X-ray Angle-resolved Photoemission Spectroscopy Research Articles

Orientation-dependent electronic structure in interfacial superconductors LaAlO3/KTaO3

Emergent superconductivity at the LaAlO3/KTaO3 interfaces exhibits a mysterious dependence on the KTaO3 crystallographic orientations. Here by soft X-ray angle-resolved photoemission spectroscopy, we directly resolve the electronic structure of the LaAlO3/KTaO3 interfacial superconductors and the non-superconducting counterpart. We find that the mobile electrons that contribute to the interfacial superconductivity show strong k⊥ dispersion. Comparing the superconducting and non-superconducting interfaces, the quasi-three-dimensional electron gas with over 5.5 nm spatial distribution ubiquitously exists and shows similar orbital occupations. The signature of electron-phonon coupling is observed and intriguingly dependent on the interfacial orientations. Remarkably, the stronger electron-phonon coupling signature correlates with the higher superconducting transition temperature. Our observations help scrutinize the theories on the orientation-dependent superconductivity and offer a plausible and straightforward explanation. The interfacial orientation effect that can modify the electron-phonon coupling strength over several nanometers sheds light on the applications of oxide interfaces in general.

Probing buried interface band dispersion of a MgO/Fe heterostructure with hard X-ray angle-resolved photoemission

Interface band dispersion of a MgO(2 nm)/Fe(50 nm) heterostructure was detected by hard X-ray angle-resolved photoemission spectroscopy (HARPES) with the excitation photon energy of 3.29 keV by utilizing X-ray total reflection (TR). By subtracting bulk-sensitive band dispersion of the buried Fe(001) obtained by HARPES in the non-TR condition from near-interface-sensitive Fe(001) band dispersion obtained by TR-HARPES, the band-folding of Fe and the O 2p -Fe 3d hybridization at the heterointerface were clearly unveiled. These results suggest that HARPES can probe not only the bulk band but also the buried interface band of heterojunctions.

Tin oxide thin films on Ag(111): Thickness and temperature dependent study of surface structure and electronic properties

Growth and structure of tin oxide films on Ag(111) from submonolayer to 10 monolayer equivalent thickness have been studied using low energy electron diffraction (LEED), x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy techniques for room temperature (RT) and high temperature (HT) (573 K). For RT growth, most of the tin is oxidised to form tin oxide (SnOx) as confirmed by XPS where no metallic component is noticed. However, the grown films are not epitaxial or ordered which can be confirmed by no spots in LEED for higher coverage. Metallic Sn along with two oxidation states of Sn are noticed for HT growth, with the surface mainly composed of SnO2. LEED pattern shows the presence of c(2 × 2) and (3 × 1) along with two different hexagonal multidomain type patterns. The electronic structure for lower coverage resembled the Sn/Ag(111) alloy structures for lower coverage. For higher coverages appearance of three bands below 2 eV binding energy was noticed.

Pioneering preparation and analysis of a clean surface on a microcrystal, mined by a focused ion beam

We demonstrate a series of procedures to prepare a clean surface of micro-sized graphite, mined from a bulk flake and securely affixed onto a macroscopic Si plate by focused ion beam scanning electron microscope. Analyses of structure and electronic (chemical) states were made using micro-beam X-ray photoelectron spectroscopy and angle-resolved photoemission spectroscopy. At the surface of the micro graphite, the band dispersion from a single-domain structure was observed. The proposed methodology showcases its capability to produce clean and high-quality micro samples suitable for surface-sensitive analyses. This technique paves the way to investigate surfaces of unexplored microcrystals embedded in complex materials.

Influence of Bi doping on the electronic structure of (Ga,Mn)As epitaxial layers

The influence of the addition of Bi to the dilute ferromagnetic semiconductor (Ga,Mn)As on its electronic structure as well as on its magnetic and structural properties has been studied. Epitaxial (Ga,Mn)(Bi,As) layers of high structural perfection have been grown using low-temperature molecular-beam epitaxy. Post-growth annealing of the samples improves their structural and magnetic properties and increases the hole concentration in the layers. Hard X-ray angle-resolved photoemission spectroscopy reveals a strongly dispersing band in the Mn-doped layers, which crosses the Fermi energy and is caused by the high concentration of Mn-induced itinerant holes located in the valence band. An increased density of states near the Fermi level is attributed to additional localized Mn states. In addition to a decrease in the chemical potential with increasing Mn doping, we find significant changes in the valence band caused by the incorporation of a small atomic fraction of Bi atoms. The spin–orbit split-off band is shifted to higher binding energies, which is inconsistent with the impurity band model of the band structure in (Ga,Mn)As. Spectroscopic ellipsometry and modulation photoreflectance spectroscopy results confirm the valence band modifications in the investigated layers.

Biofunctionalization of Porous Titanium Oxide through Amino Acid Coupling for Biomaterial Design.

Porous transition metal oxides are widely studied as biocompatible materials for the development of prosthetic implants. Resurfacing the oxide to improve the antibacterial properties of the material is still an open issue, as infections remain a major cause of implant failure. We investigated the functionalization of porous titanium oxide obtained by anodic oxidation with amino acids (Leucine) as a first step to couple antimicrobial peptides to the oxide surface. We adopted a two-step molecular deposition process as follows: self-assembly of aminophosphonates to titanium oxide followed by covalent coupling of Fmoc-Leucine to aminophosphonates. Molecular deposition was investigated step-by-step by Atomic Force Microscopy (AFM) and X-ray Photoemission Spectroscopy (XPS). Since the inherent high roughness of porous titanium hampers the analysis of molecular orientation on the surface, we resorted to parallel experiments on flat titanium oxide thin films. AFM nanoshaving experiments on aminophosphonates deposited on flat TiO2 indicate the formation of an aminophosphonate monolayer while angle-resolved XPS analysis gives evidence of the formation of an oriented monolayer exposing the amine groups. The availability of the amine groups at the outer interface of the monolayer was confirmed on both flat and porous substrates by the following successful coupling with Fmoc-Leucine, as indicated by high-resolution XPS analysis.

A new user-friendly materials science end station at the FinEstBeAMS beamline of MAX IV

FinEstBeAMS is an atmospheric and materials science beamline located at the 1.5 GeV storage ring of the MAX IV Laboratory in Lund, Sweden. It offers a very wide photon energy range 4.5-1300 eV and radiation with different polarization characteristics. The beamline has three end stations installed at two branch lines. The new solid state end station (SSES) is described in this paper. It is a high-throughput apparatus with flexible sample preparation options for X-ray photoemission, angle-resolved photoemission, and X-ray absorption spectroscopy. Three examples of experiments at room temperature demonstrate the capabilities of the SSES in the research field of surface science and condensed matter physics.

Structure and electronic properties of stable facets in the 2D material hexagonal boron nitride (hBN) on curved platinum

<h2>Abstract</h2> A hexagonal boron nitride (hBN) monolayer was grown on a curved crystal c-Pt(331) substrate including all vicinal surfaces between Pt(111) and Pt(110) faces. The surface structure has been studied by a combination of low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). We observed that the hBN monolayer covers the entire curved crystal, but induces a faceting subsequent to the growth. We encountered (111), (221), (441), (991), and (110) as stable facets. We assign this faceting to two factors i) a better lattice coincidence and ii) the existence of local covalent bonds and non-local Van der Waals interactions. The electronic structure was characterized by near-edge x-ray absorption fine structure (NEXAFS), X-ray photoemission spectroscopy and angle-resolved photoemission spectroscopy (ARPES). The hBN/Pt(111) has the weakest interacting overlayer system, while the hBN on vicinal surfaces is more strongly bound to the Pt substrate. Additionally, we determined using angle-resolved photoemission measurements (ARPES) that the π band shifts downward when going away from the hBN/Pt(111) surface towards hBN/Pt(110). In the latter case, the π band shift is the largest.

Robust Electronic Structure of Manganite-Buffered Oxide Interfaces with Extreme Mobility Enhancement.

The electronic structure as well as the mechanism underlying the high-mobility two-dimensional electron gases (2DEGs) at complex oxide interfaces remain elusive. Herein, using soft X-ray angle-resolved photoemission spectroscopy (ARPES), we present the band dispersion of metallic states at buffered LaAlO3/SrTiO3 (LAO/STO) heterointerfaces where a single-unit-cell LaMnO3 (LMO) spacer not only enhances the electron mobility but also renders the electronic structure robust toward X-ray radiation. By tracing the evolution of band dispersion, orbital occupation, and electron-phonon interaction of the interfacial 2DEG, we find unambiguous evidence that the insertion of the LMO buffer strongly suppresses both the formation of oxygen vacancies as well as the electron-phonon interaction on the STO side. The latter effect makes the buffered sample different from any other STO-based interfaces and may explain the maximum mobility enhancement achieved at buffered oxide interfaces.

Surface and dynamical properties of GeI2

GeI2 is an interesting two-dimensional wide-band gap semiconductor because of diminished edge scattering due to an absence of dangling bonds. Angle-resolved x-ray photoemission spectroscopy indicates a germanium rich surface, and a surface to bulk core-level shift of 1.8 eV in binding energy, between the surface and bulk components of the Ge 2p3/2 core-level, making clear that the surface is different from the bulk. Temperature dependent studies indicate an effective Debye temperature () of 186 ± 18 K for the germanium x-ray photoemission spectroscopy feature associated with the surface. These measurements also suggest an unusually high effective Debye temperature for iodine (587 ± 31 K), implying that iodine is present in the bulk of the material, and not the surface. From optical absorbance, GeI2 is seen to have an indirect (direct) optical band gap of 2.60 (2.8) ± 0.02 (0.1) eV, consistent with the expectations. Temperature dependent magnetometry indicates that GeI2 is moment paramagnetic at low temperatures (close to 4 K) and shows a diminishing saturation moment at high temperatures (close to 300 K and above).

Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi2B2C

Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. A broad spectral phonon line shape is often interpreted as a marker of strong electron-phonon coupling associated with Fermi surface nesting, i.e., parallel sections of the Fermi surface connected by the phonon momentum. Alternatively broad phonons are known to arise from strong atomic lattice anharmonicity. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi2B2C. This new scenario likely applies to a wide range of compounds.

Surface-to-bulk core level shift in CoFe2O4 thin films

In spite of the absence of significant segregation of either cobalt oxide or iron oxide, core level photoemission binding energy shifts tend to indicate that the surface is significantly different from the bulk for CoFe2O4(111) thin films grown on Al2O3(0001). CoFe2O4(111) thin films show a surface-to-bulk core level shift in both the Co 2p and Fe 2p core level photoemission spectra. Surface weighted components in the core level photoemission spectra of both Co 2p3/2 and Fe 2p3/2 can be distinguished from the bulk components, by angle-resolved x-ray photoemission spectroscopy, for CoFe2O4(111) thin films. The surface termination of CoFe2O4(111) contains both Co and Fe with no evidence of strong preferential surface termination of either an iron or cobalt oxide, except for CoFe2O4(111) in the thin film limit. With extensive annealing above room temperature, the cobalt oxide component of very thin CoFe2O4(111) films, grown on Al2O3 (0001), will lose oxygen.

Complexities at the Au/ZrS3(001) interface probed by x-ray photoemission spectroscopy

Interaction between the Au adlayers and ZrS3(001) has been examined via x-ray photoemission spectroscopy (XPS). The angle-resolved XPS measurements reveal that ZrS3(001) is disulfide (S2 2−) terminated and the Au thickness-dependent XPS indicates that the observed band bending, for low Au coverage, is consistent with formation of a Schottky barrier at the Au/ZrS3(001) interface. This band bending, however, appears to be suppressed as the thickness of Au adlayer is increased, indicating varying interfacial interactions at the Au/ZrS3(001) interface. Such complex interface effects between Au and ZrS3(001) may explain the observed non-ohmic I–V characteristics for a ZrS3-based device, and could suppress current injection.

Charge density wave in single-layer Pb/Ge(111) driven by Pb-substrate exchange interaction

Single layer Pb on top of (111) surfaces of group IV semiconductors hosts charge density wave and superconductivity depending on the coverage and on the substrate. These systems are normally considered to be experimental realizations of single band Hubbard models and their properties are mostly investigated using lattice models with frozen structural degrees of freedom, although the reliability of this approximation is unclear. Here, we consider the case of Pb/Ge(111) at $1/3$ coverage, for which surface x-ray diffraction and angle-resolved photoemission spectroscopy data are available. By performing first-principles calculations, we demonstrate that the nonlocal exchange between Pb and the substrate drives the system into a $3\ifmmode\times\else\texttimes\fi{}3$ charge density wave. The electronic structure of this charge ordered phase is mainly determined by two effects: The magnitude of the Pb distortion and the large spin-orbit coupling. Finally, we show that the effect applies also to the $3\ifmmode\times\else\texttimes\fi{}3$ phase of Pb/Si(111) where the Pb-substrate exchange interaction increases the bandwidth by more than a factor $1.5$ with respect to density functional theory $+\mathrm{U}$, in better agreement with scanning tunneling spectroscopy data. The delicate interplay between substrate, structural, and electronic degrees of freedom invalidates the widespread interpretation available in literature considering these compounds as physical realizations of single band Hubbard models.

X-ray photoemission studies of BiInO3: Surface termination and effective Debye temperature

BiInO3 is a potentially polar oxide with distinct optical properties whose origin could result from a surface that has not been well characterized. The surface properties of BiInO3 thin films have been characterized here by temperature dependent angle-resolved x-ray photoemission spectroscopy. A large surface to bulk core-level binding energy shift for the In 3d5/2 core-level is identified, indicating a surface very different from the bulk. BiInO3 terminates in indium oxide and loses bismuth from the surface of the film at T = 573 K. The Debye–Waller plots suggest effective Debye temperatures of 263 ± 10 and 556 ± 27 K for the surface and bulk components of In 3d core-level, respectively.

Bulk electronic structure of lanthanum hexaboride ( LaB6 ) by hard x-ray angle-resolved photoelectron spectroscopy

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter physics, and for their applications in advanced technological fields. Among these compounds, LaB$_6$ has a special place, being a traditional d-band metal without additional f- bands. In this paper we investigate the bulk electronic structure of LaB$_6$ using hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. By comparing La 3d core level spectra to cluster model calculations, we identify well-screened peak residing at a lower binding energy compared to the main poorly-screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulk-like band structure of LaB$_6$ determined by hard x-ray angle-resolved photoemission spectroscopy (HARPES). We interpret HARPES experimental results by the free-electron final-state calculations and by the more precise one-step photoemission theory including matrix element and phonon excitation effects. In addition, we consider the nature and the magnitude of phonon excitations in HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that one step theory of photoemission and HARPES experiments provide, at present, the only approach capable of probing true bulk-like electronic band structure of rare-earth hexaborides and strongly correlated materials.

Temperature-dependent change of the electronic structure in the Kondo lattice system YbRh2Si2

The heavy-fermion behavior in intermetallic compounds manifests itself in a quenching of local magnetic moments by developing Kondo spin-singlet many-body states combined with a drastic increase of the effective mass of conduction electrons, which occurs below the lattice Kondo temperature T K. This behavior is caused by interactions between the strongly localized 4f electrons and itinerant electrons. A controversially discussed question in this context is how the localized electronic states contribute to the Fermi surface upon changing the temperature. One expects that hybridization between the local moments and the itinerant electrons leads to a transition from a small Fermi surface in a non-coherent regime at high temperatures to a large Fermi surface once the coherent Kondo lattice regime is realized below T K. We demonstrate, using hard x-ray angle-resolved photoemission spectroscopy that the electronic structure of the prototypical heavy fermion compound YbRh2Si2 changes with temperature between 100 and 200 K, i.e. far above the Kondo temperature, T K = 25 K, of this system. Our results suggest a transition from a small to a large Fermi surface with decreasing temperature. This result is inconsistent with the prediction of the dynamical mean-field periodic Anderson model and supports the idea of an independent energy scale governing the change of band dispersion.

An investigation into the surface termination and near-surface bulk doping of oxygen-terminated diamond with lithium at various annealing temperatures

An alternative method of doping and surface functionalization of diamond using a chemical route was explored. The interaction of Li with the surface and bulk of oxygen-terminated diamond was investigated using Angle-Resolved X-ray Photoemission Spectroscopy (ARXPS). A stable LiO2 termination of diamond (100) surface and doping of near-surface diamond bulk was achieved up to an annealing temperature of 850 °C. The changes in interaction between the species involved (C, O, Li) and their stoichiometric ratios at the surface were investigated as a function of annealing temperature. This was done using ARXPS peak analysis.Graphic abstract

Direct-ARPES and STM Investigation of FeSe Thin Film Growth by Nd:YAG Laser

Research on ultrathin quantum materials requires full control of the growth and surface quality of the specimens in order to perform experiments on their atomic structure and electron states leading to ultimate analysis of their intrinsic properties. We report results on epitaxial FeSe thin films grown by pulsed laser deposition (PLD) on CaF2 (001) substrates as obtained by exploiting the advantages of an all-in-situ ultra-high vacuum (UHV) laboratory allowing for direct high-resolution surface analysis by scanning tunnelling microscopy (STM), synchrotron radiation X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) on fresh surfaces. FeSe PLD growth protocols were fine-tuned by optimizing target-to-substrate distance d and ablation frequency, atomically flat terraces with unit-cell step heights are obtained, overcoming the spiral morphology often observed by others. In-situ ARPES with linearly polarized horizontal and vertical radiation shows hole-like and electron-like pockets at the Γ and M points of the Fermi surface, consistent with previous observations on cleaved single crystal surfaces. The control achieved in growing quantum materials with volatile elements such as Se by in-situ PLD makes it possible to address the fine analysis of the surfaces by in-situ ARPES and XPS. The study opens wide avenues for the PLD based heterostructures as work-bench for the understanding of proximity-driven effects and for the development of prospective devices based on combinations of quantum materials.

Itinerant ferromagnetism mediated by giant spin polarization of the metallic ligand band in the van der Waals magnet Fe5GeTe2

We investigate near-Fermi-energy (EF) element-specific electronic and spin states of ferromagnetic van der Waals (vdW) metal Fe5GeTe2. The soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurement provides spectroscopic evidence of localized Fe 3d band. We also find prominent hybridization between the localized Fe 3d band and the delocalized Ge/Te p bands. This picture is strongly supported from direct observation of the remarkable spin polarization of the ligand p bands near EF, using x-ray magnetic circular dichroism (XMCD) measurements. The strength of XMCD signal from ligand element Te shows the highest value, as far as we recognize, among literature reporting finite XMCD signal for none-magnetic element in any systems. Combining SX-ARPES and elemental selective XMCD measurements, we collectively point an important role of giant spin polarization of the delocalized ligand Te states for realizing itinerant long-range ferromagnetism in Fe5GeTe2. Our finding provides a fundamental elemental selective view-point for understanding mechanism of itinerant ferromagnetism in low dimensional compounds, which also leads insight for designing exotic magnetic states by interfacial band engineering in heterostructures.