Development of a momentum microscope for time resolved band structure imaging

Asymmetric electrostatic dodecapole: compact bandpass filter with low aberrations for momentum microscopy

Imaging energy filters in photoelectron microscopes and momentum microscopes use spherical fields with deflection angles of 90°, 180° and even 2 × 180°. These instruments are optimized for high energy resolution, and exhibit image aberrations when operated in high transmission mode at medium energy resolution. Here, a new approach is presented for bandpass-filtered imaging in real or reciprocal space using an electrostatic dodecapole with an asymmetric electrode array. In addition to energy-dispersive beam deflection, this multipole allows aberration correction up to the third order. Here, its use is described as a bandpass prefilter in a time-of-flight momentum microscope at the hard X-ray beamline P22 of PETRA III. The entire instrument is housed in a straight vacuum tube because the deflection angle is only 4° and the beam displacement in the filter is only ∼8 mm. The multipole is framed by transfer lenses in the entrance and exit branches. Two sets of 16 different-sized entrance and exit apertures on piezomotor-driven mounts allow selection of the desired bandpass. For pass energies between 100 and 1400 eV and slit widths between 0.5 and 4 mm, the transmitted kinetic energy intervals are between 10 eV and a few hundred electronvolts (full width at half-maximum). The filter eliminates all higher or lower energy signals outside the selected bandpass, significantly improving the signal-to-background ratio in the time-of-flight analyzer.

Development of dual-beamline photoelectron momentum microscopy for valence orbital analysis

The soft X-ray photoelectron momentum microscopy (PMM) experimental station at the UVSOR Synchrotron Facility has been recently upgraded by additionally guiding vacuum ultraviolet (VUV) light in a normal-incidence configuration. PMM offers a very powerful tool for comprehensive electronic structure analyses in real and momentum spaces. In this work, a VUV beam with variable polarization in the normal-incidence geometry was obtained at the same sample position as the soft X-ray beam from BL6U by branching the VUV beamline BL7U. The valence electronic structure of the Au(111) surface was measured using horizontal and vertical linearly polarized (s-polarized) light excitations from BL7U in addition to horizontal linearly polarized (p-polarized) light excitations from BL6U. Such highly symmetric photoemission geometry with normal incidence offers direct access to atomic orbital information via photon polarization-dependent transition-matrix-element analysis.

Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure

In two-dimensional semiconductors, cooperative and correlated interactions determine the material's excitonic properties and can even lead to the creation of correlated states of matter. Here, we study the fundamental two-particle correlated exciton state formed by the Coulomb interaction between single-particle holes and electrons. We find that the ultrafast transfer of an exciton's hole across a type II band-aligned semiconductor heterostructure leads to an unexpected sub-200-femtosecond upshift of the single-particle energy of the electron being photoemitted from the two-particle exciton state. While energy relaxation usually leads to an energetic downshift of the spectroscopic signature, we show that this upshift is a clear fingerprint of the correlated interaction of the electron and hole parts of the exciton. In this way, time-resolved photoelectron spectroscopy is straightforwardly established as a powerful method to access electron-hole correlations and cooperative behavior in quantum materials. Our work highlights this capability and motivates the future study of optically inaccessible correlated excitonic and electronic states of matter.

Performance of a photoelectron momentum microscope in direct- and momentum-space imaging with ultraviolet photon sources

The Photoelectron-Related Image and Nano-Spectroscopy (PRINS) endstation located at the Taiwan Photon Source beamline 27A2 houses a photoelectron momentum microscope capable of performing direct-space imaging, momentum-space imaging and photoemission spectroscopy with position sensitivity. Here, the performance of this microscope is demonstrated using two in-house photon sources - an Hg lamp and He(I) radiation - on a standard checkerboard-patterned specimen and an Au(111) single crystal, respectively. By analyzing the intensity profile of the edge of the Au patterns, the Rashba-splitting of the Au(111) Shockley surface state at 300 K, and the photoelectron intensity across the Fermi edge at 80 K, the spatial, momentum and energy resolution were estimated to be 50 nm, 0.0172 Å-1 and 26 meV, respectively. Additionally, it is shown that the band structures acquired in either constant energy contour mode or momentum-resolved photoemission spectroscopy mode were in close agreement.

2D imaging spin-filter for NanoESCA based on Au/Ir(001) or Fe(001)-p(1×1)O

A two-dimensional imaging spin-filter for photo-emission electron microscopy is described. The spin-filter is capable of imaging the electron spin polarization of either real space or momentum space electron distributions. As a scattering target either Au/Ir(001) comes into use, where spin sensitivity results from using spin-orbit scattering or Fe(001)-p(1×1)O that exploits exchange interaction. Both scattering targets were characterized with respect to their working points and Sherman function in a separate setup. Spin-polarization images of secondary electrons from the magnetic domains of a poly-crystalline iron sample are shown using both scattering targets. Images with a spin-filter using Au/Ir(001) show more than 104 discrete detection channels which increases the effective two-dimensional figure-of-merit (FoM) of this spin-filter by four orders of magnitude compared to single-channel spin detectors. Using the exchange scattering target two spin-components have been imaged for the first time. A method to detect all three spin-components is also outlined.

Ultrafast Electron Microscopy of Nanoscale Charge Dynamics in Semiconductors

The ultrafast dynamics of charge carriers in solids plays a pivotal role in emerging optoelectronics, photonics, energy harvesting, and quantum technology applications. However, the investigation and direct visualization of such nonequilibrium phenomena remains as a long-standing challenge, owing to the nanometer-femtosecond spatiotemporal scales at which the charge carriers evolve. Here, we propose and demonstrate an interaction mechanism enabling nanoscale imaging of the femtosecond dynamics of charge carriers in solids. This imaging modality, which we name charge dynamics electron microscopy (CDEM), exploits the strong interaction of free-electron pulses with terahertz (THz) near fields produced by the moving charges in an ultrafast scanning transmission electron microscope. The measured free-electron energy at different spatiotemporal coordinates allows us to directly retrieve the THz near-field amplitude and phase, from which we reconstruct movies of the generated charges by comparison to microscopic theory. The CDEM technique thus allows us to investigate previously inaccessible spatiotemporal regimes of charge dynamics in solids, providing insight into the photo-Dember effect and showing oscillations of photogenerated electron-hole distributions inside a semiconductor. Our work facilitates the exploration of a wide range of previously inaccessible charge-transport phenomena in condensed matter using ultrafast electron microscopy.

Time- and angle-resolved photoemission spectroscopy (TR-ARPES) of TMDC monolayers and bilayers

Many unique properties in two-dimensional (2D) materials and their heterostructures rely on charge excitation, scattering, transfer, and relaxation dynamics across different points in the momentum space. Understanding these dynamics is crucial in both the fundamental study of 2D physics and their incorporation in optoelectronic and quantum devices. A direct method to probe charge carrier dynamics with momentum resolution is time- and angle-resolved photoemission spectroscopy (TR-ARPES). Such measurements have been challenging, since photoexcited carriers in many 2D monolayers reside at high crystal momenta, requiring probe photon energies in the extreme UV (EUV) regime. These challenges have been recently addressed by development of table-top pulsed EUV sources based on high harmonic generation, and the successful integration into a TR-ARPES and/or time-resolved momentum microscope. Such experiments will allow direct imaging of photoelectrons with superior time, energy, and crystal momentum resolution, with unique advantage over traditional optical measurements. Recently, TR-ARPES experiments of 2D transition metal dichalcogenide (TMDC) monolayers and bilayers have created unprecedented opportunities to reveal many intrinsic dynamics of 2D materials, such as bandgap renormalization, charge carrier scattering, relaxation, and wavefunction localization in moiré patterns. This perspective aims to give a short review of recent discoveries and discuss the challenges and opportunities of such techniques in the future.

Through the Lens of a Momentum Microscope: Viewing Light-Induced Quantum Phenomena in 2D Materials

Van der Waals (vdW) materials at their 2D limit are diverse, flexible, and unique laboratories to study fundamental quantum phenomena and their future applications. Their novel properties rely on their pronounced Coulomb interactions, variety of crystal symmetries and spin-physics, and the ease of incorporation of different vdW materials to form sophisticated heterostructures. In particular, the excited state properties of many 2D semiconductors and semi-metals are relevant for their technological applications, particularly those that can be induced by light. In this paper, the recent advances made in studying out-of-equilibrium, light-induced, phenomena in these materials are reviewed using powerful, surface-sensitive, time-resolved photoemission-based techniques, with a particular emphasis on the emerging multi-dimensional photoemission spectroscopy technique of time-resolved momentum microscopy. The advances this technique has enabled in studying the nature and dynamics of occupied excited states in these materials are discussed. Then, the future research directions opened by these scientific and instrumental advancements are projected for studying the physics of 2D materials and the opportunities to engineer their band-structure and band-topology by laser fields.

Expanding the momentum field of view in angle-resolved photoemission systems with hemispherical analyzers

In photoelectron spectroscopy, the measured electron momentum range is intrinsically related to the excitation photon energy. Low photon energies <10 eV are commonly encountered in laser-based photoemission and lead to a momentum range that is smaller than the Brillouin zones of most materials. This can become a limiting factor when studying condensed matter with laser-based photoemission. An additional restriction is introduced by widely used hemispherical analyzers that record only electrons photoemitted in a solid angle set by the aperture size at the analyzer entrance. Here, we present an upgrade to increase the effective solid angle that is measured with a hemispherical analyzer. We achieve this by accelerating the photoelectrons toward the analyzer with an electric field that is generated by a bias voltage on the sample. Our experimental geometry is comparable to a parallel plate capacitor, and therefore, we approximate the electric field to be uniform along the photoelectron trajectory. With this assumption, we developed an analytic, parameter-free model that relates the measured angles to the electron momenta in the solid and verify its validity by comparing with experimental results on the charge density wave material TbTe₃. By providing a larger field of view in momentum space, our approach using a bias potential considerably expands the flexibility of laser-based photoemission setups.

The 2021 ultrafast spectroscopic probes of condensed matter roadmap

In the 60 years since the invention of the laser, the scientific community has developed numerous fields of research based on these bright, coherent light sources, including the areas of imaging, spectroscopy, materials processing and communications. Ultrafast spectroscopy and imaging techniques are at the forefront of research into the light-matter interaction at the shortest times accessible to experiments, ranging from a few attoseconds to nanoseconds. Light pulses provide a crucial probe of the dynamical motion of charges, spins, and atoms on picosecond, femtosecond, and down to attosecond timescales, none of which are accessible even with the fastest electronic devices. Furthermore, strong light pulses can drive materials into unusual phases, with exotic properties. In this roadmap we describe the current state-of-the-art in experimental and theoretical studies of condensed matter using ultrafast probes. In each contribution, the authors also use their extensive knowledge to highlight challenges and predict future trends.

Projection-type electron spectroscopy collimator analyzer for charged particles and x-ray detections

We developed a compact sized device for angular and energy analysis of charged particles in a wide acceptance cone angle of nearly 1π steradian. This device is configured from an electrostatic lens comprising an axisymmetric aspherical mesh, which has a concave shape viewed from the point source, a set of axisymmetric electrodes, planar grids, microchannel plates, and a fluorescent screen positioned coaxially. The potentials of electrodes are adjusted so that the trajectories of the electrons with arbitrarily set kinetic energy are substantially parallelized by the electrostatic lens and enter the planar grid perpendicularly. Instead of the planar grid, a collimator plate with parallel holes can be used as an energy band-pass filter. The angular distribution of electrons with the selected kinetic energy is projected directly onto the fluorescent screen without converging and passing through a pinhole. This is a simple but significant electron-optical design to obtain wide-range angular distribution with high angular resolution, and the analyzer can be suitably used for the two-dimensional angular distribution measurements of electrons and ions emitted from surfaces.

Experimental measurement of the intrinsic excitonic wave function

An exciton, a two-body composite quasiparticle formed of an electron and hole, is a fundamental optical excitation in condensed matter systems. Since its discovery nearly a century ago, a measurement of the excitonic wave function has remained beyond experimental reach. Here, we directly image the excitonic wave function in reciprocal space by measuring the momentum distribution of electrons photoemitted from excitons in monolayer tungsten diselenide. By transforming to real space, we obtain a visual of the distribution of the electron around the hole in an exciton. Further, by also resolving the energy coordinate, we confirm the elusive theoretical prediction that the photoemitted electron exhibits an inverted energy-momentum dispersion relationship reflecting the valence band where the partner hole remains, rather than that of conduction band states of the electron.

An open-source, end-to-end workflow for multidimensional photoemission spectroscopy

Characterization of the electronic band structure of solid state materials is routinely performed using photoemission spectroscopy. Recent advancements in short-wavelength light sources and electron detectors give rise to multidimensional photoemission spectroscopy, allowing parallel measurements of the electron spectral function simultaneously in energy, two momentum components and additional physical parameters with single-event detection capability. Efficient processing of the photoelectron event streams at a rate of up to tens of megabytes per second will enable rapid band mapping for materials characterization. We describe an open-source workflow that allows user interaction with billion-count single-electron events in photoemission band mapping experiments, compatible with beamlines at 3rd and 4rd generation light sources and table-top laser-based setups. The workflow offers an end-to-end recipe from distributed operations on single-event data to structured formats for downstream scientific tasks and storage to materials science database integration. Both the workflow and processed data can be archived for reuse, providing the infrastructure for documenting the provenance and lineage of photoemission data for future high-throughput experiments.

Single-hemisphere photoelectron momentum microscope with time-of-flight recording

Photoelectron momentum microscopy is an emerging powerful method for angle-resolved photoelectron spectroscopy (ARPES), especially in combination with imaging spin filters. These instruments record k_x-k_y images, typically exceeding a full Brillouin zone. As energy filters, double-hemispherical or time-of-flight (ToF) devices are in use. Here, we present a new approach for momentum mapping of the full half-space, based on a large single hemispherical analyzer (path radius of 225 mm). Excitation by an unfocused He lamp yielded an energy resolution of 7.7 meV. The performance is demonstrated by k-imaging of quantum-well states in Au and Xe multilayers. The α²-aberration term (α, entrance angle in the dispersive plane) and the transit-time spread of the electrons in the spherical field are studied in a large pass-energy (6 eV-660 eV) and angular range (α up to ±7°). It is discussed how the method circumvents the preconditions of previous theoretical work on the resolution limitation due to the α²-term and the transit-time spread, being detrimental for time-resolved experiments. Thanks to k-resolved detection, both effects can be corrected numerically. We introduce a dispersive-plus-ToF hybrid mode of operation, with an imaging ToF analyzer behind the exit slit of the hemisphere. This instrument captures 3D data arrays I (E_B, k_x, k_y), yielding a gain up to N² in recording efficiency (N being the number of resolved time slices). A key application will be ARPES at sources with high pulse rates such as synchrotrons with 500 MHz time structure.

A quantitative comparison of time-of-flight momentum microscopes and hemispherical analyzers for time- and angle-resolved photoemission spectroscopy experiments

Time-of-flight-based momentum microscopy has a growing presence in photoemission studies, as it enables parallel energy- and momentum-resolved acquisition of the full photoelectron distribution. Here, we report table-top extreme ultraviolet time- and angle-resolved photoemission spectroscopy (trARPES) featuring both a hemispherical analyzer and a momentum microscope within the same setup. We present a systematic comparison of the two detection schemes and quantify experimentally relevant parameters, including pump- and probe-induced space-charge effects, detection efficiency, photoelectron count rates, and depth of focus. We highlight the advantages and limitations of both instruments based on exemplary trARPES measurements of bulk WSe₂. Our analysis demonstrates the complementary nature of the two spectrometers for time-resolved ARPES experiments. Their combination in a single experimental apparatus allows us to address a broad range of scientific questions with trARPES.

High-Mobility Epitaxial Graphene on Ge/Si(100) Substrates

Graphene was shown to reveal intriguing properties of its relativistic two-dimensional electron gas; however, its implementation to microelectronic applications is missing to date. In this work, we present a comprehensive study of epitaxial graphene on technologically relevant and in a standard CMOS process achievable Ge(100) epilayers grown on Si(100) substrates. Crystalline graphene monolayer structures were grown by means of chemical vapor deposition (CVD). Using angle-resolved photoemission spectroscopy and in situ surface transport measurements, we demonstrate their metallic character both in momentum and real space. Despite numerous crystalline imperfections, e.g., grain boundaries and strong corrugation, as compared to epitaxial graphene on SiC(0001), charge carrier mobilities of 1 × 10⁴ cm²/Vs were obtained at room temperature, which is a result of the quasi-charge neutrality within the graphene monolayers on germanium and not dependent on the presence of an interface oxide. The interface roughness due to the facet structure of the Ge(100) epilayer, formed during the CVD growth of graphene, can be reduced via subsequent in situ annealing up to 850 °C coming along with an increase in the mobility by 30%. The formation of a Ge(100)-(2 × 1) structure demonstrates the weak interaction and effective delamination of graphene from the Ge/Si(100) substrate.

Time-resolved momentum microscopy with a 1 MHz high-harmonic extreme ultraviolet beamline

Recent progress in laser-based high-repetition rate extreme ultraviolet (EUV) light sources and multidimensional photoelectron spectroscopy enables the build-up of a new generation of time-resolved photoemission experiments. Here, we present a setup for time-resolved momentum microscopy driven by a 1 MHz fs EUV table-top light source optimized for the generation of 26.5 eV photons. The setup provides simultaneous access to the temporal evolution of the photoelectron's kinetic energy and in-plane momentum. We discuss opportunities and limitations of our new experiment based on a series of static and time-resolved measurements on graphene.

Coexisting Charge States in a Unary Organic Monolayer Film on a Metal

The electronic and geometric structures of tetracene films on Ag(110) and Cu(110) have been studied with photoemission tomography and compared to that of pentacene. Despite similar energy level alignment of the two oligoacenes on these surfaces revealed by conventional ultraviolet photoelectron spectroscopy, the momentum-space resolved photoemission tomography reveals a significant difference in both structural and electronic properties of tetracene and pentacene films. Particularly, the saturated monolayer of tetracene on Ag(110) is found to consist of two molecular species that, despite having the same orientation, are electronically very different-while one molecule remains neutral, another is charged because of electron donation from the substrate.

Imaging properties of hemispherical electrostatic energy analyzers for high resolution momentum microscopy

Hemispherical deflection analyzers are the most widely used energy filters for state-of-the-art electron spectroscopy. Due to the high spherical symmetry, they are also well suited as imaging energy filters for electron microscopy. Here, we review the imaging properties of hemispherical deflection analyzers with emphasis on the application for cathode lens microscopy. In particular, it turns out that aberrations, in general limiting the image resolution, cancel out at the entrance and exit of the analyzer. This finding allows more compact imaging energy filters for momentum microscopy or photoelectron emission microscopy. For instance, high resolution imaging is possible, using only a single hemisphere. Conversely, a double pass hemispherical analyzer can double the energy dispersion, which means it can double the energy resolution at certain transmission, or can multiply the transmission at certain energy resolution.

Modification of Pb quantum well states by the adsorption of organic molecules

The successful implementation of nanoscale materials in next generation optoelectronic devices crucially depends on our ability to functionalize and design low dimensional materials according to the desired field of application. Recently, organic adsorbates have revealed an enormous potential to alter the occupied surface band structure of tunable materials by the formation of tailored molecule-surface bonds. Here, we extend this concept of adsorption-induced surface band structure engineering to the unoccupied part of the surface band structure. This is achieved by our comprehensive investigation of the unoccupied band structure of a lead (Pb) monolayer film on the Ag(1 1 1) surface prior and after the adsorption of one monolayer of the aromatic molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA). Using two-photon momentum microscopy, we show that the unoccupied states of the Pb/Ag(1 1 1) bilayer system are dominated by a parabolic quantum well state (QWS) in the center of the surface Brillouin zone with Pb p[Formula: see text] orbital character and a side band with almost linear dispersion showing Pb p[Formula: see text] orbital character. After the adsorption of PTCDA, the Pb side band remains completely unaffected while the signal of the Pb QWS is fully suppressed. This adsorption induced change in the unoccupied Pb band structure coincides with an interfacial charge transfer from the Pb layer into the PTCDA molecule. We propose that this charge transfer and the correspondingly vertical (partially chemical) interaction across the PTCDA/Pb interface suppresses the existence of the QWS in the Pb layer. Our results hence unveil a new possibility to orbital selectively tune and control the entire surface band structure of low dimensional systems by the adsorption of organic molecules.

Variable-deceleration-ratio wide-acceptance-angle electrostatic lens for two-dimensional angular and energy analysis

Variable-deceleration-ratio wide-acceptance-angle electrostatic lens (VD-WAAEL) is proposed as a potential technique for two-dimensional angular and energy analysis. The basic features of the lens are studied using the charge simulation method and ray tracing calculation. The lens uses an ellipsoidal mesh electrode and allows a wide acceptance angle of ±50°. Two possible applications of the VD-WAAEL are discussed. One is a simple combination of the VD-WAAEL-projection-lens, in which an aperture is used for energy analysis. The other is a combination of the VD-WAAEL with a conventional electron spectrometer, which is responsible for obtaining higher energy resolution. The former is discussed in detail and the latter is described briefly. While the ray tracing calculation is only for the case of an ideal mesh, a note on the disturbing effect of mesh holes is presented. The best possible energy resolution of the simple VD-WAAEL-projection-lens analyzer seems to be around 1/1000, given a fine mesh electrode to suppress the disturbing effect of mesh holes.

Nonlocal electron correlations in an itinerant ferromagnet

Our understanding of the properties of ferromagnetic materials, widely used in spintronic devices, is fundamentally based on their electronic band structure. However, even for the most simple elemental ferromagnets, electron correlations are prevalent, requiring descriptions of their electronic structure beyond the simple picture of independent quasi-particles. Here, we give evidence that in itinerant ferromagnets like cobalt these electron correlations are of nonlocal origin, manifested in a complex self-energy Σσ(E,k) that disperses as function of spin σ, energy E, and momentum vector k. Together with one-step photoemission calculations, our experiments allow us to quantify the dispersive behaviour of the complex self-energy over the whole Brillouin zone. At the same time we observe regions of anomalously large "waterfall"-like band renormalization, previously only attributed to strong electron correlations in high-TC superconductors, making itinerant ferromagnets a paradigmatic test case for the interplay between band structure, magnetism, and many-body correlations.

Direct Observation of the Band Gap Transition in Atomically Thin ReS2

ReS₂ is considered as a promising candidate for novel electronic and sensor applications. The low crystal symmetry of this van der Waals compound leads to a highly anisotropic optical, vibrational, and transport behavior. However, the details of the electronic band structure of this fascinating material are still largely unexplored. We present a momentum-resolved study of the electronic structure of monolayer, bilayer, and bulk ReS₂ using k-space photoemission microscopy in combination with first-principles calculations. We demonstrate that the valence electrons in bulk ReS₂ are-contrary to assumptions in recent literature-significantly delocalized across the van der Waals gap. Furthermore, we directly observe the evolution of the valence band dispersion as a function of the number of layers, revealing the transition from an indirect band gap in bulk ReS₂ to a direct gap in the bilayer and the monolayer. We also find a significantly increased effective hole mass in single-layer crystals. Our results establish bilayer ReS₂ as an advantageous building block for two-dimensional devices and van der Waals heterostructures.

Multi-orbital charge transfer at highly oriented organic/metal interfaces

The molecule-substrate interaction plays a key role in charge injection organic-based devices. Charge transfer at molecule-metal interfaces strongly affects the overall physical and magnetic properties of the system, and ultimately the device performance. Here, we report theoretical and experimental evidence of a pronounced charge transfer involving nickel tetraphenyl porphyrin molecules adsorbed on Cu(100). The exceptional charge transfer leads to filling of the higher unoccupied orbitals up to LUMO+3. As a consequence of this strong interaction with the substrate, the porphyrin's macrocycle sits very close to the surface, forcing the phenyl ligands to bend upwards. Due to this adsorption configuration, scanning tunneling microscopy cannot reliably probe the states related to the macrocycle. We demonstrate that photoemission tomography can instead access the Ni-TPP macrocycle electronic states and determine the reordering and filling of the LUMOs upon adsorption, thereby confirming the remarkable charge transfer predicted by density functional theory calculations.Charge transfer at molecule-metal interfaces affects the overall physical and magnetic properties of organic-based devices, and ultimately their performance. Here, the authors report evidence of a pronounced charge transfer involving nickel tetraphenyl porphyrin molecules adsorbed on copper.

Polarization driven conductance variations at charged ferroelectric domain walls

Conducting domain walls (CDWs) in ferroelectric materials are promising candidates for applications in a manifold of nanoscale, optoelectronic devices. Characterization of their microscopic properties, however, remains challenging due to their small dimension and highly insulating environment. Here, we inspect individual CDWs in single-crystalline LiNbO₃ by the combination of photoemission electron microscopy (PEEM) and second harmonic generation (SHG) microscopy. While SHG unveils the overall domain wall inclination angle α, PEEM is sensitive to local conductance variations, both at and away from the domain wall. Thus, the two imaging techniques deliver complementary information over a large field of view. In agreement with earlier theoretical predictions we find that the local conductance is dictated by α and reveal a quantitative connection between them. Our results help to elucidate the electronic structure of CDWs and underline the value of PEEM as a non-contact characterization tool for mapping local conductance variations in highly resistive environments.

Modifying the Surface of a Rashba-Split Pb-Ag Alloy Using Tailored Metal-Organic Bonds

Hybridization-related modifications of the first metal layer of a metal-organic interface are difficult to access experimentally and have been largely neglected so far. Here, we study the influence of specific chemical bonds (as formed by the organic molecules CuPc and PTCDA) on a Pb-Ag surface alloy. We find that delocalized van der Waals or weak chemical π-type bonds are not strong enough to alter the alloy, while localized σ-type bonds lead to a vertical displacement of the Pb surface atoms and to changes in the alloy's surface band structure. Our results provide an exciting platform for tuning the Rashba-type spin texture of surface alloys using organic molecules.

Nanofaceting as a stamp for periodic graphene charge carrier modulations

The exceptional electronic properties of monatomic thin graphene sheets triggered numerous original transport concepts, pushing quantum physics into the realm of device technology for electronics, optoelectronics and thermoelectrics. At the conceptual pivot point is the particular two-dimensional massless Dirac fermion character of graphene charge carriers and its volitional modification by intrinsic or extrinsic means. Here, interfaces between different electronic and structural graphene modifications promise exciting physics and functionality, in particular when fabricated with atomic precision. In this study we show that quasiperiodic modulations of doping levels can be imprinted down to the nanoscale in monolayer graphene sheets. Vicinal copper surfaces allow to alternate graphene carrier densities by several 10(13) carriers per cm(2) along a specific copper high-symmetry direction. The process is triggered by a self-assembled copper faceting process during high-temperature graphene chemical vapor deposition, which defines interfaces between different graphene doping levels at the atomic level.

Optical Generation of Hot Spin-Polarized Electrons from a Ferromagnetic Two-Dimensional Electron Gas

Linearly polarized light with an energy of 3.1 eV has been used to excite highly spin-polarized electrons in an ultrathin film of face-centered-tetragonal cobalt to majority-spin quantum well states (QWS) derived from an sp band at the border of the Brillouin zone. The spin-selective excitation process has been studied by spin- and momentum-resolved two-photon photoemission. Analyzing the photoemission patterns in two-dimensional momentum planes, we find that the optically driven transition from the valence band to the QWS acts almost exclusively on majority-spin electrons. The mechanism providing the high spin polarization is discussed by the help of a density-functional theory calculation. Additionally, a sizable effect of spin-orbit coupling for the QWS is evidenced.

Topological states on the gold surface

Gold surfaces host special electronic states that have been understood as a prototype of Shockley surface states. These surface states are commonly employed to benchmark the capability of angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling spectroscopy. Here we show that these Shockley surface states can be reinterpreted as topologically derived surface states (TDSSs) of a topological insulator (TI), a recently discovered quantum state. Based on band structure calculations, the Z₂-type invariants of gold can be well-defined to characterize a TI. Further, our ARPES measurement validates TDSSs by detecting the dispersion of unoccupied surface states. The same TDSSs are also recognized on surfaces of other well-known noble metals (for example, silver, copper, platinum and palladium), which shines a new light on these long-known surface states.

The surfaces of noble metals possess Shockley states which exhibit Rashba-type spin splitting and spin-momentum locking. Here, the authors use ab initio methods and photoemission spectroscopy to demonstrate how such Shockley states may be reinterpreted as topologically protected surface states.

Broadband evolution of phononic-crystal-waveguide eigenstates in real- and k-spaces

Control of sound in phononic band-gap structures promises novel control and guiding mechanisms. Designs in photonic systems were quickly matched in phononics, and rows of defects in phononic crystals were shown to guide sound waves effectively. The vast majority of work in such phononic guiding has been in the frequency domain, because of the importance of the phononic dispersion relation in governing acoustic confinement in waveguides. However, frequency-domain studies miss vital information concerning the phase of the acoustic field and eigenstate coupling. Using a wide range of wavevectors k, we implement an ultrafast technique to probe the wave field evolution in straight and L-shaped phononic crystal surface-phonon waveguides in real- and k-space in two spatial dimensions, thus revealing the eigenstate-energy redistribution processes and the coupling between different frequency-degenerate eigenstates. Such use of k-t space is a first in acoustics, and should have other interesting applications such as acoustic-metamaterial characterization.

Polarization sensitive surface band structure of doped BaTiO3(001)

We present a spatial and wave-vector resolved study of the electronic structure of micron sized ferroelectric domains at the surface of a BaTiO(3)(001) single crystal. The n-type doping of the BaTiO(3) 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.

Laboratory-based real and reciprocal space imaging of the electronic structure of few layer graphene on SiC(0001¯) using photoelectron emission microscopy

We present real and reciprocal space photoelectron emission microscopy (PEEM) results on few layer graphene using laboratory based He I and II radiation. The combination of a focused high-intensity source and high transmission PEEM electron optics provides good signal to noise ratios for the different modes of acquisition. We demonstrate work function mapping and secondary electron analysis, related to the graphene layer thickness, band structure imaging from micron scale regions by wave vector resolved PEEM (k-PEEM) and local secondary electron spectroscopy, giving information on the valence and conduction band states and the dispersion relations of the π bands. Dark field PEEM is done by selecting the Dirac cone corresponding to the specific rotation of each graphene layer and allows spatial mapping of the commensurate rotation angles. The use of He II radiation increases the volume of reciprocal space accessible to k-PEEM and improves signal to background. The preferential linear polarization of the light source is used to investigate aspects of the electronic chirality near the Dirac cone. Recent developments in sample manipulation and cooling are presented.

Photoexcitation of adsorbates on metal surfaces: one-step or three-step

In this essay we discuss the light-matter interactions at molecule-covered metal surfaces that initiate surface photochemistry. The hot-electron mechanism for surface photochemistry, whereby the absorption of light by a metal surface creates an electron-hole pair, and the hot electron scatters through an unoccupied resonance of adsorbate to initiate nuclear dynamics leading to photochemistry, has become widely accepted. Yet, ultrafast spectroscopic measurements of molecule-surface electronic structure and photoexcitation dynamics provide scant support for the hot electron mechanism. Instead, in most cases the adsorbate resonances are excited through photoinduced substrate-to-adsorbate charge transfer. Based on recent studies of the role of coherence in adsorbate photoexcitation, as measured by the optical phase and momentum resolved two-photon photoemission measurements, we examine critically the hot electron mechanism, and propose an alternative description based on direct charge transfer of electrons from the substrate to adsorbate. The advantage of this more quantum mechanically rigorous description is that it informs how material properties of the substrate and adsorbate, as well as their interaction, influence the frequency dependent probability of photoexcitation and ultimately how light can be used to probe and control surface femtochemistry.

Measurement of intrinsic dirac fermion cooling on the surface of the topological insulator Bi2Se3 using time-resolved and angle-resolved photoemission spectroscopy

We perform time- and angle-resolved photoemission spectroscopy of a prototypical topological insulator (TI) Bi(2)Se(3) to study the ultrafast dynamics of surface and bulk electrons after photoexcitation. By analyzing the evolution of surface states and bulk band spectra, we obtain their electronic temperature and chemical potential relaxation dynamics separately. These dynamics reveal strong phonon-assisted surface-bulk coupling at high lattice temperature and total suppression of inelastic scattering between the surface and the bulk at low lattice temperature. In this low temperature regime, the unique cooling of Dirac fermions in TI by acoustic phonons is manifested through a power law dependence of the surface temperature decay rate on carrier density.

Dark field photoelectron emission microscopy of micron scale few layer graphene

We demonstrate dark field imaging in photoelectron emission microscopy (PEEM) of heterogeneous few layer graphene (FLG) furnace grown on SiC(000-1). Energy-filtered, threshold PEEM is used to locate distinct zones of FLG graphene. In each region, selected by a field aperture, the k-space information is imaged using appropriate transfer optics. By selecting the photoelectron intensity at a given wave vector and using the inverse transfer optics, dark field PEEM gives a spatial distribution of the angular photoelectron emission. In the results presented here, the wave vector coordinates of the Dirac cones characteristic of commensurate rotations of FLG on SiC(000-1) are selected providing a map of the commensurate rotations across the surface. This special type of contrast is therefore a method to map the spatial distribution of the local band structure and offers a new laboratory tool for the characterisation of technically relevant, microscopically structured matter.

Highly efficient multichannel spin-polarization detection

Since the original work by Mott, the low efficiency of electron spin polarimeters, remaining orders of magnitude behind optical polarimeters, has prohibited many fundamental experiments. Here we report a solution to this problem using a novel concept of multichannel spin-polarization analysis that provides a stunning increase in efficiency by 4 orders of magnitude. This improvement was demonstrated in a setup using a hemispherical electron energy analyzer. An imaging setup proved the principal capability of resolving more than 10(5) data points in parallel.