Core-level Photoemission Spectroscopy Research Articles

Role of electron correlation and disorder on the electronic structure of layered nickelate (La0.5Sr0.5)2NiO4

We investigate the role of electron correlation and disorder on the electronic structure of layered nickelate ${({\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5})}_{2}{\mathrm{NiO}}_{4}$ using core level and valence band photoemission spectroscopy in conjunction with density functional theory (DFT) and dynamical mean-field theory (DMFT) calculations. Sr $3d$ and La $4d$ core level spectra exhibit multiple features associated with photoemission final-state effects. An increase of unscreened features in the Sr $3d$ and La $4d$ core level spectra with lowering temperature suggests the reduction in density of states (DOS) at the Fermi level, ${E}_{F}$. Valence band spectra collected using different photon energies reveal finite intensity at ${E}_{F}$ and overall spectra are well captured by $\mathrm{DFT}+\mathrm{DMFT}$. Strong renormalization of partially filled ${e}_{g}$ bands in $\mathrm{DFT}+\mathrm{DMFT}$ result indicates strong correlation in this system. Mass enhancement factor, ${m}^{*}/{m}_{\text{DFT}}\ensuremath{\sim}$ 3, agrees well with values obtained from specific heat measurements. High-resolution spectra in the vicinity of ${E}_{F}$ show monotonically decreasing spectral intensity with lowering temperature, which evolves to exhibit a Fermi cutoff at low temperatures indicating metallic character in contrast to insulating transport, suggesting Anderson insulating state. $|E\ensuremath{-}{E}_{F}{|}^{1/2}$ dependence of the spectral DOS and square-root temperature dependence of spectral DOS at ${E}_{F}$ evidences the role of disorder in the electronic structure of ${({\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5})}_{2}{\mathrm{NiO}}_{4}$.

Three-dimensional angle-resolved photoemission study of bulk SiGe single crystals

The composition- and structure-dependent electronic band structure of SiGe alloys facilitates a rich variety of its application in various fields. In this study, we performed high-resolution core-level and angle-resolved photoemission spectroscopy (ARPES) of unstrained SiGe single crystals with the (001) surface. Thermally cleaned surfaces of the single crystals of SiGe alloys exhibited double domain (2 × 1) reconstruction, wherein the asymmetric dimer was composed of Ge atoms, and Si atoms were located below the subsurface region. The compositional dependence of three valence bands and their constant energy contours were clearly resolved by three-dimensional ARPES measurements using the high-intensity synchrotron radiation. The valence bands of unstrained SiGe alloys could be well described by the composition-based interpolation of the band parameters of pristine Si and Ge crystals.

Optical film-thinning of graphene epitaxially grown on 4H-SiC(0001): robustness of monolayer-graphene against the photoexcitation

As the properties of graphene films depend on their stacked atomic layers, their thickness should be accurately controlled to improve their specific properties. However, by existing methods, controlling the homogeneity of graphene films at the atomic level remains difficult. In this work, photo-stimulated structural modifications of few-layer graphene epitaxially grown on 4H-SiC(0001) were studied using Raman scattering spectroscopy and core-level photoemission spectroscopy (CLPES). Iterative excitation with laser pulses (800 nm, 100 fs, p-polarized, 250 mJ cm−2) changed the graphene-related two-dimensional (2D) Raman line, which is composed of three components characterized by their different responses upon photoexcitation: two components decaying at fast and slow rates, and a component highly resistant to excitation. CLPES revealed that the observed decay of the 2D line was associated with the elimination of carbon atoms from the graphene layers, finally leaving the robust thin film of single-layer graphene by prolonged excitation. Therefore, this work clearly demonstrates the thickness-dependent structural stability of graphene to optical excitation and opens a promising new method for thinning graphene. An underlying mechanism for the photo-stimulated modifications was also proposed.

The ALBA high-stability monochromator for VUV and soft X-rays

LOREA is ALBA’s beamline devoted to the investigation of solids electronic structure by means of Angle Resolved Photo-Emission Spectroscopy (ARPES). The beamline operates in the photon energy range 10-1000 eV with tuneable linear and circular polarizations produced by an APPLE II helical undulator. Thanks to its energy range and the high photon flux, LOREA is suitable for high resolution VUV ARPES investigations in the 10-200 eV range, while it is feasible to extend ARPES measurements to the 200-600 eV energy range (soft X-ray ARPES). Core level photoemission, resonant photoemission and X-ray absorption spectroscopies will be accessible in the whole energy range. The energy selection is obtained by an Hettrick-Underwood monochromator without entrance slit. The optical arrangement, with 3 spherical mirrors (SM) and 4 plane varied line-spaced (VLS) gratings, is able to cover the entire energy range of the beamline. The monochromator includes the motions to select and do the fine adjustments of the mirrors (pitch and height), and to scan the energy and select among the different gratings (grating pitch and grating horizontal translation). The monochromator has been designed by the ALBA team of engineers, and has been fully assembled and commissioned at the facility. Besides the required range and resolution performances, it has been designed to achieve high stability and reproducibility, and optimal performance of the optical surfaces under different heat loads and conditions. The cooling circuits of mirrors and gratings are mechanically decoupled from the optical elements. In the case of the gratings, heat load is removed by flexible copper straps connected from optics to rigid water lines, through temperature controller devices based on Peltier elements. The use of Peltier element allows stabilizing the temperature of the gratings to room temperature also under quickly varying heat loads. The gradients within the grating are well below the one degree, and the thermal equilibrium with the surrounding mechanics contributes the long-term stability of the system. The water circuit and the Peltiers, rest in an independent platform inside the vacuum chamber, that allows them drift freely with no effect on the position of optical elements. In the case of the mirrors, the water tubes and cooling pads are not pressed against the mirrors, but just in contact through a 0.1 mm thick pellicle of eutectic InGa. This allows for a very efficient heat transfer using a minimum contact surface sufficient to evacuate up to 60 W, and without any deformation of the mirrors. The mechanics are also designed so that no flexible loops are required, which contributes to a better vibration stability of the system. Mirrors and gratings can be removed from the monochromator inside their holders and with the cooling scheme installed on it. This is a mandatory goal of design, as it is necessary for a careful installation and control of surface deformations at the optics laboratory. In this contribution we describe the main features of the monochromator that allow reaching the target performances, especially those concerning the cooling scheme. And also, we provide details about the positioning mechanics of the optical elements, the energy scanning mechanism and the vacuum system. The monochromator has been already mounted and installed and it is already in operation. The first results of the He photoionization spectra shows an energy resolution better than 10meV at 60eV, with a strong ionization signal and very low noise.

Valence and core-level X-ray photoemission spectroscopy of light-sensitive molecules: Lumazine and alloxazine

Valence and core-level photoemission spectra of two gaseous nitrogen containing heterocycles, lumazine and alloxazine, have been studied experimentally and theoretically. Comprehensive information about the electronic structure of these aromatic molecules has been acquired. The outer-valence Green’s function (OVGF) approach was used to calculate vertical ionization energies and relative intensities (pole strengths) of the valence band spectra. The C, N, and O 1s core level spectra of both compounds were measured and assigned on the basis of Δ-Kohn–Sham calculations. The relative stability of different tautomers of lumazine and alloxazine was investigated through DFT calculations. Only the single, most stable tautomer for lumazine and alloxazine was populated at the present experimental conditions and this finding is confirmed by the core-level photoemission signal, which is able to discriminate between different isomers of a molecule. Other tautomers of lumazine and alloxazine have not been clearly identified spectroscopically, however their relative free energies and Boltzmann populations were calculated and compared with previous studies.

Homogeneous Spatial Distribution of Deuterium Chemisorbed on Free-Standing Graphene.

Atomic deuterium (D) adsorption on free-standing nanoporous graphene obtained by ultra-high vacuum D molecular cracking reveals a homogeneous distribution all over the nanoporous graphene sample, as deduced by ultra-high vacuum Raman spectroscopy combined with core-level photoemission spectroscopy. Raman microscopy unveils the presence of bonding distortion, from the signal associated to the planar sp configuration of graphene toward the sp tetrahedral structure of graphane. The establishment of D–C sp hybrid bonds is also clearly determined by high-resolution X-ray photoelectron spectroscopy and spatially correlated to the Auger spectroscopy signal. This work shows that the low-energy molecular cracking of D in an ultra-high vacuum is an efficient strategy for obtaining high-quality semiconducting graphane with homogeneous uptake of deuterium atoms, as confirmed by this combined optical and electronic spectro-microscopy study wholly carried out in ultra-high vacuum conditions.

Investigation of quinoline derivatives by photoemission spectroscopy and theoretical calculations

The electronic structure of gaseous quinoline and three substituted quinolines, 4-chloroquinoline, 4-aminoquinoline and 4-amino-7-chloroquinoline, have been investigated by valence and core level photoemission spectroscopy. The complete assignment of the spectral features in the valence region has been performed with the aid of the outer valence Green’s function (OVGF) method, providing an acceptable description of the measured spectra. The C 1s, N 1s, and Cl 2p core level spectra of quinoline and its derivatives were also measured and compared with theoretical calculations based on density functional theory, which predicted ionization potentials in good agreement with the experimental data. For the case of the Cl 2p ionization the spin-orbit coupling was also included in the calculation. This work provides a full assignment of the relative binding energies of the core level features, and an analysis of the electronic structure of substituted quinolines in comparison with the parent molecule.

On-Surface Synthesis of Unsaturated Hydrocarbon Chains through C-S Activation.

We use an on‐surface synthesis approach to drive the homocoupling reaction of a simple dithiophenyl‐functionalized precursor on Cu(111). The C−S activation reaction is initiated at low annealing temperature and yields unsaturated hydrocarbon chains interconnected in a fully conjugated reticulated network. High‐resolution atomic force microscopy imaging reveals the opening of the thiophenyl rings and the presence of trans‐ and cis‐oligoacetylene chains as well as pentalene units. The chemical transformations were studied by C 1s and S 2p core level photoemission spectroscopy and supported by theoretical calculations. At higher annealing temperature, additional cyclization reactions take place, leading to the formation of small graphene flakes.

Coexistence of the hourglass and nodal-line dispersions in Nb3SiTe6 revealed by ARPES

SummaryThe non-symmorphic crystal symmetry protection in the layered topological semimetal Nb3SiTe6 can generate exotic band crossings. Herein, high-quality Nb3SiTe6 single crystal was synthesized via chemical vapor transport. The lattice structure of Nb3SiTe6 was characterized by scanning transmission electron microscopy, X-ray diffraction, core-level photoemission, and Raman spectroscopies. Angle-resolved photoemission spectroscopy was used to reveal its topological properties by presenting band structures along different high-symmetry directions. Our data show that nontrivial band features coexist in Nb3SiTe6, including an hourglass-type dispersion formed by two bands along the S-R high-symmetry line, two node lines along the S-X path and the S-R-U path, respectively. These results provide a context for the understanding and exploration of the exotic topological properties of Nb3SiTe6.

Intercalation and reactions of CO under single layer graphene/Ni(111): the role of vacancies

We use synchrotron radiation-induced core level photoemission spectroscopy to investigate the influence of vacancies, produced by ion bombardment, on monolayer graphene/Ni(111) exposed to CO at pressures ranging from ultra-high vacuum (10-10 mbar) up to near ambient (5.6 mbar) conditions. CO intercalates at a rate which is comparable to the one observed in absence of defects and reacts via the Boudouard reaction producing additional carbon atoms and CO2. While the former attach to the graphene layer and extend it over areas previously covered by carbide, the CO2 molecules bind to the graphene vacancies forming epoxy-like bonds across them, thus mending the defects. The so-formed complexes give rise to a peak at 533.4 eV which persists upon evacuating the vacuum chamber at room temperature and which we assign to a covalently bonded species containing C and O.

Positional and Conformational Isomerism in Hydroxybenzoic Acid: A Core-Level Study and Comparison with Phenol and Benzoic Acid.

Three positional isomers of hydroxybenzoic acid, as well as phenol and benzoic acid, were studied using core-level photoemission and X-ray absorption spectroscopies, supported by quantum chemical calculations. While 2-hydroxybenzoic (salicylic) acid exists as a single conformer with an internal hydrogen bond, 3- and 4-hydroxybenzoic acids are mixtures of multiple conformers. The effects due to isomerism are clearly seen in the C 1s and O 1s photoelectron spectra, whereas the conformational effects on the binding energies are less pronounced. The O 1s photoelectron spectrum of salicylic acid is significantly different from that of the other two isomers, providing a signature of the hydrogen bond. In contrast, the oxygen K edge X-ray absorption spectra of the three hydroxybenzoic acids show only minor differences. The salicylic acid absorption spectrum at the carbon K edge shows a more resolved vibrational structure than the spectra of the other molecules, which can be explained in part by the existence of a single conformer. Our theoretical study of vibrational excitations in the lowest C 1s absorption bands of salicylic and 4-hydroxybenzoic acids indicates that the observed structure can be assigned to 0-0 lines of various electronic transitions since most of the totally symmetric vibrational modes with sufficiently large frequencies to be resolved are predicted to be inactive. Significant sensitivity of the C 1s excitations in 3-hydroxybenzoic acid to rotational conformerism was predicted but not observed due to spectral crowding.

Core-Level X-Ray Spectroscopy of Infinite-Layer Nickelate: LDA+DMFT Study

Motivated by recent core-level x-ray photoemission spectroscopy, x-ray absorption spectroscopy (XAS), and resonant inelastic x-ray scattering (RIXS) experiments for the newly discovered superconducting infinite-layer nickelate, we investigate the core-level spectra of the parent compounds NdNiO2 and LaNiO2 using the combination of local density approximation and dynamical mean-field theory (LDA+DMFT). Adjusting a charge-transfer energy to match the experimental spectra, we determine the optimal model parameters and discuss the nature of the NdNiO2 ground state. We find that self-doping from the Nd 5d states in the vicinity of the Fermi energy prohibits opening of a Mott-Hubbard gap in NdNiO2. The present Ni L3 XAS and RIXS calculation for LaNiO2 cannot explain the difference from NdNiO2 spectra.1 MoreReceived 11 May 2021Revised 4 August 2021Accepted 7 September 2021Corrected 23 November 2021Corrected 11 November 2021DOI:https://doi.org/10.1103/PhysRevX.11.041009Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectronic structurePhysical SystemsCupratesMott insulatorsStrongly correlated systemsSuperconductorsTransition metal oxidesTechniquesDensity functional theoryDynamical mean field theoryResonant inelastic x-ray scatteringX-ray absorption spectroscopyX-ray photoelectron spectroscopyCondensed Matter, Materials & Applied Physics

Investigation of single-domain Au silicide nanowires on Si(110) formed for Au coverages in the monolayer regime

The self-organized formation of single domain Au silicide nanowires is observed on Si(110). These nanowires are analysed using scanning tunnelling microscopy (STM) and spectroscopy (STS) as well as photoemission spectroscopy (PES). Core-level PES is utilised to confirm the formation of Au silicide and establish its presence as the top most surface structure, i.e., the nanowires. The growth of the Au silicide nanowires and their dimensions are studied by STM. They form for Au coverages of about 1 monolayer and are characterized by widths of about 2 to 3 nm and heights below 1 nm while reaching lengths exceeding 500 nm when choosing appropriate annealing temperatures. Valence band PES and STS indicate a small but finite density of states at the Fermi level typical for compound metals.

Electronic structure of Tb0.5Sr0.5MnO3

We study the electronic structure of single-crystal ${\mathrm{Tb}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$, a non-charge-ordered mixed-valent semiconductor which exhibits a glassy magnetic ground state. We use the techniques of soft x-ray photoemission, hard x-ray photoemission, x-ray absorption, and resonant photoemission spectroscopy to investigate the occupied and unoccupied electronic states of ${\mathrm{Tb}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$. Core level photoemission and x-ray absorption spectroscopy allow us to determine the valence states of Tb, Sr, and Mn ions in ${\mathrm{Tb}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$. Model charge transfer multiplet calculations of core level photoemission and x-ray absorption spectra are employed to separate out the ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ states and confirm their relative concentrations. Resonant photoemission spectroscopy across the Mn $2p\text{\ensuremath{-}}3d$ threshold shows clear resonant enhancement of the Mn $3d$ partial density of states and two-hole correlation satellites. A Cini-Sawatzky analysis gives on-site Coulomb energy ${U}_{dd}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}5.5\ifmmode\pm\else\textpm\fi{}0.2$ eV for the Mn $3{d}^{n}$ states and ${U}_{pd}$ = 0.7 $\mathrm{eV}\ifmmode\pm\else\textpm\fi{}0.2$ eV for the Mn $3{d}^{n+1}{\underline{L}}^{1}$ states. The O $1s\text{\ensuremath{-}}2p$ resonant photoemission is used to identify the O $2p$ two-hole correlation satellite which provides ${U}_{pp}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}3.4\ifmmode\pm\else\textpm\fi{}0.2$ eV for the O $2p$ states. Valence band photoemission indicates a small-gap semiconductor ($<100$ meV) consistent with electrical transport measurements. The estimated electronic structure parameters of the on-site Coulomb energies, in combination with the charge transfer energy and the hybridization strength obtained from the model calculations, indicate that ${\mathrm{Tb}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ is a strongly correlated charge transfer type semiconductor.

Electronic structure of the Si(111)3×3R30∘−B surface from theory and photoemission spectroscopy

The $\mathrm{Si}(111)\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R{30}^{\ensuremath{\circ}}\text{\ensuremath{-}}\mathrm{B}\phantom{\rule{0.28em}{0ex}}[\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{B}]$ surface has evolved into a particularly interesting surface in the context of on-surface molecular self-assembly. Photoemission spectroscopy is a powerful tool to understand the interaction between the surface and the adsorbates. Previous studies of $\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{B}$ contain many inconsistencies with regard to the Si $2p$ core level and valence-band dispersion. Here we employ synchrotron-based core-level and angle-resolved photoemission spectroscopy measurements in combination with density functional theory (DFT) calculations to address these issues. DFT calculations of the Si $2p$ core-level spectra accurately identify contributions from one bulk and five surface components, which allows us to obtain a comprehensive understanding of the spectra recorded at different photon energies. As an archetypal example, this refined decomposition is employed to understand the changes in Si $2p$ spectra upon the adsorption of cobalt phthalocyanine molecules. Regarding the valence-band dispersion of the clean $\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{B}$ surface, our comprehensive DFT and photoemission investigations are able to reconcile the inconsistencies appearing in previous studies and reveal several yet unreported surface states. Furthermore, we are able to theoretically and experimentally resolve the distribution of these surface states in constant energy plots.

Observation of an Excitonic Mott Transition through Ultrafast Core-cum-Conduction Photoemission Spectroscopy.

Time-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe_{2}. Wepresent a many-body approximation for the Green's function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions.

Evolution of Electronic States and Emergence of Superconductivity in the Polar Semiconductor GeTe by Doping Valence-Skipping Indium

GeTe is a chemically simple IV-VI semiconductor which bears a rich plethora of different physical properties induced by doping and external stimuli. Here, we report a superconductor-semiconductor-superconductor transition controlled by finely-tuned In doping. Our results reveal the existence of a critical doping concentration x_{c}=0.12 in Ge_{1-x}In_{x}Te, where various properties, including structure, resistivity, charge carrier type, and the density of states, take either an extremum or change their character. At the same time, we find indications of a change in the In-valence state from In^{3+} to In^{1+} with increasing x by core-level photoemission spectroscopy, suggesting that this system is a new promising playground to probe valence fluctuations and their possible impact on structural, electronic, and thermodynamic properties of their host.

Growth and characterization of Tb silicide nanostructures on Si(hhk) substrates

The growth of Tb-induced nanostructures on vicinal Si(111) substrates with an offcut toward the $\left[11\overline{2}\right]$ direction, so-called Si(hhk) substrates, was studied. The nanostructures were analyzed using scanning tunneling microscopy and spectroscopy as well as core-level photoemission spectroscopy. For low Tb coverages, terraces covered by a Tb-induced submonolayer structure are observed. At higher coverages, metallic Tb disilicide monolayer structures are dominant. Their appearance strongly depends on the offcut direction. Well-defined nanowires with straight edges are only present for Si(hhk) substrates with $hlk$. Furthermore, only these substrates may host very narrow metallic nanolines. In general, narrower nanostructures are observed for larger offcut angles enabling the formation of Tb disilicide nanowires with average widths as low as $2.3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ for Si(335) substrates.

Probing single-unit-cell resolved electronic structure modulations in oxide superlattices with standing-wave photoemission

Control of structural couplings at the complex-oxide interfaces is a powerful platform for creating new ultrathin layers with electronic and magnetic properties unattainable in the bulk. However, with the capability to design and control the electronic structure of such buried layers and interfaces at a unit-cell level, a new challenge emerges to be able to probe these engineered emergent phenomena with depth-dependent atomic resolution as well as element- and orbital selectivity. Here, we utilize a combination of core-level and valence-band soft x-ray standing-wave photoemission spectroscopy, in conjunction with scanning transmission electron microscopy, to probe the depth-dependent and single-unit-cell resolved electronic structure of an isovalent manganite superlattice [Eu0.7Sr0.3MnO3/La0.7Sr0.3MnO3]x15 wherein the electronic-structural properties are intentionally modulated with depth via engineered oxygen octahedra rotations/tilts and A-site displacements. Our unit-cell resolved measurements reveal significant transformations in the local chemical and electronic valence-band states, which are consistent with the layer-resolved first-principles theoretical calculations, thus opening the door for future depth-resolved studies of a wide variety of hetero-engineered material systems.

Linear polarization-dependent core-level photoemission spectroscopy in Yb-based valence fluctuating system

In this paper, we show the linearly polarized hard X-ray core-level photoemission spectroscopy of valence fluctuating system β-YbAlB4. Interestingly, we observed subtle but distinct linear dichroisms in Yb2+ 3d5/2 core-level spectra at several angles of β-YbAlB4. Basically linear dichroism in Yb2+ 3d5/2 core-level spectrum is not expected in the ionic picture because Yb2+ 4f orbital is fully occupied. These contradiction indicates the breakdown of ionic picture in β-YbAlB4.