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The superconducting and magnetic properties of phase-separated AxFe2-y,Sex compounds are known to depend on postgrowth heat treatments and cooling profiles. This paper focuses on the evolution of microstructure on annealing and how this influences the superconducting properties of RbxFe2-ySe2 single crystals. We find that the minority phase in the as-grown crystal has increased unit cell anisotropy (c/a ratio), reduced Rb content, and increased Fe content compared to the main phase. The microstructure is rather complex, with two-phase mesoscopic plate-shaped features aligned along {113} habit planes. The minority phases are strongly faceted on the {113} planes, which we have shown to be driven by minimizing the volume strain energy introduced as a result of the phase transformation. Annealing at 488 K results in coarsening of the mesoscopic plate-shaped features and the formation of a third distinct phase. The subtle differences in structure and chemistry of the minority phase(s) in the crystals are thought to be responsible for changes in the superconducting transition temperature. In addition, scanning photoemission microscopy has clearly shown that the electronic structure of the minority phase has a higher occupied density of states of the low binding energy Fe3d orbitals, which is characteristic of crystals that exhibit superconductivity. This demonstrates a clear correlation between the Fe-vacancy-free phase with high c/a ratio and the electronic structure characteristics of the superconducting phase. [ABSTRACT FROM AUTHOR]

Abstract: The influence of nanoscale morphology of the Rh(110) and Ru(0001) surfaces on their oxidation by molecular oxygen has been studied by scanning tunneling microscopy (STM) and scanning photoelectron microscopy (SPEM). The STM results clearly show that after roughening by ion sputtering, these surfaces consist of step-bunching areas and almost atomically flat plateaus. For both metals SPEM measurements reveal that the step-bunching areas, consisting of narrow terraces separated by single atomic steps, oxidize more promptly than plateaus. The locally higher chemical activity of the step-bunching areas is tentatively attributed to the higher density of defects, which promotes the penetration of oxygen underneath the surface. [Copyright &y& Elsevier]

The formation conditions, morphology, and reactivity of thin oxide films, grown on a Rh(110) surface in the ambient of atomic or molecular oxygen, have been studied by means of laterally resolved core level spectroscopy, scanning tunneling microscopy and low energy electron diffraction. Exposures of Rh(110) to atomic oxygen lead to subsurface incorporation of oxygen even at room temperature and facile formation of an ordered, laterally uniform surface oxide at ∼520 K, with a quasi-hexagonal structure and stoichiometry close to that of RhO2. In the intermediate oxidation stages, the surface oxide coexists with areas of high coverage adsorption phases. After a long induction period, the reduction of the Rh oxide film with H2 is very rapid and independent of the coexisting adsorption phases. The growth of the oxide film by exposure of a Rh(110) surface to molecular oxygen requires higher pressures and temperatures. The important role of the O2 dissociation step in the oxidation process is reflected by the complex morphology of the oxide films grown in O2 ambient, consisting of microscopic patches of different Rh and oxygen atomic density. [ABSTRACT FROM AUTHOR]

We have investigated the reduction of initial C[sub 60] electronic structure under the influence of a growing number of attached fluorines in C[sub 60]F[sub 48], C[sub 60]F[sub 36], C[sub 60]F[sub 24], and C[sub 60]F[sub 18]. The reduction of the low-energy π-electron subsystem reveals itself in lowering of the optical absorption in the visible region and in gradual blueshift of the luminescence. The absorption of C[sub 60]F[sub 24] and C[sub 60]F[sub 18] films strongly resembles that of C[sub 60], thus showing the similarity of electronic structures of C[sub 60] and these compounds. This resemblance disappears in the absorption spectra of C[sub 60]F[sub 48] and C[sub 60]F[sub 36]. The absorption of these compounds looks like that of C[sub 6]H[sub 6], because the π-bonds are isolated. The luminescence of C[sub 60]F[sub 18] film has an unexpected two-component structure, with the first component close to the C[sub 60] luminescence spectra, and the second one lying approximately in the same region with C[sub 60]F[sub 36]. We attribute this splitting of luminescence spectra to the two different channels for relaxation of the electronic excitation. The first one is correlated with the fluorinated part of the C[sub 60]F[sub 18] molecule, the second one with the part free of fluorine atoms. [ABSTRACT FROM AUTHOR]

Dirac semimetals are classified into different phases based on the types of Dirac fermions. Tuning the transition among different types of Dirac fermions in one system remains a challenge. Recently, KMgBi was predicted to be located at a critical state in which various types of Dirac fermions can be induced owing to the existence of a flatband. Here, we carried out systematic studies on the electronic structure of KMgBi single crystals by combining angle-resolve photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. The flatband was clearly observed near the Fermi level. We also revealed a small bandgap of ∼20 meV between the flatband and the conduction band. These results demonstrate the critical states of KMgBi that transition among various types of Dirac fermions can be tuned in one system. [ABSTRACT FROM AUTHOR]

The spatially resolved ARPES (nanoARPES) is a development of conventional ARPES technique achieved with the focusing of light on the sample into the spot with submicron sizes. This development is used in research of essentially small samples, for example, heterostructures build of flakes of 2D materials, micro-crystals or polycrystalline samples, different crystal termination, domains in electronic structure. ANTARES is delivering the nanoARPES technique to user community since 2010, up to now the instrument was used with samples of various types, and that was useful to accumulate the specific experience. In this paper we report the current layout and actual performance of the ANTARES instrument at Synchrotron SOLEIL, as well as the most typical application areas. The most important and recent upgrades include focusing optics and in-operando setup. The new optical units deliver the increased flux on sample as well as the option to vary the photon energy. The in-operando setup offers the option of electrical connection to the sample being studied with nanoARPES, that promises various applications, where the most demanded now is the control of charge carrier density with applied voltage. This "gate-doping" is often applied to the heterostructures of 2D materials, that could be designed and build on purpose. The 2D heterostructures is probably the most typical field of application of the instrument, with or without in-operando option. At the same time the use case of the ANTARES instrument is still in the development by the realisation of new kinds of samples and of the new types of the experiments. • NanoARPES is a technique providing the angles resolved photoemission with spatial resolution. • ANTARES is a nanoARPES instrument launched about 10 years ago among the first instruments. • The instrument is delivering performance adequate for studies in various research areas. • Research of 2D materials and heterostructures, topological materials, superconductors, and other subjects. • In-operando electrical connections to the sample under measurement provides opportunity to study micro-devices. [ABSTRACT FROM AUTHOR]

A synchrotron radiation beamline in the photon energy range of 18-240 eV and an electron spectroscopy end station have been constructed at the 3 GeV Diamond Light Source storage ring. The instrument features a variable polarisation undulator, a high resolution monochromator, a re-focussing system to form a beam spot of 50 × 50 μm2, and an end station for angle-resolved photoelectron spectroscopy (ARPES) including a 6-degrees-of-freedom cryogenic sample manipulator. The beamline design and its performance allow for a highly productive and precise use of the ARPES technique at an energy resolution of 10-15 meV for fast k-space mapping studies with a photon flux up to 2 . 1013 ph/s and well below 3 meV for high resolution spectra. [ABSTRACT FROM AUTHOR]

We present a spectromicroscopy study of the two distinct metal-insulator transitions in (V1-xCrx)2O3, x = 0.011. The coexistence of metallic and insulating domains was observed with scanning photoelectron microscopy for both the paramagnetic insulator-paramagnetic metal and paramagnetic metal-antiferromagnetic insulator transitions, evidencing a clear correlation between their nucleation regions. Although these two transitions are very different in nature and underlying mechanism, in both cases the morphology of their phase separation is influenced by structural inhomogeneities. These results demonstrate the general relevance of strain caused by local lattice distortions in guiding the intrinsic tendency towards phase separation in Mott materials. [ABSTRACT FROM AUTHOR]

The initial oxidation of the Rh(110) surface was studied by scanning tunneling microscopy, core level spectroscopy, and density functional theory. The experiments were carried out exposing the Rh(110) surface to molecular or atomic oxygen at temperatures in the 500-700 K range. In molecular oxygen ambient, the oxidation terminates at oxygen coverage close to a monolayer with the formation of alternating islands of the (10×2) one-dimensional surface oxide and (2×1)p2mg adsorption phases. The use of atomic oxygen facilitates further oxidation until a structure with a c(2×4) periodicity develops. The experimental and theoretical results reveal that the c(2×4) structure is a “surface oxide” very similar to the hexagonal O-Rh-O trilayer structures formed on the Rh(111) and Rh(100) substrates. Some of the experimentally found adsorption phases appear unstable in the phase diagram predicted by thermodynamics, which might reflect kinetic hindrance. The structural details, core level spectra, and stability of the surface oxides formed on the three basal planes are compared with those of the bulk RhO2 and Rh2O3. [ABSTRACT FROM AUTHOR]

Weyl semimetals are crystalline solids that host emergent relativistic Weyl fermions and have characteristic surface Fermi-arcs in their electronic structure. Weyl semimetals with broken time reversal symmetry are difficult to identify unambiguously. In this work, using angle-resolved photoemission spectroscopy, we visualized the electronic structure of the ferromagnetic crystal Co3Sn2S2 and discovered its characteristic surface Fermi-arcs and linear bulk band dispersions across the Weyl points. These results establish Co3Sn2S2 as a magnetic Weyl semimetal that may serve as a platform for realizing phenomena such as chiral magnetic effects, unusually large anomalous Hall effect and quantum anomalous Hall effect. [ABSTRACT FROM AUTHOR]

We report an angle-resolved photoemission spectroscopy study of a ThCr2Si2-type superconductor YFe2Ge2 (Tc ≈ 1.8 K) in the normal state. The resolved low-energy band structure mainly consists of several flat Fe bands, while the renormalization factors of different bands range from 1.1 to 2.8. Around the Fermi level, these bands contribute to a high density of states, which may account for the unusually high Sommerfeld coefficient Y. Besides, a 30 meV kink is observed, which is probably induced by electron-phonon interactions. Our results indicate that YFe2Ge2 is a moderately correlated compound, while electron-phonon coupling should be taken into account to better understand its properties. [ABSTRACT FROM AUTHOR]

We report the detailed electronic structure of WTe2 by high resolution angle-resolved photoemission spectroscopy. We resolved a rather complicated Fermi surface of WTe2. Specifically, there are in total nine Fermi pockets, including one hole pocket at the Brillouin zone center г, and two hole pockets and two electron pockets on each side of г along the г-X direction. Remarkably, we have observed circular dichroism in our photoemission spectra, which suggests that the orbital angular momentum exhibits a rich texture at various sections of the Fermi surface. гhis is further confirmed by our density-functional-theory calculations, where the spin texture is qualitatively reproduced as the conjugate consequence of spin-orbital coupling. Since the spin texture would forbid backscatterings that are directly involved in the resistivity, our data suggest that the spin-orbit coupling and the related spin and orbital angular momentum textures may play an important role in the anomalously large magnetoresistance of WTe2. Furthermore, the large differences among spin textures calculated for magnetic fields along the in-plane and out-of-plane directions also provide a natural explanation of the large field-direction dependence on the magnetoresistance. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) topological Dirac semimetals (TDSs) are a recently proposed state of quantum matter that have attracted increasing attention in physics and materials science. A 3D TDS is not only a bulk analogue of graphene; it also exhibits non-trivial topology in its electronic structure that shares similarities with topological insulators. Moreover, a TDS can potentially be driven into other exotic phases (such as Weyl semimetals, axion insulators and topological superconductors), making it a unique parent compound for the study of these states and the phase transitions between them. Here, by performing angle-resolved photoemission spectroscopy, we directly observe a pair of 3D Dirac fermions in Cd3As2, proving that it is a model 3D TDS. Compared with other 3D TDSs, for example, β-cristobalite BiO2 (ref. ) and Na3Bi (refs , ), Cd3As2 is stable and has much higher Fermi velocities. Furthermore, by in situ doping we have been able to tune its Fermi energy, making it a flexible platform for exploring exotic physical phenomena. [ABSTRACT FROM AUTHOR]

Abstract: The extraction of the light produced by an organic light emitting diode has been made possible by the use of transparent conductive materials which should have well defined electronic and optical properties. All the requirements are satisfied by indium tin oxide which has rapidly become the most common conductive substrate used for the growth of organic light emitting sources. Atomic force microscope, conventional X-ray photoemission spectroscopy and scanning photoemission spectromicroscopy have been used to investigate the morphology and the chemical properties of commercial thin indium tin oxide films after several treatments commonly used prior to the organic layer growth for smoothing/cleaning/patterning the surface. Unambiguous smoothing effects of the potassium hydroxide-based solutions have not been observed while Si contaminations of the surfaces have been found after the application of different patterning procedures. [Copyright &y& Elsevier]

Abstract: In situ core level photoelectron spectroscopy and mass spectrometry have been utilized to study the methanol oxidation on a model RuO2 catalyst at pressures ranging from 10−6 to 10−1 mbar. The experiments were carried out varying the O2/CH3OH molecular mixing ratio from 0.25 to 3.3 and the reaction temperature from 350 to 720K. The Ru 3d5/2 and O 1s core level spectra were used to characterise the dynamic changes in the Ru oxidation state by exposing the oxide pre-catalyst to different reagents partial pressures and temperatures. Full oxidation to CO2 +H2O or partial oxidation to CO+H2O+H2 have been observed in the whole pressure range for specific reaction conditions, which preserve the oxide catalyst state or reduce the oxide to metallic Ru. The selective oxidation to formaldehyde is observed only at pressures in the 10−1 mbar range, catalyzed by a RuO x surface oxide formed by partial reduction of the oxide pre-catalyst. [Copyright &y& Elsevier]

Abstract: Identifying the composition of the catalytically active state of metal catalysts under dynamic operating conditions is of particular importance for oxidation catalysis. Here we report new insights into the chemical identity of different catalytically active states formed on a Ru(0001) catalyst during CO oxidation at various reaction temperatures. The changes in the surface composition of the Ru catalyst and the CO2 yield under varying reaction conditions in the mbar pressure range were followed in situ by synchrotron-based high-pressure X-ray photoelectron spectroscopy and mass spectroscopy. The results reveal that the catalytic activity of a few layers thick surface oxide without well-defined stoichiometry and structure is comparable with that of the stoichiometric RuO2(110) phase. This surface oxide forms under reaction conditions when RuO2 formation is kinetically hindered and can coexist with RuO2 in wide temperature and pressure ranges. [Copyright &y& Elsevier]

The mechanism of high superconducting transition temperatures (Tc) in bismuthates remains under debate despite more than 30 years of extensive research. Our angle-resolved photoemission spectroscopy studies on Ba0.51K0.49BiO3 reveal an unexpectedly 34% larger bandwidth than in conventional density functional theory calculations. This can be reproduced by calculations that fully account for long-range Coulomb interactions--the first direct demonstration of bandwidth expansion due to the Fock exchange term, a long-accepted and yet uncorroborated fundamental effect in many body physics. Furthermore, we observe an isotropic superconducting gap with 2Δ0/kBTc=3.51±0.05, and strong electron-phonon interactions with a coupling constant λ~1.3±0.2. These findings solve a long-standing mystery--Ba0.51K0.49BiO3 is an extraordinary Bardeen-Cooper-Schrieffer superconductor, where long-range Coulomb interactions expand the bandwidth, enhance electron-phonon coupling, and generate the high Tc. Such effects will also be critical for finding new superconductors. [ABSTRACT FROM AUTHOR]

van der Waals heterostructures (VDWHs) exhibit rich properties and thus has potential for applications, and charge transfer between different layers in a heterostructure often dominates its properties and device performance. It is thus critical to reveal and understand the charge transfer effects in VDWHs, for which electronic structure measurements have proven to be effective. Using angle-resolved photoemission spectroscopy, we studied the electronic structures of (PbSe)1.16(TiSe2)m (m=1, 2), which are naturally occurring VDWHs, and discovered several striking charge transfer effects. When the thickness of the TiSe2 layers is halved from m=2 to m=1, the amount of charge transferred increases unexpectedly by more than 250%. This is accompanied by a dramatic drop in the electron-phonon interaction strength far beyond the prediction by first-principles calculations and, consequently, superconductivity only exists in the m=2 compound with strong electron-phonon interaction, albeit with lower carrier density. Furthermore, we found that the amount of charge transferred in both compounds is nearly halved when warmed from below 10 K to room temperature, due to the different thermal expansion coefficients of the constituent layers of these misfit compounds. These unprecedentedly large charge transfer effects might widely exist in VDWHs composed of metal-semiconductor contacts; thus, our results provide important insights for further understanding and applications of VDWHs. [ABSTRACT FROM AUTHOR]

A key issue in heavy fermion research is how subtle changes in the hybridization between the 4f (5f) and conduction electrons can result in fundamentally different ground states. CeRhIn5 stands out as a particularly notable example: when replacing Rh with either Co or Ir, antiferromagnetism gives way to superconductivity. In this photoemission study of CeRhIn5, we demonstrate that the use of resonant angle-resolved photoemission spectroscopy with polarized light allows us to extract detailed information on the 4f crystal field states and details on the 4f and conduction electron hybridization, which together determine the ground state. We directly observe weakly dispersive Kondo resonances of f electrons and identify two of the three Ce 4f¹5/2 crystal-electric-field levels and band-dependent hybridization, which signals that the hybridization occurs primarily between the Ce 4f states in the CeIn3 layer and two more three-dimensional bands composed of the Rh 4d and In 5p orbitals in the RhIn2 layer. Our results allow us to connect the properties observed at elevated temperatures with the unusual low-temperature properties of this enigmatic heavy fermion compound. [ABSTRACT FROM AUTHOR]

Heavy-fermion systems share some of the strange metal phenomenology seen in other unconventional superconductors, providing a unique opportunity to set strange metals in a broader context. Central to understanding heavy-fermion systems is the interplay of localization and itinerancy. These materials acquire high electronic masses and a concomitant Fermi volume increase as the f electrons delocalize at low temperatures. However, despite the wide-spread acceptance of this view, a direct microscopic verification has been lacking. Here we report high-resolution angle-resolved photoemission measurements on CeCoIn5, a prototypical heavy-fermion compound, which spectroscopically resolve the development of band hybridization and the Fermi surface expansion over a wide temperature region. Unexpectedly, the localized-to-itinerant transition occurs at surprisingly high temperatures, yet f electrons are still largely localized even at the lowest temperature. These findings point to an unanticipated role played by crystal-field excitations in the strange metal behavior of CeCoIn5. Our results offer a comprehensive experimental picture of the heavy-fermion formation, setting the stage for understanding the emergent properties, including unconventional superconductivity, in this and related materials. [ABSTRACT FROM AUTHOR]

Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations, we explore the electronic structure and properties of Sm atoms at the surface and in the bulk of the antiferromagnet SmRh2Si2. We show that the Sm atoms reveal weak mixed-valent behavior both in the bulk and at the surface. Although trivalent 4f emission strongly dominates, a small divalent 4f signal near the Fermi energy can be clearly resolved for surface and bulk Sm atoms. This behavior is quite different to most other Sm-based materials which typically experience a surface valence transition to a divalent state of Sm atoms at the surface. This phenomenon is explained in analogy to the isostructural Ce compound, where strong 4f hybridization stabilizes mixed-valent ground state both in the bulk and at the surface, and which were described in the light of the single-impurity Anderson model. Implications for other RERh2Si2 (RE = rare-earth elements) compounds are discussed. [ABSTRACT FROM AUTHOR]

PtBi2 with a layered hexagonal crystal structure was recently reported to exhibit an unconventional large linear magnetoresistance, while the mechanism involved is still elusive. Using high-resolution angle-resolved photoemission spectroscopy, we present a systematic study on its bulk and surface electronic structure. Through careful comparison with first-principle calculations, our experiment distinguishes the low-lying bulk bands from entangled surface states, allowing the estimation of the real composition of samples. We find significant electron doping in PtBi2, implying a substantial Bi-deficiency-induced disorder therein. Intriguingly, we discover a Dirac-cone-like surface state on the boundary of the Brillouin zone, which is identified as an accidental Dirac band without topological protection. Our findings exclude linear band dispersion in the quantum limit as the cause of the unconventional large linear magnetoresistance but give support to the classical disorder model from the perspective of the electronic structure. [ABSTRACT FROM AUTHOR]