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Effective models focused on pertinent low-energy degrees of freedom have substantially contributed to our qualitative understanding of quantum materials. An iconic example, the Kondo model, was key to demonstrating that the rich phase diagrams of correlated metals originate from the interplay of localized and itinerant electrons. Modern electronic structure calculations suggest that to achieve quantitative material-specific models, accurate consideration of the crystal field and spin-orbit interactions is imperative. This poses the question of how local high-energy degrees of freedom become incorporated into a collective electronic state. Here, we use resonant inelastic x-ray scattering (RIXS) on CePd3 to clarify the fate of all relevant energy scales. We find that even spin-orbit excited states acquire pronounced momentum-dependence at low temperature—the telltale sign of hybridization with the underlying metallic state. Our results demonstrate how localized electronic degrees of freedom endow correlated metals with new properties, which is critical for a microscopic understanding of superconducting, electronic nematic, and topological states. The fate of high-energy degrees of freedom, such as spin-orbit interactions, in the coherent state of Kondo lattice materials remains unclear. Here, the authors use resonant inelastic x-ray scattering in CePd3 to show how Kondo-quasiparticle excitations are renormalized and develop a pronounced momentum dependence, while maintaining a largely unchanged spin-orbit gap. [ABSTRACT FROM AUTHOR]

The present article highlights two aspects at the intersection between Rashba physics and topological matter. Topologically nontrivial matter has been in the focus for almost two decades. It depends strongly on spin–orbit coupling but, in contrast to large parts of modern solid state physics, strong electron correlation does not play a major role. In this context, SmB6 has been suggested as the first topological insulator driven by strong electron correlation and the first topological Kondo insulator. We review the important role of the Rashba splitting in determining that the observed surface states are not topological. Moreover, we point out that the Rashba splitting of SmB6 represents the extreme case of a large splitting in momentum space at a small Rashba parameter. [ABSTRACT FROM AUTHOR]

V-doped (Bi,Sb)2Te3 has a ten times higher magnetic coercivity than its Cr-doped counterpart and, therefore, is believed to be advantageous for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic bandgap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K. By x-ray magnetic circular dichroism (XMCD), we directly probe the magnetism at the V site and derive spin and orbital magnetic moments of 1.69 and –0.22 μ B /atom. Hysteresis curves of the XMCD signal show a strong dependence of the coercivity on the ramping velocity of the magnetic field. The XMCD signal decays on a timescale of minutes, which we conclude contributes to the absence of a detectable magnetic gap at the Dirac point. [ABSTRACT FROM AUTHOR]

The adsorption and decomposition of NH3 on Ir(110) has been studied in the temperature range from 80 K to 700 K. By using high-energy resolution x-ray photoelectron spectroscopy it is possible to distinguish chemically different surface species. At low temperature a NH3 multilayer, which desorbs at ∼110 K, was observed. The second layer of NH3 molecules desorbs around 140 K, in a separate desorption peak. Chemisorbed NH3 desorbs in steps from the surface and several desorption peaks are observed between 200 and 400 K. A part of the NH3ad decomposes into NHad between 225 and 300 K. NHad decomposes into Nad between 400 K and 500 K and the hydrogen released in this process immediately desorbs. N2 desorption takes place between 500 and 700 K via Nad combination. The steady state decomposition reaction of NH3 starts at 500 K. The maximum reaction rate is observed between 540 K and 610 K. A model is presented to explain the occurrence of a maximum in the reaction rate. Hydrogenation of Nad below 400 K results in NHad. No NH2ad or NH3ad/NH3 were observed. The hydrogenation of NHad only takes place above 400 K. On the basis of the experimental findings an energy scheme is presented to account for the observations. [ABSTRACT FROM AUTHOR]

Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE), which provides quantized edge states for lossless charge-transport applications1–8. The edge states are hosted by a magnetic energy gap at the Dirac point2, but hitherto all attempts to observe this gap directly have been unsuccessful. Observing the gap is considered to be essential to overcoming the limitations of the QAHE, which so far occurs only at temperatures that are one to two orders of magnitude below the ferromagnetic Curie temperature, TC (ref. 8). Here we use low-temperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi2Te3, which displays ferromagnetic out-of-plane spin texture and opens up only below TC. Surprisingly, our analysis reveals large gap sizes at 1 kelvin of up to 90 millielectronvolts, which is five times larger than theoretically predicted9. Using multiscale analysis we show that this enhancement is due to a remarkable structure modification induced by Mn doping: instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3 quintuple layers is formed. This enhances the wavefunction overlap and size of the magnetic gap10. Mn-doped Bi2Se3 (ref. 11) and Mn-doped Sb2Te3 form similar heterostructures, but for Bi2Se3 only a nonmagnetic gap is formed and the magnetization is in the surface plane. This is explained by the smaller spin–orbit interaction by comparison with Mn-doped Bi2Te3. Our findings provide insights that will be crucial in pushing lossless transport in topological insulators towards room-temperature applications. In theory, the anomalous quantum Hall effect is observed in edge channels of topological insulators when there is a magnetic energy gap at the Dirac point; this gap has now been observed by low-temperature photoelectron spectroscopy in Mn-doped Bi2Te3. [ABSTRACT FROM AUTHOR]

SmB6 is predicted to be the first member of the intersection of topological insulators and Kondo insulators, strongly correlated materials in which the Fermi level lies in the gap of a many-body resonance that forms by hybridization between localized and itinerant states. While robust, surface-only conductivity at low temperature and the observation of surface states at the expected high symmetry points appear to confirm this prediction, we find both surface states at the (100) surface to be topologically trivial. We find the Γ̅ state to appear Rashba split and explain the prominent X̅ state by a surface shift of the many-body resonance. We propose that the latter mechanism, which applies to several crystal terminations, can explain the unusual surface conductivity. While additional, as yet unobserved topological surface states cannot be excluded, our results show that a firm connection between the two material classes is still outstanding. [ABSTRACT FROM AUTHOR]

In this article, we review our angle- and time-resolved photoemission studies (ARPES and trARPES) on various ferropnictides. In the ARPES studies, we focus first on the band structure as a function of control parameters. We find near optimally 'doped' compounds a Lifshitz transition of hole/electron pocket vanishing type. Second, we investigated the inelastic scattering rates as a function of the control parameter. In contrast to the heavily discussed quantum critical scenario, we find no enhancement of the scattering rate near optimally 'doping.' Correlation effects which show up by the non-Fermi-liquid behavior of the scattering rates, together with the Lifshitz transition offer a new explanation for the strange normal state properties and suggests an interpolating superconducting state between BCS and BE condensation. Adding femtosecond time resolution to ARPES provides complementary information on electron and lattice dynamics. We report on the response of the chemical potential by a collective periodic variation coupled to coherent optical phonons in combination with incoherent electron and phonon dynamics described by a three temperature heat bath model. We quantify electron phonon coupling in terms of [ABSTRACT FROM AUTHOR]

The phase diagram of Sr3Ru2O7 shows hallmarks of strong electron correlations despite the modest Coulomb interaction in the Ru 4d shell. We use angle-resolved photoelectron spectroscopy measurements to provide microscopic insight into the formation of the strongly renormalized heavy d-electron liquid that controls the physics of Sr3Ru2O7. Our data reveal itinerant Ru 4d-states confined over large parts of the Brillouin zone to an energy range of <6 meV, nearly three orders of magnitude lower than the bare band width. We show that this energy scale agrees quantitatively with a characteristic thermodynamic energy scale associated with quantum criticality and illustrate how it arises from a combination of back-folding due to a structural distortion and the hybridization of light and strongly renormalized, heavy quasiparticle bands. The resulting heavy Fermi liquid has a marked k-dependence of the renormalization which we relate to orbital mixing along individual Fermi surface sheets. [ABSTRACT FROM AUTHOR]

Over the recent years, electronic surface states have been used for a detailed spectroscopic study of the electron-phonon (e-ph) interaction, both experimentally and theoretically. This review discusses the basic physics of e-ph coupling and how information can be obtained from angle-resolved photoemission experiments and first principles calculations. Several recent results for clean and adsorbate-covered surfaces, quantum wells and freestanding monolayers are also discussed. [ABSTRACT FROM AUTHOR]

We performed a high-resolution angle-resolved photoemission spectroscopy study on superconducting (SC) Tl0.63K0.37Fe1.78Se2 (Tc = 29 K) in the whole Brillouin zone (BZ). In addition to a nearly isotropic ∼8.2 meV 2-dimensional (2D) SC gap (2Δ/kBTc ∼ 7) on quasi-2D electron Fermi surfaces (FSs) located around M(π,0,0)-A(π,0,π), we observe a ∼6.2 meV isotropic SC gap (2Δ/kBTc ∼ 5) on the Z-centred electron FS that rules out any d-wave pairing symmetry and rather favors an s-wave symmetry. All isotropic SC gap amplitudes can be fit by a single-gap function derived from a local strong-coupling approach suggesting an enhancement of the next-nearest neighbor exchange interaction in the ferrochalcogenide superconductors. [ABSTRACT FROM AUTHOR]