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To generate and manipulate spin-polarized electronic states in solids are crucial for modern spintronics. The textbook routes employ quantum well states or Shockley/topological type surface states whose spin degeneracy is lifted by strong spin-orbit coupling and inversion symmetry breaking at the surface/interface. The resultant spin polarization is usually truncated because of the intertwining between multiple orbitals. Here a unique type of surface states is realized, namely, dangling bond surface states in a family of ternary rare-earth pnictide tellurides RePnTe (Re = La, Gd, Ce; Pn = Sb, Bi), with robust band structure and sizeable spin splitting. Spin and angle-resolved photoemission spectroscopy measurements reveal high spin polarization and distinct spin-momentum locking texture, which, according to the theoretical analysis, arise from local site asymmetry and surface-purified spin-orbital texture. The work extends the so-called "hidden spin polarization" from the bulk to the surface, presenting an intriguing spin-orbital-momentum-layer locking phenomenon, which may shed lights on potential spintronic applications., (© 2024 Wiley‐VCH GmbH.)

Condensed matter physics has often provided a platform for investigating the interplay between particles and fields in cases that have not been observed in high-energy physics. Here, using angle-resolved photoemission spectroscopy, we provide an example of this by visualizing the electronic structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs and bulk Weyl fermion dispersion as well as the emergence of new Weyl fermions in the ferrimagnetic state. Our results establish NdAlSi as a magnetic Weyl semimetal and provide an experimental observation of ferrimagnetic regulation of Weyl fermions in condensed matter., (© 2023. The Author(s).)

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 CePd 3 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., (© 2022. The Author(s).)

Artificial graphene based on molecular networks enables the creation of novel 2D materials with unique electronic and topological properties. Landau quantization has been demonstrated by CO molecules arranged on the two-dimensional electron gas on Cu(111) and the observation of electron quantization may succeed based on the created gauge fields. Recently, it was reported that instead of individual manipulation of CO molecules, simple deposition of nonpolar C 60 molecules on Cu(111) and Au(111) produces artificial graphene as evidenced by Dirac cones in photoemission spectroscopy. Here, we show that C 60 -induced Dirac cones on Au(111) have a different origin. We argue that those are related to umklapp diffraction of surface electronic bands of Au on the molecular grid of C 60 in the final state of photoemission. We test this alternative explanation by precisely probing the dimensionality of the observed conical features in the photoemission spectra, by varying both the incident photon energy and the degree of charge doping via alkali adatoms. Using density functional theory calculations and spin-resolved photoemission we reveal the origin of the replicating Au(111) bands and resolve them as deep leaky surface resonances derived from the bulk Au sp-band residing at the boundary of its surface projection. We also discuss the manifold nature of these resonances which gives rise to an onion-like Fermi surface of Au(111).

Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE), which provides quantized edge states for lossless charge-transport applications 1-8 . The edge states are hosted by a magnetic energy gap at the Dirac point 2 , 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, T C (ref. 8 ). Here we use low-temperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi 2 Te 3 , which displays ferromagnetic out-of-plane spin texture and opens up only below T C . Surprisingly, our analysis reveals large gap sizes at 1 kelvin of up to 90 millielectronvolts, which is five times larger than theoretically predicted 9 . 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 MnBi 2 Te 4 septuple and Bi 2 Te 3 quintuple layers is formed. This enhances the wavefunction overlap and size of the magnetic gap 10 . Mn-doped Bi 2 Se 3 (ref. 11 ) and Mn-doped Sb 2 Te 3 form similar heterostructures, but for Bi 2 Se 3 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 Bi 2 Te 3 . Our findings provide insights that will be crucial in pushing lossless transport in topological insulators towards room-temperature applications.

SmB 6 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 [Formula: see text] state to appear Rashba split and explain the prominent [Formula: see text] 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.

Combining multiple emergent correlated properties such as superconductivity and magnetism within the topological matrix can have exceptional consequences in garnering new and exotic physics. Here, we study the topological surface states from a noncentrosymmetric α-BiPd superconductor by employing angle-resolved photoemission spectroscopy and first-principles calculations. We observe that the Dirac surface states of this system have several interesting and unusual properties, compared to other topological surface states. The surface state is strongly anisotropic and the in-plane Fermi velocity varies rigorously on rotating the crystal about the y axis. Moreover, it acquires an unusual band gap as a function of k_{y}, possibly due to hybridization with bulk bands, detected upon varying the excitation energy. The coexistence of all the functional properties in addition to the unusual surface state characteristics make this an interesting material.

We use polarization-dependent angle-resolved photoemission spectroscopy (ARPES) to study the high-energy anomaly (HEA) in the dispersion of Nd(2-x)Ce(x)CuO₄, x=0.123. We find that at particular photon energies the anomalous, waterfall-like dispersion gives way to a broad, continuous band. This suggests that the HEA is a matrix element effect: it arises due to a suppression of the intensity of the broadened quasiparticle band in a narrow momentum range. We confirm this interpretation experimentally, by showing that the HEA appears when the matrix element is suppressed deliberately by changing the light polarization. Calculations of the matrix element using atomic wave functions and simulation of the ARPES intensity with one-step model calculations provide further evidence for this scenario. The possibility to detect the full quasiparticle dispersion further allows us to extract the high-energy self-energy function near the center and at the edge of the Brillouin zone.

We reveal a giant Rashba effect (α(R)≈1.3  eV Å) on a surface state of Ir(111) by angle-resolved photoemission and by density functional theory. It is demonstrated that the existence of the surface state, its spin polarization, and the size of its Rashba-type spin-orbit splitting remain unaffected when Ir is covered with graphene. The graphene protection is, in turn, sufficient for the spin-split surface state to survive in ambient atmosphere. We discuss this result along with indications for a topological protection of the surface state.

We report a Rashba spin splitting of a two-dimensional electron gas in the topological insulator Bi(2)Se(3) from angle-resolved photoemission spectroscopy. We further demonstrate its electrostatic control, and show that spin splittings can be achieved which are at least an order-of-magnitude larger than in other semiconductors. Together these results show promise for the miniaturization of spintronic devices to the nanoscale and their operation at room temperature., (© 2011 American Physical Society)

An energy gap between the valence and the conduction band is the defining property of a semiconductor, and the gap size plays a crucial role in the design of semiconductor devices. We show that the presence of a two-dimensional electron gas near to the surface of a semiconductor can significantly alter the size of its band gap through many-body effects caused by its high electron density, resulting in a surface band gap that is much smaller than that in the bulk. Apart from reconciling a number of disparate previous experimental findings, the results suggest an entirely new route to spatially inhomogeneous band-gap engineering.

The (114) surface of the semimetal Bi is found to support a quasi-one-dimensional, metallic surface state. As required by symmetry, the state is degenerate along the Gamma-Y line of the surface Brillouin zone with a highest binding energy of approximately 150 meV. In the Gamma-X direction the degeneracy is lifted by the strong spin-orbit splitting in Bi, as directly shown by spin-resolved photoemission. This results in a Fermi contour consisting of two closely separated, parallel lines of opposite spin direction. It is argued that similar states on related insulators would give rise to a one-dimensional quantum spin Hall effect.

High energy resolution fast XPS combined with TPD experiments were used to study the effect of chemisorbed oxygen on the adsorption and dissociation of NH(3) on Ir(110). Below 250 K the presence of O(ad) does not influence NH(3) decomposition. Above 250 K O(ad) enhances NH(3) dissociation, which results in three times as much N(2) formation and less molecular NH(3) desorption compared to the experiments without O(ad). The effect of O(ad) can be attributed to destabilization of NH(ad) on the surface, resulting in a further dehydrogenation towards N(ad). The presence of O(ad) on the surface lowers the temperature at which the N(ad) combination reaction takes place by as much as 200 K, due to repulsive interaction between N(ad) and O(ad). NO is formed above 450 K if both N(ad) and O(ad) are present on the surface.

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 approximately 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 NH(ad) between 225 and 300 K. NH(ad) decomposes into N(ad) 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 N(ad) 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 N(ad) below 400 K results in NH(ad). No NH2ad or NH3ad/NH3 were observed. The hydrogenation of NH(ad) only takes place above 400 K. On the basis of the experimental findings an energy scheme is presented to account for the observations.