Hard X-ray Photoemission Spectroscopy Research Articles

Local moment behaviors of the valence fluctuating systems β-YbAlB4 and α-YbAlB4

β-YbAlB4 is the first example of an Yb-based heavy Fermion superconductor with Tc = 80 mK and exhibits pronounced non-Fermi-liquid behavior above Tc. On the other hand, recent hard x-ray photoemission spectroscopy measurements have revealed strong intermediate valence of both β-YbAlB4 and its locally isostructural polymorph α-YbAlB4 . Here we present the results of the specific heat and magnetization measurements of β-YbAlB4 and α-YbAlB4 . The results indicate a Fermi liquid ground state for α-YbAlB4 in contrast to the quantum criticality found in β-YbAlB4 . Interestingly, both systems exhibit Kondo lattice behavior with large Wilson ratio below T* ~ 8K, through the renormalization of a high valence fluctuation scale ~ 200 K.

Band Gap and Electronic Structure of an Epitaxial, SemiconductingCr0.80Al0.20Thin Film

Cr(1-x)Al(x) exhibits semiconducting behavior for x = 0.15-0.26. This Letter uses hard x-ray photoemission spectroscopy and density functional theory to further understand the semiconducting behavior. Photoemission measurements of an epitaxial Cr(0.80)Al(0.20) thin film show several features in the valence band region, including a gap at the Fermi energy (E(F)) for which the valence band edge is 95 ± 14 meV below E(F). Theory agrees well with the valence band measurements, and shows an incomplete gap at E(F) due to the hole band at M shifting almost below E(F).

X-dependent electronic structure of Yb XCu 4 ( X = Cd, In, Sn) investigated by hard X-ray photoemission spectroscopy

We have carried out hard X-ray photoemission spectroscopy on YbCdCu 4 and YbSnCu 4 at hν ∼ 6 keV in order to investigate electronic structure, and compared the results with that of the valence transition compound YbInCu 4. Based on fitting analyses of the Yb 3d spectra, the Yb valences of YbCdCu 4 and YbSnCu 4 are estimated to be 2.96 ± 0.01 and 2.86 ± 0.01 at 300 K, respectively, and the valences slightly decrease becoming closer to divalent on cooling. The Cu 2p 3/2 level does not follow the chemical trend on going from YbCdCu 4, to YbInCu 4, and to YbSnCu 4. The lowest binding energy of the Cu 2p 3/2 level for YbInCu 4 suggests that the large number of conduction electrons distributes over the Cu site in YbInCu 4 and the excess conduction electrons play an important role for the valence transition of YbInCu 4.

X-ray photoemission spectroscopy analysis of N-containing carbon-based cathode catalysts for polymer electrolyte fuel cells

We report on the electronic structure of three different types of N-containing carbon-based cathode catalysts for polymer electrolyte fuel cells observed by hard X-ray photoemission spectroscopy. Prepared samples are derived from: (1) melamine and poly(furfuryl alcohol), (2) nitrogen-doped carbon black and (3) cobalt phthalocyanine and phenolic resin. C 1 s spectra show the importance of sp 2 carbon network formation for the oxygen reduction reaction (ORR) activity. N 1 s spectra of the carbon-based cathode catalysts are decomposed into four components identified as pyridine-like, pyrrole- or cyanide-like, graphite-like, and oxide nitrogen. Samples having high oxygen reduction reaction activity in terms of oxygen reduction potential contain high concentration of graphite-like nitrogen. O 1 s spectra are similar among carbon-based cathode catalysts of different oxygen reduction reaction activity. There is no correlation between the ORR activity and oxygen content. Based on a quantitative analysis of our results, the oxygen reduction reaction activity of the carbon-based cathode catalysts will be improved by increasing concentration of graphite-like nitrogen in a developed sp 2 carbon network.

Strong Valence Fluctuation in the Quantum Critical Heavy Fermion Superconductorβ−YbAlB4: A Hard X-Ray Photoemission Study

Electronic structures of the quantum critical superconductor β-YbAlB4 and its polymorph α-YbAlB4 are investigated by using bulk-sensitive hard x-ray photoemission spectroscopy. From the Yb 3d core level spectra, the values of the Yb valence are estimated to be ∼2.73 and ∼2.75 for α- and β-YbAlB4, respectively, thus providing clear evidence for valence fluctuations. The valence band spectra of these compounds also show Yb2+ peaks at the Fermi level. These observations establish an unambiguous case of a strong mixed valence at quantum criticality for the first time among heavy fermion systems, calling for a novel scheme for a quantum critical model beyond the conventional Doniach picture in β-YbAlB4.

Electronic structure ofIn2O3and Sn-dopedIn2O3by hard x-ray photoemission spectroscopy

The valence and core levels of In(2)O(3) and Sn-doped In(2)O(3) have been studied by hard x-ray photoemission spectroscopy (hv = 6000 eV) and by conventional Al K alpha (hv = 1486.6 eV) x-ray photoemission spectroscopy. The experimental spectra are compared with density-functional theory calculations. It is shown that structure deriving from electronic levels with significant In or Sn 5s character is selectively enhanced under 6000 eV excitation. This allows us to infer that conduction band states in Sn-doped samples and states at the bottom of the valence band both contain a pronounced In 5s contribution. The In 3d core line measured at hv = 1486.6 eV for both undoped and Sn-doped In(2)O(3) display an asymmetric lineshape, and may be fitted with two components associated with screened and unscreened final states. The In 3d core line spectra excited at hv = 6000 eV for the Sn-doped samples display pronounced shoulders and demand a fit with two components. The In 3d core line spectrum for the undoped sample can also be fitted with two components, although the relative intensity of the component associated with the screened final state is low, compared to excitation at 1486.6 eV. These results are consistent with a high concentration of carriers confined close to the surface of nominally undoped In(2)O(3). This conclusion is in accord with the fact that a conduction band feature observed for undoped In(2)O(3) in Al K alpha x-ray photoemission is much weaker than expected in hard x-ray photoemission.

Anomalous State Sandwiched between Fermi Liquid and Charge Ordered Mott-Insulating Phases ofTi4O7

The Magnéli phase Ti(4)O(7) exhibits two sharp jumps in resistivity with coupled structural transitions as a function of temperature at T(c1) approximately 142 K and T(c2) = 154 K. We have studied electronic structure changes across the two transitions using 7 eV laser, soft x-ray, and hard x-ray (HX) photoemission spectroscopy (PES). Ti 2p-3d resonant PES and HX PES show a clear metallic Fermi edge and mixed valency above T(c2). The low temperature phase below T(c1) shows a clear insulating gap of approximately 100 meV. The intermediate phase between T(c1) and T(c2) indicates a pseudogap coexisting with remnant coherent states. HX PES and complementary calculations have confirmed the coherent screening in the strongly correlated intermediate phase. The results suggest the existence of a highly anomalous state sandwiched between the mixed-valent Fermi liquid and charge ordered Mott-insulating phase in Ti(4)O(7).

Doping dependence of the chemical potential and surface electronic structure inYBa2Cu3O6+xandLa2−xSrxCuO4using hard x-ray photoemission spectroscopy

The electronic structure of YBa2Cu3O6+x and La2-xSrxCuO4 for various values of x has been investigated using hard x-ray photoemission spectroscopy. The experimental results establish that the cleaving of YBa2Cu3O6+x compounds occurs predominantly in the BaCuO3 complex leading to charged surfaces at higher x and to uncharged surfaces at lower x values. The bulk component of the core level spectra exhibits a shift in binding energy as a function of x, from which a shift of the chemical potential as a function of hole concentration in the CuO2 layers could be derived. The doping dependence of the chemical potential across the transition from a Mott-Hubbard insulator to a Fermi-liquid-like metal is very different in these two series of compounds. In agreement with previous studies in the literature the chemical potential shift in La2-xSrxCuO4 is close to zero for small hole concentrations. In YBa2Cu3O6+x, similar to all other doped cuprates studied so far, a strong shift of the chemical potential at low hole doping is detected. However, the results for the inverse charge susceptibility at small x shows a large variation between different doped cuprates. The results are discussed in view of various theoretical models. None of these models turns out to be satisfactory.

Pressure-induced change in the electronic structure of epitaxially strainedLa1−xSrxMnO3thin films

We report the observation of pressure-induced changes in the electronic structures of ${\text{La}}_{1\ensuremath{-}x}{\text{Sr}}_{x}{\text{MnO}}_{3}$ (LSMO) by hard x-ray photoemission spectroscopy. Application of compressive and tensile strains results in the formation of a gap at the Fermi level $({E}_{\text{F}})$ and suppression of spectral weight at ${E}_{\text{F}}$, respectively, across magnetic phase transitions. In contrast, no detectable change is observed in the absence of phase transitions even upon application of pressure. These results indicate that the change in the electronic structure of LSMO does not originate from the lattice distortions alone, but is induced by subtle interplay among the lattice, magnetic, and orbital degrees of freedom.

Effect of Oxide Charge Trapping on X-ray Photoelectron Spectroscopy of HfO2/SiO2/Si Structures

We examined the effects of interfacial SiO2 layers and a surface metal layer on the photoelectron spectra of HfO2/SiO2/Si structures by hard X-ray photoemission spectroscopy with synchrotron radiation as well as conventional X-ray photoelectron spectroscopy (XPS). The Hf 4f and Hf 3d photoelectron peaks broadened and shifted toward a higher binding energy with increasing thickness of the interfacial SiO2 layer, even though photoelectrons may have been emitted from the HfO2 layer with the same chemical composition. Thinning the interfacial Si oxide layer to approximately one monolayer and depositing a metal layer on the HfO2 surface suppressed these phenomena. The O 1s photoelectron spectra revealed marked differences between the metal- and nonmetal-deposited HfO2/SiO2/Si structures; HfO2 and SiO2 components in the O 1s photoelectron spectra for the metal-deposited structures were observed at reasonably separated binding energies, but those for the nonmetal-deposited structures were not separated clearly. From this behavior concerning the effects of interfacial SiO2 and surface metal layers, we concluded that the Hf 4f, Hf 3d, and O 1s spectra measured from the HfO2/SiO2/Si structures did not reflect actual chemical bonding states. We consider that potential variations in the HfO2 film owing to charge trapping strongly affect the measured photoelectron spectra. On the basis of angle-resolved XPS measurements, we propose that positive charges are trapped at the HfO2 surface and negative charges are trapped inside the HfO2 layer.

Fabrication of (Fe,Mn)3O4 ferromagnetic oxide nanostructure using molybdenum nanomask atomic force microscope lithography

We reported the fabrication of Fe2.5Mn0.5O4 (FMO) ferromagnetic oxide nanostructure using molybdenum (Mo) nanomask atomic force microscope (AFM) lithography technique for oxide nanospintronics applications. This technique enables us to prepare transition metal oxide nanostructures down to 200nm. The hard x-ray photoemission spectroscopy revealed that the resulting nanostructures preserve a fine electric structure as spintronics oxide same as unprocessed films. The resulting FMO nanostructure showed the magnetoresistance of about −0.11% to −0.14% at the magnetic field of 3000Oe, which value was similar to that of FMO film. These indicate that this technique can construct well defined nanoscale oxides both in structure and physical properties.

Tunable optical cavity for an x-ray free-electron-laser oscillator

An x-ray free-electron laser oscillator proposed recently for hard x rays [K. Kim, Y. Shvyd'ko, and S. Reiche, Phys. Rev. Lett. 100, 244802 (2008)] can be made tunable by using an x-ray cavity composed of four crystals, instead of two. The tunability of x-ray energy will significantly enhance the usefulness of an x-ray free-electron laser oscillator. We present a detailed analysis of the four-crystal optical cavity and choice of crystals for several applications: inelastic x-ray scattering, nuclear resonant scattering, bulk-sensitive hard x-ray photoemission spectroscopy, other high-energy-resolution ($\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{meV}$) spectroscopic probes, and for imaging with hard x rays at near-atomic resolution ($\ensuremath{\simeq}1\text{ }\text{ }\mathrm{nm}$).

Investigation on the electronic state of In-doped ZnO nanocrystals by hard X-ray photoemission spectroscopy

ZnO:In nanocrystals (NCs) with various In contents were synthesized by using a mixed source vapor transportation method. The morphological variation and electronic structure of ZnO:In NCs were studied by bulk sensitive hard X-ray photoemission spectroscopy (HX-PES). ZnO:In NCs are classified into ZnO-like hexagonal and In 2O 3-like cubic groups, based on the crystal structure, and we have estimated the In content for the phase transformation to be as high as 35 atomic% (atm.%). A filled electronic state in the conduction band was observed from the ZnO-like group, which is regarded as a metallic state, due to an increase of extrinsic carrier density.

Study of electronic states of a pyrochlore-type molybdate (Sm2Mo2O7) at low temperature by means of hard x-ray photoemission spectroscopy

We have carried out the measurement for a pyrochlore-type Mo oxide (Sm2Mo2O7) by means of hard-x-ray and soft-x-ray photoemission spectroscopy (PES) with high energy resolution. The PES spectrum obtained with the hard x-ray clearly shows a coherent part and a incoherent part. And its boundary is relatively distinguishable. The ratio of the coherent part to the incoherent part is more enhanced in the hard-x-ray PES compared to the soft-x-ray PES. The results indicate that hard-x-ray photoemission spectroscopy (HAXPES) is powerful method for investigating the bulk electronic state.

Thickness dependent electronic structure of La0.6Sr0.4MnO3 layer in SrTiO3/La0.6Sr0.4MnO3/SrTiO3 heterostructures studied by hard x-ray photoemission spectroscopy

The authors have investigated changes in the electronic structures of digitally controlled La0.6Sr0.4MnO3 (LSMO) layers sandwiched between SrTiO3 as a function of LSMO layer thickness in terms of hard x-ray photoemission spectroscopy (HX-PES). The HX-PES spectra show the evolution of Mn 3d derived states near the Fermi level and the occurrence of metal-insulator transition at 8 ML. The detailed analysis for the thickness dependent HX-PES spectra reveals the existence of the less conducting and nonmagnetic transition layer with a film thickness of about 4 ML in the interface region owing to significant interaction through the interface.

Hard X-ray photoemission spectroscopy

Except in the very early stage of the development of X-ray photoemission spectroscopy (XPS) by Kai Siegbahn and his coworkers, the excitation sources for XPS studies have predominantly been the Al Kα and Mg Kα emission lines. The advent of synchrotron radiation sources opened up the possibility of tuning the excitation photon energy with much higher throughputs for photoemission spectroscopy, however the excitation energy range was limited to the vacuum ultra violet and soft X-ray regions. Over the past 5–6 years, bulk-sensitive hard X-ray photoemission spectroscopy using high-brilliance high-flux X-rays from third generation synchrotron radiation facilities has been developed. This article reviews the history of HXPES covering the period from Kai Siegbahn and his coworkers’ pioneering works to the present, and describes the fundamental aspects, instrumentation, applications to solid state physics, applied physics, materials science, and industrial applications of HXPES. Finally, several challenging new developments which have been conducted at SPring-8 by collaborations among several groups are introduced.

Ex-site Characterization of Thin File Materials for Advanced Devices by Hard X-ray Photoemission Spectroscopy

Hard X-ray photoemission spectroscopy (HXPS) with high-resolution, high-throughput and large photoelectron attenuation length has been developed at SPring-8 utilizing high brilliant undulator X-rays. This enhanced versatility of the photoemission spectroscopy, enabling to apply to laboratory prepared thin films and nano-structured samples of advanced materials. We will introduce wide varieties of experimental results on advance materials performed at JASRI public beamline BL47XU and NIMS contract beamline BL15XU of SPring-8.

Narrowing the search for potential hydrogen-generating photocatalysts

The sun offers an immense amount of free and clean power, which we have not yet begun to seriously exploit. In addition to directly converting sunlight to electrical energy, we could also generate storable chemical energy, to be used at a time of our choosing. The photoelectrochemical (PEC) decomposition of water to form hydrogen and oxygen is ideal for this purpose, since hydrogen can be generated, stored, and later converted to emission-free electricity using fuel cell technologies. The missing link in this idealized cycle is the first step: an efficient and affordable way to generate hydrogen from water and sunlight. The challenge is to find a low band-gap semiconductor that is stable in solution, catalytically active, and has high overall solar-to-hydrogen conversion efficiency. Metal oxides are ideal candidate photocatalyts because of their low cost and high stability in aqueous solutions.1 Unfortunately their optical band-gaps generally lie outside the visible range (> 3eV), making them good for transparent electronics,2 but bad for solar applications. Despite immense effort, attempts to reduce the band-gaps of catalytically active oxides such as TiO2 through doping or co-doping have not produced a promising material. We must therefore find new avenues of oxide design. Millions of potential multi-ternary oxides exist, but researchers need a strategy to identify the most likely material classes. The valence bands of most oxides generally lie well below the water oxidation potential, while the conduction bands straddle the hydrogen reduction potential. Thus, the task requires raising the valence band, not lowering the conduction band. One approach could harness low binding energy cations with an nd10 electron configuration, such as Cu and Ag. Alternatively, scientists could use low binding energy ns2 cations such as Sn(II), Sb(III), Pb(II), Bi(III). We are currently focusing on the latter. Typical metal oxides such as MgO and ZnO have band edges formed from occupied O 2p and unoccupied cation s orbitals, respectively. Heavier post-transition metal cations are unique because the binding energy of the cation s orbital increases, giving rise to two accessible oxidation states. These orbitals are either nominally filled or empty, such as Sn(II)O/Sn(IV)O2 and Sb(III)2O3/Sb(V)2O5. The filled orbitals were historically thought chemically inert, but stereochemically active. However, we recently showed that cation s − anion p hybridization dominates in these systems and second order cation p coupling with the antibonding cation s − anion p states occurs towards the top of the valence band.3–7 We also verified this theoretical understanding with hard x-ray photoemission spectroscopy.8 Bismuth vanadate (BiVO4) is a ternary oxide combining Bi(III) 6s2 and V(V) 3d0 cations, as shown in Figure 1. It has a welldemonstrated potential for water photodecomposition with the presence of both a low band-gap (2.4eV) and reasonable catalytic activity.1 We used density functional theory to analyze its electronic structure and explain its unusual properties.9 BiVO4 has a direct fundamental band-gap, with the Bi 6s states helping to raise the valence band by 0.4eV, while the conduction band remains low because of the V 3d states. Both the electron and hole effective masses are relatively light, indicating the potential for both p and n-type conductivity. The combination of ns2 and nd0 cations generally results in unique crystal structures because the ns2 ones have a strong preference for disordered coordination that varies with the cation s binding energy. This is demonstrated by BiVO4 , which has distorted 2D stacking of Bi−O−V layers with C6 2h symmetry. The generality of this cation combination is apparent from a recent optical evaluation of ternary tungstenates by

Electronic States in Zinc Magnesium Oxide Alloy Semiconductors: Hard X-ray Photoemission Spectroscopy and Density Functional Theory Calculations

The electronic states of zinc magnesium oxide (Zn1−xMgx)O thin films were determined exactly by hard X-ray photoemission spectroscopy (HX-PES) using synchrotron radiation. The Zn 2p core level was shifted to a higher binding energy along with widening of the bandgap by alloying with MgO, whereas the shift of the O 1s peak was less than that of the Zn 2p peak. Density functional theory (DFT) calculations revealed that the electronic state of an O ion bonding with an adjacent Mg ion is remarkably different from that not bonding with a Mg ion. As a result, the energy shift observed in the O 1s peak results from a combination of the expansion of the bandgap energy and the chemical shift due to a change in the ionicity. The Fermi level is always situated just below the conduction band; this suggests that a shallow donor can be added, even in the alloy film with very high MgO fraction, for example, x = 0.47.

Chemical potential shift ofFe3−xVxSistudied by hard x-ray photoemission

Core-level photoemission spectra of ${\text{Fe}}_{3\ensuremath{-}x}{\text{V}}_{x}\text{Si}$ alloys with inequivalent ${\text{Fe}}_{\text{I}}$ and ${\text{Fe}}_{\text{II}}$ sites are investigated via hard x-ray photoemission spectroscopy over the entire doping range $x=0--1$. All the measured $1s$ core-level peaks are found to shift to higher binding energy with increasing V concentration. First-principles, all electron charge- and spin-self-consistent electronic structure computations within the framework of the local-spin-density approximation are used to interpret the experimental results. The measured size of energy shift in going from $x=0$ to 1 is consistent with the corresponding theoretical value for the ${\text{Fe}}_{\text{II}}$ and $\text{Si}\text{ }1s$ core levels, whereas for the ${\text{Fe}}_{\text{I}}$ and V core levels the computed shifts are generally larger than the experimental values. We ascribe these discrepancies to the effects of the core-hole screening in the final state which are not accounted for in the computations. In a rigid-band model the chemical potential and the core-level binding energies are expected to decrease with V doping as electrons are depleted from the Fermi energy. The observed increase in the binding energy of core levels thus supports a picture of the electronic structure where V doping induces a ``pseudogap'' or a region of reduced density of states in the vicinity of the Fermi energy.