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International audience; Oxide interfaces exhibit a broad range of physical effects stemming from broken inversion symmetry. In particular, they can display non-reciprocal phenomena when time reversal symmetry is also broken, e.g., by the application of a magnetic field. Examples include the direct and inverse Edelstein effects (DEE, IEE) that allow the interconversion between spin currents and charge currents. The DEE and IEE have been investigated in interfaces based on the perovskite SrTiO$_3$ (STO), albeit in separate studies focusing on one or the other. The demonstration of these effects remains mostly elusive in other oxide interface systems despite their blossoming in the last decade. Here, we report the observation of both the DEE and IEE in a new interfacial two-dimensional electron gas (2DEG) based on the perovskite oxide KTaO$_3$. We generate 2DEGs by the simple deposition of Al metal onto KTaO$_3$ single crystals, characterize them by angle-resolved photoemission spectroscopy and magnetotransport, and demonstrate the DEE through unidirectional magnetoresistance and the IEE by spin-pumping experiments. We compare the spin-charge interconversion efficiency with that of STO-based interfaces, relate it to the 2DEG electronic structure, and give perspectives for the implementation of KTaO$_3$ 2DEGs into spin-orbitronic devices.

One way to induce insulator-to-metal transitions in the spin-orbit Mott insulator ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ is to substitute iridium with transition metals (Ru, Rh). However, this creates intriguing inhomogeneous metallic states, which cannot be described by a simple doping effect. We detail the electronic structure of the Ru-doped case with angle-resolved photoemission and show that, in contrast to Rh, it cannot be connected to the undoped case by a rigid shift. We further identify bands below ${E}_{F}$ coexisting with the metallic ones that we assign to nonbonding Ir sites. We rationalize the differences between Rh and Ru by a different hybridization with oxygen, which mediates the coupling to Ir and sensitively affects the effective doping. We argue that the spin-orbit coupling does not control either the charge transfer or the transition threshold.

Hybridization between $f$ electrons and conduction bands ($c$-$f$ hybridization) is a driving force for many unusual phenomena. To provide insight into it, systematic studies of CeCoIn$_5$ heavy fermion superconductor have been performed by angle-resolved photoemission spectroscopy (ARPES) in a large angular range at temperature of $T=6$ K. The used photon energy of 122 eV corresponds to Ce $4d$-$4f$ resonance. Calculations carried out with relativistic multiple scattering Korringa-Kohn-Rostoker method and one-step model of photoemission yielded realistic simulation of the ARPES spectra indicating that Ce-In surface termination prevails. Surface states, which have been identified in the calculations, contribute significantly to the spectra. Effects of the hybridization strongly depend on wave vector. They include a dispersion of heavy electrons and bands gaining $f$-electron character when approaching Fermi energy. We have also observed a considerable variation of $f$-electron spectral weight at $E_F$, which is normally determined by both matrix element effects and wave vector dependent $c$-$f$ hybridization. Fermi surface scans covering a few Brillouin zones revealed large matrix element effects. A symmetrization of experimental Fermi surface, which reduces matrix element contribution, yielded a specific variation of $4f$-electron enhanced spectral intensity at $E_F$ around $\bar{\Gamma}$ and $\bar{M}$ points. Tight-binding approximation calculations for Ce-In plane provided the same universal distribution of $4f$-electron density for a range of values of the parameters used in the model., Comment: 15 pages, 9 figures

Oxide interfaces exhibit a broad range of physical effects stemming from broken inversion symmetry. In particular, they can display non-reciprocal phenomena when time reversal symmetry is also broken, e.g., by the application of a magnetic field. Examples include the direct and inverse Edelstein effects (DEE, IEE) that allow the interconversion between spin currents and charge currents. The DEE and IEE have been investigated in interfaces based on the perovskite SrTiO

International audience; This paper demonstrates that highly reflective Cr/B 4 C multilayer interference coatings with nano-metric layer thicknesses, designed to operate in the soft X-ray photon energy range, have stable reflective performance for a period of 3 years after deposition. The microstructure and chemical composition of layers and interfaces within Cr/B 4 C multilayers is also examined, with emphasis on the B 4 Con -Cr interface where a significant diffusion layer is formed and on the oxide in the top B 4 C layer. Multiple characterization techniques (X-ray reflectivity at different photon energies, X-ray pho-toelectron spectroscopy, transmission electron microscopy, electron diffraction and X-ray diffraction) are employed and the results reveal a consistent picture of the Cr/B 4 C layer structure.

We report the observation of a non-trivial spin texture in Dirac node arcs, novel topological objects formed when Dirac cones of massless particles extend along an open one-dimensional line in momentum space. We find that such states are present in all the compounds of the tetradymite M$_2$Te$_2$X family (M$=$Ti, Zr or Hf and X$=$P or As), regardless of the weak or strong character of the topological invariant. The Dirac node arcs in tetradymites are thus the simplest possible, textbook example, of a type-I Dirac system with a single spin-polarized node arc., Main Text + Supplementary Information, 12 pages, 11 figures

International audience; van der Waals materials offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Among them, As2Te3 has attracted attention due to the promise of outstanding electronic properties, and high photo-response. Precise experimental determinations of the electronic properties of As2Te3 are yet sorely needed for better understanding of potential properties and device applications. Here, we study the structural and electronic properties of α-As2Te3. Scanning transmission electron microscopy coupled to energy X-ray dispersion (STEM-EDX), and micro-Raman spectroscopy all confirm that our specimens correspond to α-As2Te3. Scanning tunneling spectroscopy (STS) at 4.2K demonstrates that α-As2Te3 exhibits an electronic band gap of about 0.4 eV. The material can be exfoliated, revealing the (100) anisotropic surface. Transport measurements on a thick exfoliated sample (bulk-like) confirm the STS results. These findings allows for a deeper understanding of the As2Te3 electronic properties, underlying the potential of V-VI semiconductors for electronic and photonic technologies.

By adjusting the incidence angle of incoming X-ray near the critical angle of X-ray total reflection, the photoelectron intensity is strongly modulated due to the variation of X-ray penetration depth. Photoelectron spectroscopy (PES) combining with near total reflection (NTR) exhibit tunable surface sensitivity, providing depth-resolved information. In this review article, we first describe the experimental setup and specific data analysis process. We then review three different examples which show the broad application of this method. The emphasis is on its applications to correlated oxide heterostructures, especially quantitative depth analyses of compositions and electronic states. In a last part, we discussed the limitations of this technique, mostly in terms of range of samples which can be studied., Comment: 28 pages, 5 figures, review paper on NTR HAXPES

Vicinal surfaces exhibiting arrays of atomic steps are frequently investigated due to their diverse physical-chemical properties and their use as growth templates. However, surfaces featuring steps with a large number of low-coordinated kink-atoms have been widely ignored, despite their higher potential for chemistry and catalysis. Here, the equilibrium structure and the electronic states of vicinal Ag(111) surfaces with densely kinked steps are investigated in a systematic way using a curved crystal. With scanning tunneling microscopy we observe an exceptional structural homogeneity of this class of vicinals, reflected in the smooth probability distribution of terrace sizes at all vicinal angles. This allows us to observe, first, a subtle evolution of the terrace-size distribution as a function of the terrace-width that challenges statistical models of step lattices, and second, lattice fluctuations around resonant modes of surface states. As shown in angle resolved photoemission experiments, surface states undergo stronger scattering by fully-kinked step-edges, which triggers the full depletion of the two-dimensional band at surfaces with relatively small vicinal angles., We acknowledge the financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2016-78293-C6 and Severo Ochoa No. SEV-2013-0295), the Basque Government (Grant No. IT-621-13), the regional Government of Aragon (RASMIA project), the CERCA Programme/Generalitat de Catalunya, and the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA194/16 TNSI).

Epitaxial bilayer silicon oxide is a transferable two-dimensional material predicted to be a wide band gap semiconductor, with potential applications for deep UV optoelectronics, or as a building block of van der Waals heterostructures. The prerequisite to any sort of such applications is the knowledge of the electronic band structure, which we unveil using angle-resolved photoemission spectroscopy and rationalize with the help of density functional theory (DFT) calculations. We discover dispersing bands related to electronic delocalization within the top and bottom planes of the material, with two linear crossings reminiscent of those predicted in bilayer AA-stacked graphene, and semi-flat bands stemming from the chemical bridges between the two planes. This band structure is robust against exposure to air, and can be controlled by exposure to oxygen. We provide an experimental lower-estimate of the band gap size of 5 eV and predict a full gap of 7.36 eV using DFT calculations.

Physical review / B 101(8), 085121 (1-10) (2020). doi:10.1103/PhysRevB.101.085121, We report the observation of a two-dimensional electron system (2DES) at the (110) surface of the transparent bulk insulator SnO$_2$ and the tunability of its carrier density by means of temperature or Eu deposition. The 2DES is insensitive to surface reconstructions and, surprisingly, it survives even after exposure to ambient conditions—an extraordinary fact recalling the well known catalytic properties SnO$_2$. Our data show that surface oxygen vacancies are at the origin of such 2DES, providing key information about the long-debated origin of n-type conductivity in SnO$_2$, at the basis of a wide range of applications. Furthermore, our study shows that the emergence of a 2DES in a given oxide depends on a delicate interplay between its crystal structure and the orbital character of its conduction band., Published by Inst., Woodbury, NY

The need for new materials is constantly growing as their use becomes more and more critical for technological advances. Materials come in a wide variety of structures, dimensionalities, and phases...

Transition metal oxide heterostructures are interesting due to the distinctly different properties that can arise from their interfaces, such as superconductivity, high catalytic activity, and magnetism. Oxygen point defects can play an important role at these interfaces in inducing potentially novel properties. The design of oxide heterostructures in which the oxygen defects are manipulated to attain specific functionalities requires the ability to resolve the state and concentration of local oxygen defects across buried interfaces. In this work, we utilized a novel combination of hard X-ray photoelectron spectroscopy (HAXPES) and high resolution X-ray diffraction (HRXRD) to probe the local oxygen defect distribution across the buried interfaces of oxide heterolayers. This approach provides a nondestructive way to qualitatively probe locally the oxygen defects in transition metal oxide heterostructures. We studied two trilayer structures as model systems: the La0.8Sr0.2CoO3−δ/(La0.5Sr0.5)2CoO4−δ/La0.8Sr0...

Structural defects in the molybdenum disulfide (MoS2) monolayer are widely known for strongly altering its properties. Therefore, a deep understanding of these structural defects and how they affect MoS2 electronic properties is of fundamental importance. Here, we report on the incorporation of atomic hydrogen in mono-layered MoS2 to tune its structural defects. We demonstrate that the electronic properties of single layer MoS2 can be tuned from the intrinsic electron (n) to hole (p) doping via controlled exposure to atomic hydrogen at room temperature. Moreover, this hydrogenation process represents a viable technique to completely saturate the sulfur vacancies present in the MoS2 flakes. The successful incorporation of hydrogen in MoS2 leads to the modification of the electronic properties as evidenced by high resolution X-ray photoemission spectroscopy and density functional theory calculations. Micro-Raman spectroscopy and angle resolved photoemission spectroscopy measurements show the high quality of the hydrogenated MoS2 confirming the efficiency of our hydrogenation process. These results demonstrate that the MoS2 hydrogenation could be a significant and efficient way to achieve tunable doping of transition metal dichalcogenides (TMD) materials with non-TMD elements., 15 pages, 6 figures + SI 3 pages 3 figures. Pre-print published with the authorization of ACS Publications

${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ is characterized by a large spin-orbit coupling, which gives rise to bands with strongly entangled spin and orbital characters, called ${\mathrm{J}}_{1/2}$ and ${\mathrm{J}}_{3/2}$. We use light-polarization dependent ARPES to study directly the orbital character of these bands and fully map out their dispersion. We observe bands in very good agreement with our cluster dynamical mean-field theory calculations. We show that the ${\mathrm{J}}_{1/2}$ band, the closest to the Fermi level ${E}_{F}$, is dominated by ${d}_{xz}$ character along ${k}_{x}$ and ${d}_{yz}$ along ${k}_{y}$. This is actually in agreement with an isotropic ${\mathrm{J}}_{1/2}$ character on average, but this large orbital dependence in $\mathbf{k}$ space was mostly overlooked before. It gives rise to strong modulations of the ARPES intensity that we explain and carefully take into account to compare dispersions in equivalent directions of the Brillouin zone. Although the latter dispersions look different at first, suggesting possible symmetry breakings, they are found essentially similar, once corrected for these intensity variations. In particular, the pseudogaplike features close to the $X$ point appearing in the nearly metallic 15% Rh-doped ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ strongly depend on experimental conditions. We reveal that there is nevertheless an energy scale of 30 meV below which spectral weight is suppressed, independent of the experimental conditions, which gives a reliable basis to analyze this behavior. We suggest it is caused by disorder.

International audience; By means of angle-resolved photoemission spectroscopy and density functional theory calculations, we investigate the electronic structure of a Br or I atom overlayer on the Cu(111) surface produced by the well-known Ullmann coupling reaction. We found that the iodine adsorbate induces two spin–orbit split highly dispersive bands, which are well-separated from the bulk Cu 3d bands, whereas the bromine-induced bands are flat and largely hybridized with the copper states. Also, our measured constant energy maps show that the I-induced bands have a parabolic shape in the whole surface Brillouin zone, which is confirmed by our calculations. Overall, the agreement between theory and experiments is excellent, giving new insights into the electronic structure of halogen atoms on noble metals and their possible influence on the molecular electronic structure.

The reduction of magnetic damping is one of the biggest challenges in low-energy-consumption spintronics and magnonics, in the pursuit of $e.g.$ low switching current for spin-transfer-torque-based technology, and long-range spin-wave propagation. This experimental study highlights ultralow damping values in high-quality epitaxial Co${}_{2}$Mn$Z$ ($Z$ = Al, Si, Ga, Ge, Sn, Sb) Heusler half-metallic magnets. As predicted theoretically, these ultralow values are intrinsically coupled to the underlying electronic structure. The width of the spin gap, as well as the location of the Fermi energy within it, play key roles in the relaxation of precessing magnetization.

Silicon oxide can be formed in a crystalline form, when prepared on a metallic substrate. It is a candidate support catalyst and possibly the ultimately-thin version of a dielectric host material for two-dimensional materials (2D) and heterostructures. We determine the atomic structure and chemical bonding of the ultimately thin version of the oxide, epitaxially grown on Ru(0001). In particular, we establish the existence of two sub-lattices defined by metal-oxygen-silicon bridges involving inequivalent substrate sites. We further discover four electronic bands below Fermi level, at high binding energies, two of them forming a Dirac cone at K point, and two others forming semi-flat bands. While the latter two correspond to hybridized states between the oxide and the metal, the former relate to the topmost silicon-oxygen plane, which is not directly coupled to the substrate. Our analysis is based on high resolution X-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and density functional theory calculations., Comment: Main part : 31 pages, 6 figures / Supporting information : 13 pages, 11 figures

Metal mono-chalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers and stacking-order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photo-response. Metal mono-chalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers and stacking-order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photo-response. Precise experimental determination of the electronic structure of InSe is sorely needed for better understanding of potential properties and device applications. Here, combining scanning tunneling spectroscopy (STS) and two-photon photoemission spectroscopy (2PPE), we demonstrate that InSe exhibits a direct band gap of about 1.25 eV located at the Gamma point of the Brillouin zone (BZ). STS measurements underline the presence of a finite and almost constant density of states (DOS) near the conduction band minimum (CBM) and a very sharp one near the maximum of the valence band (VMB). This particular DOS is generated by a poorly dispersive nature of the top valence band, as shown by angle resolved photoemission spectroscopy (ARPES) investigation. technologies. In fact, a hole effective mass of about m/m0 = -0.95 gammaK direction) was measured. Moreover, using ARPES measurements a spin-orbit splitting of the deeper-lying bands of about 0.35 eV was evidenced. These findings allow a deeper understanding of the InSe electronic properties underlying the potential of III-VI semiconductors for electronic and photonic

Single crystalline bismuth (Bi) is known to have a peculiar electronic structure which is very close to the topological phase transition. The modification of the surface states of Bi depending on the temperature are revealed by angle-resolved photoelectron spectroscopy (ARPES). At low temperature, the upper branch of the surface state merged to the projected bulk conduction bands around the $\bar{M}$ point of the surface Brillouin zone (SBZ). In contrast, the same branch merged to the projected bulk valence bands at high temperature (400 K). Such behavior could be interpreted as a topological phase transition driven by the temperature, which might be applicable for future spin-thermoelectric devices. We discuss the possible mechanisms to cause such transition, such as the thermal lattice distortion and electron-phonon coupling., 15 pages with 6 figures (single column)

A synergistic effect between strong electron correlation and spin-orbit interaction (SOI) has been theoretically predicted to result in a new topological state of quantum matter on Kondo insulators (KIs), so-called topological Kondo insulators (TKIs). One TKI candidate has been experimentally observed on the KI SmB6(001), and the origin of the surface states (SS) and the topological order of SmB6 has been actively discussed. Here, we show a metallic SS on the clean surface of another TKI candidate YbB12(001), using angle-resolved photoelectron spectroscopy. The SS showed temperature-dependent reconstruction corresponding with the Kondo effect observed for bulk states. Despite the low-temperature insulating bulk, the reconstructed SS with c-f hybridization was metallic, forming a closed Fermi contour surrounding $\bar{\Gamma}$ on the surface Brillouin zone and agreeing with the theoretically expected behavior for SS on TKIs. These results demonstrate the temperature-dependent holistic reconstruction of two-dimensional states localized on KIs surface driven by the Kondo effect., Comment: 15 pages, 4 figures

At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO3 and SrTiO3. However, many oxides have a more complex electronic structure, with charge, orbital and/or spin orders arising from correlations between transition metal and oxygen ions. Strong correlations thus offer a rich playground to engineer functional interfaces but their compatibility with the classical band alignment picture remains an open question. Here we show that beyond differences in electron affinities and polar effects, a key parameter determining charge transfer at correlated oxide interfaces is the energy required to alter the covalence of the metaloxygen bond. Using the perovskite nickelate (RNiO3) family as a template, we probe charge reconstruction at interfaces with gadolinium titanate GdTiO3. X-ray absorption spectroscopy shows that the charge transfer is thwarted by hybridization effects tuned by the rare-earth (R) size. Charge transfer results in an induced ferromagnetic-like state in the nickelate, exemplifying the potential of correlated interfaces to design novel phases. Further, our work clarifies strategies to engineer two-dimensional systems through the control of both doping and covalence., Work supported by ERC Consolidator grant MINT (Contract No. 615759)

In ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ and ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, correlations, magnetism, and spin-orbit coupling compete on similar energy scales, creating a new context to study metal-insulator transitions (MIT). We use here angle-resolved photoemission to investigate the MIT as a function of hole and electron doping in ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, obtained respectively by Ir/Rh and Sr/La substitutions. We show that there is a clear reduction as a function of doping of the gap between a lower and upper band on both sides of the Fermi level, from 0.2 to 0.05 eV. Although these two bands have a counterpart in band structure calculations, they are characterized by a very different degree of coherence. The upper band exhibits clear quasiparticle peaks, while the lower band is very broad and loses weight as a function of doping. Moreover, their ARPES spectral weights obey different periodicities, reinforcing the idea of their different nature. We argue that a very similar situation occurs in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ and conclude that the physics of the two families is essentially the same.

Author(s): Kuo, CT; Lin, SC; Conti, G; Pi, ST; Moreschini, L; Bostwick, A; Meyer-Ilse, J; Gullikson, E; Kortright, JB; Nemsak, S; Rault, JE; Le Fevre, P; Bertran, F; Santander-Syro, AF; Vartanyants, IA; Pickett, WE; Saint-Martin, R; Taleb-Ibrahimi, A; Fadley, CS | Abstract: A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the CuO2 layers and those of the intermediate layers. The large c axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-layer depth resolution of these properties. Applying SW photoemission to Bi2Sr2CaCu2O8+δ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the SrO layers and oxygen bonding to three different cations. The layer-resolved valence electronic structure is found to be strongly influenced by the atomic supermodulation structure, as determined by comparison to density functional theory calculations, by Ca-Sr intermixing, and by correlation effects associated with the Cu 3d-3d Coulomb interaction, further clarifying the complex interactions in this prototypical cuprate. Measurements of this type for other quasi-two-dimensional materials with large c represent a promising future direction.

This paper demonstrates that highly reflective Cr/B₄C multilayer interference coatings with nanometric layer thicknesses, designed to operate in the soft X-ray photon energy range, have stable reflective performance for a period of 3 years after deposition. The microstructure and chemical composition of layers and interfaces within Cr/B₄C multilayers is also examined, with emphasis on the B₄C-on-Cr interface where a significant diffusion layer is formed and on the oxide in the top B₄C layer. Multiple characterization techniques (X-ray reflectivity at different photon energies, X-ray photoelectron spectroscopy, transmission electron microscopy, electron diffraction and X-ray diffraction) are employed and the results reveal a consistent picture of the Cr/B₄C layer structure.

$\mathrm{LaCr}{\mathrm{O}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{LCO})/\mathrm{SrTi}{\mathrm{O}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{STO})$ heterojunctions are intriguing due to a polar discontinuity along $[001]$, exhibiting two distinct and controllable charged interface structures [${(\mathrm{LaO})}^{+}/{(\mathrm{Ti}{\mathrm{O}}_{2})}^{0}$ and ${(\mathrm{SrO})}^{0}/{(\mathrm{Cr}{\mathrm{O}}_{2})}^{\ensuremath{-}}$] with induced polarization, and a resulting depth-dependent potential. In this study, we have used soft- and hard-x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to quantitatively determine the elemental depth profile, interface properties, and depth distribution of the polarization-induced built-in potentials. We observe an alternating charged interface configuration: a positively charged ${(\mathrm{LaO})}^{+}/{(\mathrm{Ti}{\mathrm{O}}_{2})}^{0}$ intermediate layer at the $\mathrm{LC}{\mathrm{O}}_{\mathrm{top}}/\mathrm{ST}{\mathrm{O}}_{\mathrm{bottom}}$ interface and a negatively charged ${(\mathrm{SrO})}^{0}/{(\mathrm{Cr}{\mathrm{O}}_{2})}^{\ensuremath{-}}$ intermediate layer at the $\mathrm{ST}{\mathrm{O}}_{\mathrm{top}}/\mathrm{LC}{\mathrm{O}}_{\mathrm{bottom}}$ interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy-loss spectroscopy mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO and STO contributions to the valence-band (VB) spectra. Using a two-step analytical approach involving first SW-induced core-level binding-energy shifts and then VB modeling, the variation in potential across the complete superlattice is determined in detail. This potential is in excellent agreement with density functional theory models, confirming this method as a generally useful tool for interface studies.

${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ is the archetype of the spin-orbit Mott insulator, but the nature of the metallic states that may emerge from this type of insulator is still not very well known. We study with angle-resolved photoemission the insulator-to-metal transition observed in ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}{\mathrm{O}}_{4}$ when Ir is substituted by Rh ($0.02lxl0.35$). The originality of the Rh doping is that Ir and Rh, which are formally isovalent, adopt different charge states, a rather unusual and inhomogeneous situation. We show that the evolution to the metallic state can be essentially understood as a shift of the Fermi level into the lower Hubbard band of ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$. The Mott gap appears quite insensitive to the introduction of up to $\ensuremath{\sim}20%$ holes in this band. The metallic phase, which forms for $xg0.07$, is not a Fermi liquid. It is characterized by the absence of quasiparticles, unrenormalized band dispersion compared to calculations, and an $\ensuremath{\sim}30\text{\ensuremath{-}}\mathrm{meV}$ pseudogap on the entire Fermi surface.

Combining single-layer two-dimensional semiconducting transition metal dichalcogenides (TMDs) with graphene layer in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these heterostructures. Here, we report the electronic and structural properties of transferred single layer WS2 on epitaxial graphene using micro-Raman spectroscopy, angle-resolved photoemission spectroscopy measurements (ARPES) and Density Functional Theory (DFT) calculations. The results show good electronic properties as well as well-defined band arising from the strong splitting of the single layer WS2 valence band at K points, with a maximum splitting of 0.44 eV. By comparing our DFT results with local and hybrid functionals, we find the top valence band of the experimental heterostructure is close to the calculations for suspended single layer WS2. . Our results provide an important reference for future studies of electronic properties of WS2 and its applications in valleytronic devices., Comment: 11 pages, 3 figures + SI 7 pages 8 figures

The electric field control of functional properties is a crucial goal in oxide-based electronics. Non-volatile switching between different resistivity or magnetic states in an oxide channel can be achieved through charge accumulation or depletion from an adjacent ferroelectric. However, the way in which charge distributes near the interface between the ferroelectric and the oxide remains poorly known, which limits our understanding of such switching effects. Here we use a first-of-a-kind combination of scanning transmission electron microscopy with electron energy loss spectroscopy, near-total-reflection hard X-ray photoemission spectroscopy, and ab-initio theory to address this issue. We achieve a direct, quantitative, atomic-scale characterization of the polarization-induced charge density changes at the interface between the ferroelectric BiFeO3 and the doped Mott insulator Ca1-xCexMnO3, thus providing insight on how interface-engineering can enhance these switching effects., Work supported by ERC Consolidator grant MINT (Contract No. 615759)

The electronic structure of the reentrant superconductor $\mathrm{Eu}({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14}$)${}_{2}{\mathrm{As}}_{2}$ (${T}_{c}=22$ K) with coexisting ferromagnetic order (${T}_{M}=18$ K) is investigated using angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. We study the in-plane and out-of-plane band dispersions and Fermi surface (FS) of $\mathrm{Eu}{({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14})}_{2}{\mathrm{As}}_{2}$. The near-${E}_{F}$ Fe-$3d$-derived band dispersions near the $\mathrm{\ensuremath{\Gamma}}$ and $X$ high-symmetry points show changes due to Ir substitution, but the FS topology is preserved. From momentum-dependent measurements of the superconducting gap measured at $T=5$ K, we estimate an essentially isotropic $s$-wave gap ($\mathrm{\ensuremath{\Delta}}\ensuremath{\sim}5.25\ifmmode\pm\else\textpm\fi{}0.25$ meV), indicative of strong-coupling superconductivity with $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}\ensuremath{\simeq}5.8$. The gap gets closed at temperatures $T\ensuremath{\ge}10$ K, and this is attributed to the resistive phase which sets in at ${T}_{M}=18$ K due to the ${\mathrm{Eu}}^{2+}$-derived magnetic order. The modification of the FS with Ir substitution clearly indicates an effective hole doping with respect to the parent compound.

Hybrid heterostructures based on bulk GaN and two-dimensional (2D) materials offer novel paths toward nanoelectronic devices with engineered features. Here, we study the electronic properties of a mixed-dimensional heterostructure composed of intrinsic n-doped MoS2 flakes transferred on p-doped GaN(0001) layers. Based on angle-resolved photoemission spectroscopy (ARPES) and high resolution x-ray photoemission spectroscopy (HR-XPS), we investigate the electronic structure modification induced by the interlayer interactions in MoS2/GaN heterostructure. In particular, a shift of the valence band with respect to the Fermi level for MoS2/GaN heterostructure is observed, which is the signature of a charge transfer from the 2D monolayer MoS2 to GaN. The ARPES and HR-XPS revealed an interface dipole associated with local charge transfer from the GaN layer to the MoS2 monolayer. Valence and conduction band offsets between MoS2 and GaN are determined to be 0.77 and −0.51eV, respectively. Based on the measured work functions and band bendings, we establish the formation of an interface dipole between GaN and MoS2 of 0.2 eV.

Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied properties. They range from metals and superconductors to strongly spin-orbit-coupled semiconductors and charge-density-wave systems, with their single-layer variants one of the most prominent current examples of two-dimensional materials beyond graphene. Their varied ground states largely depend on the transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date. Here, we focus on the chalcogen-derived states. From density-functional theory calculations together with spin- and angle- resolved photoemission, we find that these generically host type-II three-dimensional bulk Dirac fermions as well as ladders of topological surface states and surface resonances. We demonstrate how these naturally arise within a single p-orbital manifold as a general consequence of a trigonal crystal field, and as such can be expected across a large number of compounds. Already, we demonstrate their existence in six separate TMDs, opening routes to tune, and ultimately exploit, their topological physics., 10 pages, 4 figures

Using highly controlled coverages of graphene on SiC(0001), we have studied the structure of the first graphene layer that grows on the SiC interface. This layer, known as the buffer layer, is semiconducting. Using x-ray reflectivity and x-ray standing waves analysis we have performed a comparative study of the buffer layer structure with and without an additional monolayer graphene layer above it. We show that no more than 26\% of the buffer carbon is covalently bonded to Si in the SiC interface. We also show that the top SiC bilayer is Si depleted and is the likely the cause of the incommensuration previously observed in this system. When a monolayer graphene layer forms above the buffer, the buffer layer becomes less corrugated with signs of a change in the bonding geometry with the SiC interface. At the same time, the entire SiC interface becomes more disordered, presumably due to entropy associated with the higher growth temperature., Comment: 12 pages, 9 figures

We investigate electronic band-structure images in reciprocal space of few-layer graphene epitaxially grown on SiC(000-1). In addition to the observation of commensurate rotation angles of the graphene layers, the k-space images recorded near the Fermi edge highlight structures originating from diffraction of the Dirac cones due to the relative rotation of adjacent layers. The 21.9° and 27° rotation angles between two sheets of graphene are responsible for a periodic pattern that can be described with a superlattice unit cells. The superlattice generates replicas of Dirac cones with smaller wave vectors, because of a Brillouin zone folding. Copyright © 2014 John Wiley & Sons, Ltd.

In this article the Wells-Dawson polyoxometalate K6[P2W18O62] is grown as an interfacial layer between indium tin oxide and bulk heterojunction of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The structure of the POM layers depends on the thickness and shows a highly anisotropic surface organization. The films have been characterized by atomic force microscopy and X-ray photoelectron spectroscopy (XPS) to gain insight into their macroscopic organization and better understand their electronic properties. Then, they were put at the anodic interface of a P3HT:PCBM organic solar cell and characterized on an optical bench. The photovoltaic efficiency is discussed in terms of the benefit of the polyoxometalate at the anodic interface of an organic photovoltaic cell., Comment: 7 pages, 6 figures

Members of the Mo${}_{x}$W${}_{1-x}$Te${}_{2}$ series are predicted to be Weyl semimetals, hosting type-II Weyl fermions, which have yet to be experimentally realized and which are unusual because they strongly violate Lorentz invariance. Crucially, the Weyl points in this system are predicted to sit above the Fermi level. Here, the authors show that for a type-II Weyl cone, although not for a type-I Weyl cone, if the Weyl point is above the Fermi level, then it's necessary to see the band structure above the Fermi level to observe a topological Fermi arc. The authors also discover that pump-probe angle-resolved photoemission beautifully displays the unoccupied band structure in Mo${}_{x}$W${}_{1-x}$Te${}_{2}$. Their work sets the stage for demonstrating that this system is the first type-II Weyl semimetal, as well as the first tunable Weyl semimetal.

Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design.

Micron scale ferroelectric (FE) domains have been written into a 20-nm-thick epitaxial thin film of BaTiO3 (001) [BTO(001)] on a Nb-doped SrTiO3 substrate using a piezoforce microscope (PFM). The domain-dependent electronic structure has been studied using fully energy-filtered photoemission electron microscopy (PEEM) and synchrotron radiation. Shifts, induced by FE polarization, of up to 300 meV were observed in the work function of the sample. The surface is Ba-O terminated. Polarization-induced distortion of the electronic structure was observed in the valence band and on the Ba 3d, Ti 2p and O 1s core levels of BTO. Polarization-dependent surface adsorption was observed. A simple electrostatic model based on net surface charge is not sufficient to explain the observed modifications in the electronic levels. Copyright © 2010 John Wiley & Sons, Ltd.

© 2018 Author(s). Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a (Si/Mo) multilayer mirror substrate are characterized by hard x-ray photoemission spectroscopy (HXPS) and by standing-wave HXPS (SW-HXPS). Information on the chemical composition and on the chemical states of the elements within the nanoribbons was obtained by HXPS and on the quantitative depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves to x-ray optical calculations, the chemical depth profile of the InAs(QM) and its interfaces were quantitatively derived with ångström precision. We determined that (i) the exposure to air induced the formation of an InAsO4 layer on top of the stoichiometric InAs(QM); (ii) the top interface between the air-side InAsO4 and the InAs(QM) is not sharp, indicating that interdiffusion occurs between these two layers; (iii) the bottom interface between the InAs(QM) and the native oxide SiO2 on top of the (Si/Mo) substrate is abrupt. In addition, the valence band offset (VBO) between the InAs(QM) and the SiO2/(Si/Mo) substrate was determined by HXPS. The value of VBO = 0.2 ± 0.04 eV is in good agreement with literature results obtained by electrical characterization, giving a clear indication of the formation of a well-defined and abrupt InAs/SiO2 heterojunction. We have demonstrated that HXPS and SW-HXPS are non-destructive, powerful methods for characterizing interfaces and for providing chemical depth profiles of nanostructures, quantum membranes, and 2D layered materials.

We have studied the chemical and electronic properties of LaAlO3/SrVO3 ultrathin films by combining hard x-ray photoemission spectroscopy and transport measurements. We compare single SrVO3 (SVO) ultrathin films and SrVO3 buried below a polar LaAlO3 (LAO) thin layer, both epitaxially grown on SrTiO3. While ultrathin films (4 unit cells) of SVO do show insulating behavior over the entire temperature range, the LAO/SVO interface has a resistivity minimum at 250 K. When increasing the SVO layer thickness, the minimum is observed to shift to higher temperatures, but the resistivity stays always smaller than that of comparable SVO single films. Hard x-ray photoemission spectroscopy reveals a surface or interface related V5+ component in the V 2p spectra for SVO films and LAO/SVO heterostructures, respectively, attributed to a strongly oxidized component. This chemical reconstruction is weaker in LAO/SVO heterostructures compared to single SVO films. We show that this dead layer in SVO ultrathin films has to be considered when the film thickness reaches the few unit-cells limit and propose solutions on how to prevent this detrimental effect., Comment: 15 pages, 4 figures + SI 2 pages, 1 figure

For certain conditions of layer thickness, the interface between GdTiO$_3$ (GTO) and SrTiO$_3$ (STO) in multilayer samples has been found to form a two-dimensional electron gas (2DEG) with very interesting properties including high mobilities and ferromagnetism. We have here studied two trilayer samples of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001) (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ (LSAT), with the STO layer thicknesses being at what has been suggested is the critical thickness for 2DEG formation. We have studied these with Ti-resonant angle-resolved (ARPES) and angle-integrated photoemission and find that the spectral feature in the spectra associated with the 2DEG is present in the 1.5 unit cell sample, but not in the 1.0 unit cell sample. We also observe through core-level spectra additional states in Ti and Sr, with the strength of a low-binding-energy state for Sr being associated with the appearance of the 2DEG, and we suggest it to have an origin in final-state core-hole screening., 12 pages, 4 figures

We have used angle-resolved photoemission spectroscopy to investigate the band structure of ${\mathrm{ReS}}_{2}$, a transition-metal dichalcogenide semiconductor with a distorted 1T crystal structure. We find a large number of narrow valence bands, which we attribute to the combined influence of structural distortion and spin-orbit coupling. We further show how this leads to a strong in-plane anisotropy of the electronic structure, with quasi-one-dimensional bands reflecting predominant hopping along zigzag Re chains. We find that this does not persist up to the top of the valence band, where a more three-dimensional character is recovered with the fundamental band gap located away from the Brillouin zone center along ${k}_{z}$. These experiments are in good agreement with our density-functional theory calculations, shedding light on the bulk electronic structure of ${\mathrm{ReS}}_{2}$, and how it can be expected to evolve when thinned to a single layer.

The typical Kondo insulator, YbB12, is a candidate of topological Kondo insulators. To clarify the surface electronic structure of not only bulk but also surface by photoelectron measurements, we developed a method to obtain a clean surface of YbB12(001) by annealing at 1650 K in an ultra-high vacuum. By surface-sensitive photoelectron measurements, it was known that the surface consists of only B atoms without Yb. In the sub-surface region, Yb atoms exist and the temperature dependence of the valence agrees with those of previous HAXPES results. Here we report the cleaning method to obtain well-defined YbB12(001) surfaces and the temperature-dependent Yb valence obtained from Yb 3d core-level and valence photoelectron spectra.

International audience; The switchable bipolar ground state is at the heart of research into ferroelectrics for future, low-energy electronics. Polarization switching by an applied field is a complex phenomenon which depends on the initial domain ordering, defect concentration, electrical boundary conditions and charge screening. Injected free charge may also to be used to reversibly switch in-plane polarized domains. We show that the interaction between the initial domain order and the bulk screening provided by very low energy electrons switches the polarization without the collateral radiation damage which occurs when employing a beam of high energy electrons. Polarization switching during charge injection adds a new dimension to the multifunctionality of ferroelectric oxides.

We report on the charge spill-out and work function of epitaxial few-layer graphene on 6H-SiC(0001). Experiments from high-resolution, energy-filtered X-ray photoelectron emission microscopy (XPEEM) are combined with ab initio Density Functional Theory calculations using a relaxed interface model. Work function values obtained from theory and experiments are in qualitative agreement, reproducing the previously observed trend of increasing work function with each additional graphene plane. Electrons transfer at the SiC/graphene interface through a buffer layer causes an interface dipole moment which is at the origin of the graphene work function modulation. The total charge transfer is independent of the number of graphene layers, and is consistent with the constant binding energy of the SiC component of the C 1s core-level measured by XPEEM. Charge leakage into vacuum depends on the number of graphene layers explaining why the experimental, layer-dependent C 1s-graphene core-level binding energy shift does not rigidly follow that of the work function. Thus, a combination of charge transfer at the SiC/graphene interface and charge spill-out into vacuum resolves the apparent discrepancy between the experimental work function and C1s binding energy., 14 pages, 9 figures

We present a spatial and wave-vector resolved study of the electronic structure of micron sized ferroelectric domains at the surface of a BaTiO3(001) single crystal. The n-type doping of the BaTiO3 is controlled by in-situ vacuum and oxygen annealing, providing experimental evidence of a surface paraelectric-ferroelectric transition below a critical doping level. Real space imaging of photoemission threshold, core level and valence band spectra show contrast due to domain polarization. Reciprocal space imaging of the electronic structure using linearly polarized light provides unambiguous evidence for the presence of both in and out-of plane polarization with two and fourfold symmetry, respectively. The results agree well with first principles calculations., Comment: 6 pages, 5 figures

In thin film ferroelectric capacitor the chemical and electronic structure of the electrode/FE interface can play a crucial role in determining the kinetics of polarization switching. We investigate the electronic structure of a Pt/BaTiO3/SrTiO3:Nb capacitor using time-resolved photoemission spectroscopy. The chemical, electronic and depth sensitivity of core level photoemission is used to probe the transient response of different parts of the upper electrode/ferroelectric interface to voltage pulse induced polarization reversal. The linear response of the electronic structure agrees quantitatively with a simple RC circuit model. The non-linear response due to the polarization switch is demonstrated by the time-resolved response of the characteristic core levels of the electrode and the ferroelectric. Adjustment of the RC circuit model allows a first estimation of the Pt/BTO interface capacitance. The experiment shows the interface capacitance is at least 100 times higher than the bulk capacitance of the BTO film, in qualitative agreement with theoretical predictions from the literature., Comment: 7 pages, 10 figures. Submitted to Phys. Rev. B