Explore the vast range of titles available.

The helicity of three‐dimensional (3D) topological insulator surface states has drawn significant attention in spintronics owing to spin‐momentum locking where the carriers' spin is oriented perpendicular to their momentum. This property can provide an efficient method to convert charge currents into spin currents, and vice‐versa, through the Rashba–Edelstein effect. However, experimental signatures of these surface states to the spin‐charge conversion are extremely difficult to disentangle from bulk state contributions. Here, spin‐ and angle‐resolved photo‐emission spectroscopy, and time‐resolved THz emission spectroscopy are combined to categorically demonstrate that spin‐charge conversion arises mainly from the surface state in Bi1 − xSbx ultrathin films, down to few nanometers where confinement effects emerge. This large conversion efficiency is correlated, typically at the level of the bulk spin Hall effect from heavy metals, to the complex Fermi surface obtained from theoretical calculations of the inverse Rashba–Edelstein response. Both surface state robustness and sizeable conversion efficiency in epitaxial Bi1 − xSbx thin films bring new perspectives for ultra‐low power magnetic random‐access memories and broadband THz generation. Spin‐angle‐resolved photoemission spectroscopy and THz‐TDS are used to categorically demonstrate that spin‐charge conversion (SCC) arises at the surface of the Bi1‐xSbx topological insulator family ultrathin films owing to their spin‐momentum locking property. SCC occurs via the inverse Rashba–Edelstein effect down to few nanometers. This is shown to be correlated to the complex Fermi surface as obtained from advanced tight‐binding calculations.

We have studied the (110) surface of Fe$$_{3}$$O$$_{4}$$single crystals by means of X-ray Photoemission Electron Microscopy (PEEM) and Low-Energy Electron Microscopy (LEEM) to characterize its structural and magnetic properties. After sputtering and annealing a well defined surface was achieved. This preparation method resulted in a one-dimensional reconstruction formed by rows aligned in the [001] direction. By acquiring X-ray magnetic circular dichroism PEEM images at various azimuthal angles, the vector magnetization map of the (110) surface was obtained. Domains were observed with their magnetization aligned along the two$$\\langle 111\\rangle$$type bulk easy axes which are in the (110) surface plane, featuring 180$$^{\\circ }$$, 109$$^{\\circ }$$, and 71$$^{\\circ }$$domain walls. The domain walls are of Néel type. Using the sum rules we estimated an iron spin and orbital magnetic moment of 3.4 $$\\mu _B$$and 0.6 $$\\mu _B$$respectively for the reconstructed surface. At the oxygen K edge we observe dichroic contrast of close to 1%, which is reversed relative of the contrast detected from octahedral iron in the L$$_3$$edge.

We report a novel approach for coherent multi-photon photoemission in the entire Brillouin zone with infrared light that is readily implemented in a laboratory setting. We excite a solid state material, Ag(110), with intense femtosecond laser pulses to excite higher-order multi-photon photoemission; angle-resolved electron spectroscopic acquisition records photoemission at large in-plane momenta involving optical transitions from the occupied to unoccupied bands of the sample that otherwise might remain hidden by the photoemission horizon. We propose this as a complementary ultrafast method to time- and angle-resolved two-color, e.g. infrared pump and extreme ultraviolet probe, photoemission spectroscopy, with the advantage of being able to measure and control the coherent electron dynamics.

Recently, unconventional antiferromagnets that enable the spin splitting (SS) of electronic states have been theoretically proposed and experimentally realized, where the magnetic sublattices containing moments pointing at different directions are connected by a novel set of symmetries. Such SS is substantial, k‐dependent, and independent of the spin–orbit coupling (SOC) strength, making these magnets promising materials for antiferromagnetic spintronics. Here, combined with angle‐resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations, a systematic study on CrSb, a metallic spin‐split antiferromagnet candidate with Néel temperature TN = 703 K, is conducted. The data reveal the electronic structure of CrSb along both out‐of‐plane and in‐plane momentum directions, rendering an anisotropic k‐dependent SS that agrees well with the calculational results. The magnitude of such SS reaches up to at least 0.8 eV at non‐high‐symmetry momentum points, which is significantly higher than the largest known SOC‐induced SS. This compound expands the choice of materials in the field of antiferromagnetic spintronics and is likely to stimulate subsequent investigations of high‐efficiency spintronic devices that are functional at room temperature. A substantial antiferromagnetism‐induced momentum‐dependent band splitting is observed in room‐temperature metallic antiferromagnet CrSb. Using high‐resolution angle‐resolved photoemission spectroscopy measurements, clear evolution of band splitting along both out‐of‐plane and in‐plane directions is realized. This work provides spectroscopic evidence for CrSb as a spin‐split antiferromagnet and showcases its potential for applications in the evolving landscape of antiferromagnetic spintronics.

The ability to modulate the collective properties of correlated electron systems at their interfaces and surfaces underpins the burgeoning field of “designer” quantum materials. Here, we show how an electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability to itinerant ferromagnetism at the Pd-terminated surface of the nonmagnetic delafossite oxide metal PdCoO 2 . Combining angle-resolved photoemission spectroscopy and density-functional theory calculations, we show how this leads to a rich multiband surface electronic structure. We find similar surface state dispersions in PdCrO 2 , suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.

Topological insulators: no turning back Topological insulators are materials in which a relativistic effect known as spin–orbit coupling gives rise to a bulk insulating gap and surface states that resemble so-called chiral edge states in the quantum Hall effect. It has been theoretically suggested that the quantum mechanical spin degree of freedom of such surface edge states may be protected against scattering due to topology, which could be useful for spintronics and quantum computing. Now Roushan et al . provide the experimental confirmation of this important prediction. Using scanning tunnelling and angle-resolved photoemission microscopy they are able to demonstrate that, despite strong atomic scale disorder in their system, backscattering between surface states with opposite momentum and opposite spin is absent. Topological insulators are materials in which a relativistic effect known as spin–orbit coupling gives rise to surface states that resemble chiral edge modes in quantum Hall systems, but with unconventional spin textures. It has been suggested that a feature of such spin-textured boundary states is their insensitivity to spin-independent scattering, which is thought to protect them from backscattering. Here, scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy are used to confirm this prediction. Topological insulators are a new class of insulators in which a bulk gap for electronic excitations is generated because of the strong spin–orbit coupling 1 , 2 , 3 , 4 , 5 inherent to these systems. These materials are distinguished from ordinary insulators by the presence of gapless metallic surface states, resembling chiral edge modes in quantum Hall systems, but with unconventional spin textures. A key predicted feature of such spin-textured boundary states is their insensitivity to spin-independent scattering, which is thought to protect them from backscattering and localization. Recently, experimental and theoretical efforts have provided strong evidence for the existence of both two- and three-dimensional classes of such topological insulator materials in semiconductor quantum well structures 6 , 7 , 8 and several bismuth-based compounds 9 , 10 , 11 , 12 , 13 , but so far experiments have not probed the sensitivity of these chiral states to scattering. Here we use scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy to visualize the gapless surface states in the three-dimensional topological insulator Bi 1- x Sb x , and examine in detail the influence of scattering from disorder caused by random alloying in this compound. We show that, despite strong atomic scale disorder, backscattering between states of opposite momentum and opposite spin is absent. Our observations demonstrate that the chiral nature of these states protects the spin of the carriers. These chiral states are therefore potentially useful for spin-based electronics, in which long spin coherence is critical 14 , and also for quantum computing applications, where topological protection can enable fault-tolerant information processing 15 , 16 .

Excitons, Coulomb‐bound electron–hole pairs, are the fundamental excitations governing the optoelectronic properties of semiconductors. Although optical signatures of excitons have been studied extensively, experimental access to the excitonic wave function itself has been elusive. Using multidimensional photoemission spectroscopy, we present a momentum‐, energy‐, and time‐resolved perspective on excitons in the layered semiconductor WSe2. By tuning the excitation wavelength, we determine the energy–momentum signature of bright exciton formation and its difference from conventional single‐particle excited states. The multidimensional data allow to retrieve fundamental exciton properties like the binding energy and the exciton–lattice coupling and to reconstruct the real‐space excitonic distribution function via Fourier transform. All quantities are in excellent agreement with microscopic calculations. Our approach provides a full characterization of the exciton properties and is applicable to bright and dark excitons in semiconducting materials, heterostructures, and devices. Key points The full life cycle of excitons is recorded with time‐ and angle‐resolved photoemission spectroscopy. The real‐space distribution of the excitonic wave function is visualized. Direct measurement of the exciton‐phonon interaction. Real‐space density distribution of an excitonic wave function retrieved from time‐ and angle‐resolved photoemission spectroscopy.

The active sites over commercial copper/zinc oxide/aluminum oxide (Cu/ZnO/Al₂O₃) catalysts for carbon dioxide (CO₂) hydrogenation to methanol, the Zn-Cu bimetallic sites or ZnO-Cu interfacial sites, have recently been the subject of intense debate. We report a direct comparison between the activity of ZnCu and ZnO/Cu model catalysts for methanol synthesis. By combining x-ray photoemission spectroscopy, density functional theory, and kinetic Monte Carlo simulations, we can identify and characterize the reactivity of each catalyst. Both experimental and theoretical results agree that ZnCu undergoes surface oxidation under the reaction conditions so that surface Zn transforms into ZnO and allows ZnCu to reach the activity of ZnO/Cu with the same Zn coverage. Our results highlight a synergy of Cu and ZnO at the interface that facilitates methanol synthesis via formate intermediates.

High-order nonlinear multiphoton absorption is usually inefficient, but can be enhanced by designing resonant excitations between occupied and unoccupied energy levels. We conducted angle-resolved multi-photon photoemission (mPPE) studies on the SnSe 2(001) surfaces excited by ultrashort laser pulses. By tuning photon energy and light polarization, we demonstrate the presence of a resonant four-photon photoemission (4PPE) process involving the occupied valence band (VB), the unoccupied second conduction band (CB2) and the unoccupied image-potential state (IPs) of SnSe 2. In this 4PPE process, VB electrons of SnSe 2 are resonantly excited into CB2 by adsorbing two photons, followed by the adsorption of another photon to populate the n = 1 IPs before being emitted out to the vacuum by adsorbing one more photon. This results in a double-resonant 4PPE process, which exhibits approximately a 40 times enhancement in photoemission yields compared to cases where one of the resonant pathways, CB2 → IPs, is inhibited by involving a virtual state instead of the IPs in the 4PPE. The double-resonant 4PPE process efficiently excite the bulk VB electrons outside the vacuum, like taking advantage of resonant “ladders” through two real empty electronic states of SnSe 2. Our results highlight the important applications of mPPE in probing the band-structure, particularly the unoccupied states, of recently emerging main group dichalcogenide semiconductors. Furthermore, the discovered resonant mPPE process contributes to the exploration of their promising optoelectronic applications.

Three-dimensional analogues of graphene have recently been synthesized. The transport properties of such a Dirac semimetal, Cd 3 As 2 , have been studied, revealing an unexpected mechanism that suppresses backscattering dramatically. Dirac and Weyl semimetals are 3D analogues of graphene in which crystalline symmetry protects the nodes against gap formation 1 , 2 , 3 . Na 3 Bi and Cd 3 As 2 were predicted to be Dirac semimetals 4 , 5 , and recently confirmed to be so by photoemission experiments 6 , 7 , 8 . Several novel transport properties in a magnetic field have been proposed for Dirac semimetals 2 , 9 , 10 , 11 . Here, we report a property of Cd 3 As 2 that was unpredicted, namely a remarkable protection mechanism that strongly suppresses backscattering in zero magnetic field. In single crystals, the protection results in ultrahigh mobility, 9 × 10 6 cm 2 V −1 s −1 at 5 K. Suppression of backscattering results in a transport lifetime 10 4 times longer than the quantum lifetime. The lifting of this protection by the applied magnetic field leads to a very large magnetoresistance. We discuss how this may relate to changes to the Fermi surface induced by the applied magnetic field.