i}}_{1.1}\mathrm{Ga}$, respectively. Even after annealing at 773 K, the residual stress stabilized martensite phase does not fully revert to the equilibrium cubic austenite phase as the magnetostructural transition is only partially restored with a reduced value of $\mathrm{\ensuremath{\Delta}}M$. Our results have a very significant bearing on the application of such alloys as inverse magnetocaloric and barocaloric materials.

The surface of Ni 2+x Mn 1-x Ga have been studied using X-ray photoelectron spectroscopy under different conditions of surface preparation. We find that the surface becomes Ni rich by argon ion sputtering. However, the change in surface composition due to sputtering can be reverted by annealing at appropriate temperature so that the bulk composition is restored at the surface. The origin of a satellite feature observed both in the valence band and the Ni 2p core-level spectra is discussed. The magnetic microstructures at the surface have been observed by using magnetic force microscopy.

The modifications in surface composition of Ni2.1Mn0.9Ga ferromagnetic shape memory alloy have been investigated using X-ray photoelectron spectroscopy (XPS) under various sputtering and annealing conditions. XPS core-level spectra show that sputtering makes the surface Ni rich. However, by annealing, the Mn content at the surface increases and at about 390 ° C the bulk composition is restored. The valence band spectra show evidence of Ni related extra states for the sputtered surface, which decrease with annealing. This behavior is in agreement with the change in surface composition derived from the core-level spectra.

The electronic structure of quasicrystalline Al–Pd–Mn is investigated by means of valence and core level photoelectron spectroscopy. Variations of the photoionization cross section in the constituents’ valence electronic levels as a function of photon energy are used to identify contributions from the different atomic species, in particular near the Pd 4d Cooper minimum. Resonant photoemission at the Mn 2p absorption edge shows the contribution of the Mn 3d states to the density of states in a region near the Fermi level. The asymmetry of Pd 3d and Mn 2p core level photoemission lines, and its difference for emission fro mm etallic and quasicrystalline phases, are utilized to infer the contributions of the different constituents to the density of states at the Fermi level.

The surface electronic structure of Ag has been studied using angle and energy resolved photoyield (AERPY) using low photon energies (2.7-18 eV). The adlayer standing-wave-like bulk plasmon mode is clearly observed in Ag(1 1 1) and Ag(1 0 0) thin films. A feature at 3.7 eV, observed both on Ag adlayers and bulk crystal surface, is assigned to the multipole plasmon mode. No signature of the multipole plasmon is observed around 6.7 eV, which is in disagreement with the prediction of the s-d polarization model.

Argon ion bombardment on Al(1 1 1) leads to formation of adatoms and sub-surface bubbles. From X-ray photoemission spectroscopy study, we find that the Ar core-levels exhibit a systematic shift in binding energy and change in the line shape as function of ion incidence energy. This is related to the increase in size of the sub-surface bubbles created by argon ion bombardment where the Ar atoms are trapped. Low energy electron diffraction pattern for Al(1 1 1) exhibits broadened spots and enhanced background intensity after bombardment which is related to the formation of random steps and terraces on the surface.

We study the unoccupied region of the electronic structure of the fivefold symmetric surface of an icosahedral (i) Al-Pd-Mn quasicrystal. A feature that exhibits parabolic dispersion with an effective mass of (1.15±0.1)me and tracks the change in the work function is assigned to an image potential resonance because our density functional calculation shows an absence of band gap in the respective energy region. We show that Sn grows pseudomorphically on i−Al−Pd−Mn as predicted by density functional theory calculations, and the energy of the image potential resonance tracks the change in the work function with Sn coverage. The image potential resonance appears much weaker in the spectrum from the related crystalline Al-Pd-Mn surface, demonstrating that its strength is related to the compatibility of the quasiperiodic wave functions in i−Al−Pd−Mn with the free-electron-like image potential states. Our investigation of the energy region immediately above EF provides unambiguous evidence for the presence of a pseudogap, in agreement with our density functional theory calculations.

Photons are an alternative probe to electrons or ions for the detection of surface and interface collective modes. We have used synchrotron radiation in conjunction with angle resolved photoyield measurements to study the evolution of overlayer excitation modes in thin alkali layers. Our experimental results show clear evidence of collective modes such as the multipole plasmon, bulk overlayer plasmon and threshold excitation. Both the energy and lineshape of these photoyield resonances show strong thickness dependence. The response of the hidden substrate is rather different. The Al multipole peak shows no energy shift as a function of coverage confirming the absence of an intermediate charge density. We discuss our results in terms of previous dynamical calculations of the surface response.

The annealing temperature dependence of the core-level spectra of the rare gas bubbles implanted in the sub-surface region of Al(111) has been investigated by x-ray photoelectron spectroscopy. A systematic change in the binding energy of the rare gas core-level has been observed, as a function of annealing temperature. The change in binding energy is explained by the increase in bubble size with annealing temperature. Al conduction electron screening of the rare gas core-hole decreases in the photoemission final state with increase in bubble size. We have also observed the decrease in concentration of the rare gas with annealing temperature. The Ne 1s spectra show two components separated by about 1.2 eV and change in intensity with temperature, which is possibly related to a near-surface and a sub-surface distribution of Ne bubbles. The near-surface component gets desorbed faster than the sub-surface component with annealing.

The use of angle- and energy-resolved photoyield experiments for the study of collective excitations at clean metal surfaces and in thin metal films is discussed by recourse to recent experiments. It is shown that photoyield experiments, when compared with theoretical predictions based on the nonlocal corrections to the Fresnel optics, can provide a detailed interpretation of the experimental results for clean surfaces and thin films of simple metals such as Al and the alkali metals. The excitation of so-called multipole plasmons, bulk-like plasmons in thin films, and related multielectron excitations in Al and the heavier alkalis can be quantitatively understood on the basis of jellium-type calculations, whereas lattice effects are shown to be important for a description of the photoyield in Li. Apart from multielectron excitations, features above the plasmon excitations threshold can be understood in terms of single-particle transitions, and the excitation of bulk plasmons with q≠0 by the incident light. Finally, experimental observations of collective modes in more complex surface structures are presented, and the influence of surface electronic structure on the energy of the multipole plasmon is discussed.

The electronic bulk and surface band structure of cubic zinc sulphide (``zinc blende'') has been studied by angle-resolved photoelectron spectroscopy using synchrotron radiation. The $s\ensuremath{-}p$ derived bands along the $\ensuremath{\Gamma}\ensuremath{-}K\ensuremath{-}X$ high-symmetry direction have been determined, and the region of the Zn $3d$ line has been examined. Spectra at fixed photon energy and variation of polar electron emission angle were used to determine the dispersion of the surface states along the $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{X}$ within the surface Brillouin zone. The experimental data for bulk and surface bands are compared with results of a recent density-functional calculation, which includes the interaction between the $s\ensuremath{-}p$ and the cation d-derived states.

We report on the evolution of L 3 – and L 2 – M 45 M 45 Auger spectral regions in Cu and CuO as a function of photon energy used to create the initial hole states. It is shown that the satellite features in Auger spectra are completely suppressed close to the threshold; intensities of these satellites increase dramatically when photon energy is swept through deeper core level thresholds. We discuss various decay channels responsible for the spectral features in the L 23 – M 45 M 45 regions of Cu and CuO and obtain quantitative estimates for their contributions to the total spectrum.

Collective surface excitations in alkali-metal overlayers are observed using photoyield spectroscopy. Spectra for Na and K on Al(111) reveal a multipole surface plasmon and bulklike overlayer plasmon. In contrast, Li on Al exhibits only the multipole mode. In the submonolayer regime, all three alkali metals provide evidence for the threshold excitation. Time-dependent density-functional calculations for realistic alkali-metal overlayers agree well with these observations.

We present experimental x-ray-absorption spectra at the oxygen and $3d$ transition-metal $K$ edges of ${\mathrm{LaFeO}}_{3}$ and ${\mathrm{LaCoO}}_{3}$. We interpret the experimental results in terms of detailed theoretical calculations based on multiple-scattering theory. Along with providing an understanding of the origin of various experimental features, we investigate the effects of structural distortions and the core-hole potential in determining the experimental spectral shape. The results indicate that the core-hole potential as well as many-body effects within the valence electrons do not have any strong effect on the spectra suggesting that the spectral features can be directly interpreted in terms of the electronic structure of such compounds.

High-energy electron energy-loss spectroscopy in transmission has been applied to obtain the absorption spectra at the B 1s, C 1s and Ni 2 p 3 2 excitation edges of bulk polycrystalline YNi2B2C. We find a close similarity in the near-edge structures and confirm the strongly covalent character of the bonding between Ni, B and C. We compare our results to band structure calculations of the local unoccupied density of states generated within the local density approximation. From x-ray photoemission spectroscopy measurements we determine the core level binding energies and from a comparison of photoemission and Auger spectroscopic measurements we derive a value of the Ni d-d Coulomb energy UA of about 4eV.

We discuss high-energy spectroscopic results for systems representing electron doping near the d 0 configuration and establish that the spectral evolution with doping is inconsistent with expectations based on the Hubbard model. We argue that this failure arises due to the neglect of disorder in the real system. We present a new theoretical approach which takes into account the inhomogeneity of real systems; this approach provides results which are qualitatively different from the standard homogeneous models, but are in agreement with the experimental observations.

The electronic structure of ${\mathrm{Ag}}_{2+\ensuremath{\delta}}\mathrm{S}$ single crystals is investigated as a function of temperature and nonstoichiometry, $\ensuremath{\delta}$. It is shown that the stoichiometric, low-temperature $\ensuremath{\alpha}$-${\mathrm{Ag}}_{2}$S phase is a band insulator with correlation effects further increasing the band gap. The results suggest that the metal-insulator transition observed with increase in $\ensuremath{\delta}$ from the stoichiometric ($\ensuremath{\delta}=0$) phase in the high-temperature $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ag}}_{2+\ensuremath{\delta}}\mathrm{S}$ is due to the tuning of the Fermi energy through the mobility edge. The increase in conductivity associated with the insulator-to-insulator transition across the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\beta}$ phase transformation in the stoichiometric ${\mathrm{Ag}}_{2}$S is suggested to arise from the order-disorder transition and the consequent redistribution of states near the top of the valence band.

It is shown that the local spin-density approximation describes well electronic structures of perovskite $\mathrm{La}M{\mathrm{O}}_{3}$ ( $M\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}$ Mn, Fe, Co, Ni) for the ground-state and single-electron excited-state properties. Inclusion of details of crystal structures distorted from the cubic structure is crucial to reproduce different magnetic structures and insulating states. Calculated x-ray photoemission spectra are in very good agreement with experimental ones, and its angular momentum characters are in conformity with ultraviolet photoemission spectra.

Electronic structures of gallium and indium in the solid and liquid states have been investigated using photoemission and bremsstrahlung isochromat spectroscopies yielding information concerning the occupied and unoccupied parts of the density of states. Experimental results are compared with various calculations in order to obtain a detailed description. X-ray photoemission spectra of Ga 2p and In 3d core levels exhibit shifts across the solid- liquid transition, arising from changes in screening.

The austenite to martensite phase transition in Ni–Mn–Ga ferromagnetic shape memory alloys was studied by extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectroscopy. The spectra at all the three elements', namely, Mn, Ga and Ni, K-edges in several Ni–Mn–Ga samples (with both Ni and Mn excess) were analyzed at room temperature and low temperatures. The EXAFS analysis suggested a displacement of Mn and Ga atoms in opposite direction with respect to the Ni atoms when the compound transforms from the austenite phase to the martensite phase. The first coordination distances around the Mn and Ga atoms remained undisturbed on transition, while the second and subsequent shells showed dramatic changes indicating the presence of a modulated structure. The Mn rich compounds showed the presence of antisite disorder of Mn and Ga. The XANES results showed remarkable changes in the unoccupied partial density of states corresponding to Mn and Ni, while the electronic structure of Ga remained unperturbed across the martensite transition. The post-edge features in the Mn K-edge XANES spectra changed from a double peak like structure to a flat peak like structure upon phase transition. The study establishes strong correlation between the crystal structure and the unoccupied electronic structure in these shape memory alloys.

We use hard x-ray photoemission to resolve a controversial issue regarding the mechanism for the formation of quasicrystalline solids, i.e., the existence of a pseudogap at the Fermi level. Our data from icosahedral fivefold Al-Pd-Mn and Al-Cu-Fe quasicrystals demonstrate the presence of a pseudogap, which is not observed in surface sensitive low energy photoemission because the spectrum is affected by a metallic phase near the surface. In contrast to Al-Pd-Mn, we find that in Al-Cu-Fe the pseudogap is fully formed; i.e., the density of states reaches zero at E(F) indicating that it is close to the metal-insulator phase boundary.