Christian Tusche

Forschungszentrum Jülich · Peter Grünberg Institute (PGI)

Materials with unique quantum characteristics–quantum materials—have become of great importance for information technology. Among others, their unique transport phenomena are in many cases closely connected to details of the electronic structure. Exploring the electronic states and the interplay of the interactions in this material class down to th...

The discovery of topological states of matter has led to a revolution in materials research. When external or intrinsic parameters break symmetries, global properties of topological materials change drastically. A paramount example is the emergence of Weyl nodes under broken inversion symmetry. While a rich variety of non-trivial quantum phases cou...

Spin-polarized electrons confined in low-dimensional structures are of high interest for spintronics applications. Here, we investigate the electronic structure of an ordered array of Bi monomer and dimer chains on the Ag(110) surface. By means of spin-resolved photoemission spectroscopy, we find Rashba-Bychkov split bands crossing the Fermi level...

Topological semimetals have recently attracted great attention due to prospective applications governed by their peculiar Fermi surfaces. Weyl semimetals host chiral fermions that manifest as pairs of non-degenerate massless Weyl points in their electronic structure, giving rise to novel macroscopic quantum phenomena such as the chiral anomaly, an...

Spin-momentum locking in topological insulators and materials with Rashba-type interactions is an extremely attractive feature for novel spintronic devices and is therefore under intense investigation. Significant efforts are underway to identify new material systems with spin-momentum locking, but also to create heterostructures with new spintroni...

Many material properties such as the electronic transport characteristics depend on the details of the electronic band structure in the vicinity of the Fermi level. For an accurate ab initio description of the material properties, the electronic band structure must be known and theoretically reproduced with high fidelity. Here, we ask a question wh...

We show that the presence of a transiently excited hot electron gas in graphene leads to a substantial broadening of the C 1s line probed by time-resolved x-ray photoemission spectroscopy. The broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron gas, rather than by vibrat...

The spin of the electron is nowadays replacing the charge as basic carrier of information not only in spintronics applications, but also in the emerging field of quantum information. Topological quantum materials, where spin-momentum locking is believed to lead to particularly long spin lifetimes, are regarded as a promising platform for such appli...

In this chapter, the theoretical background necessary for the discussions of electronic structures in solids based on the photoemission spectra is described. Although the photoemission process itself is not a main issue in this book, the processes taking place in solids should briefly be explained to help the understanding of the discussions given...

In the past decades, photoelectron spectroscopy has evolved as a powerful tool to understand the valence electronic structure and band-dispersion of solid state systems. As discussed in Chaps. 6 and 7, experimental work covers almost all areas of modern solid-state and surface physics. In this chapter we describe a novel way of performing momentum...

It is well know that each emitted photoelectron has its own spin. In the case of PES, however, many electrons in an instrumental acceptance angle and energy window are detected simultaneously. Then the spin information is often averaged out in the case of PES from non-magnetic materials even when the spin detector is used after the energy analysis.

Photoelectron spectroscopy (PES) has progressed gradually and extensively in the last 5 decades with astonishing improvement in the light sources, electron energy analyzers and detection methods.

Nowadays the electronic structure of solids is very effectively probed by the PES and ARPES as discussed so far and complemented by the results of IPES and angle resolved IPES.

The progress of photoelectron spectroscopy has strongly been backed up by the development of synchrotron radiationsynchrotron radiation (SR) from the late 1970s and, in particular, undulator radiation since the 1980s.

Occupied electronic states are effectively probed by photoelectron spectroscopy as already explained. On the other hand, unoccupied electronic states can be probed either by the absorption spectroscopy or by the inverse photoemission spectroscopy (IPES) to be explained in this chapter.

Due to the relatively short inelastic mean free path (λmp) of photoelectrons in solids in a wide Ek region, photoelectron spectroscopy is rather surface sensitive. The surface electronic structure is known to be noticeably different from the bulk electronic structure in the case of strongly correlatated electron systems.

In order to realize as high bulk sensitivity as possible in PES studies of strongly correlated electron systems (SCES materials), hard X-ray photoelectron spectroscopy (HAXPES) at hν and EK above ~ a few keV is now recognized to be inevitable in spite of rather low count rates compared with PES and ARPES even in the soft X-ray region.

The photoelectrons are emitted into the vacuum with the information on energy, momentum (wave vector) as well as spin. The counting rate in angle-integrated photoelectron spectroscopy is orders of magnitude higher than that in angle-resolved photoelectron spectroscopy (ARPES). However, momentum information is smeared out or averaged in the angle-in...

The studies of micro and nano materials from the view point of topography and electronic structure are now very important for applications. Some techniques are already explained in Sect. 3.6.d. For topographic imaging of nano-materials, atomic force microscopy (AFM) as well as scanning tunneling microscopy (STM) are widely applied. In the case of m...

Angle-resolved photoelectron spectroscopy (ARPES) is performed in most cases on single crystal sample surfaces. Momentum information becomes available in ARPES, facilitating the experimental evaluation of band dispersions. By fully utilizing the energy distribution curves (EDCs) and momentum distribution curves (MDCs), the Fermi surface (FS) topolo...

Photoelectron spectroscopy is mostly used for the analysis of electronic structures. ARPES is a powerful tool for band mapping and Fermiology as explained already. It is also widely known that the photoelectrons are diffracted by neighboring atoms and provide characteristic diffraction patterns.

In contrast to the hard X-ray PEShard X-ray PES (HAXPES) handled in Chap. 8, we call the low energy PES below hν ≲ 10 eV as extremely low energy PESextremely low energy PES (ELEPES) in this book. According to the roughly calculated results or empirically acquired knowledge, the λmp or escape depth of photoelectrons in this very low energy region wa...

As discussed in Chaps. 4 and 5, the bulk sensitivity becomes essential in photoelectron spectroscopy when one studies the electronic structure of strongly correlated electron systems where the effect of the short range Coulomb repulsion U becomes decisive.

The discovery of topological states of matter has led to a revolution in materials research. When external or intrinsic parameters break certain symmetries, global properties of topological materials change drastically. A paramount example is the emergence of Weyl nodes under broken inversion symmetry, acting like magnetic monopoles in momentum spa...

Core level binding energies and absorption edges are at the heart of many experimental techniques concerned with element-specific structure, electronic structure, chemical reactivity, elementary excitations and magnetism. X-ray photoemission spectroscopy (XPS) in particular, can provide information about the electronic and vibrational many-body int...

We investigate the coupled spin and orbital textures of the topological surface state in Bi2(Te,Se)3(0001) across full momentum space using spin- and angle-resolved photoelectron spectroscopy and relativistic one-step photoemission theory. For an approximately isotropic Fermi surface in Bi2Te2Se, the measured intensity and spin momentum distributio...

We investigate the coupled spin and orbital textures of the topological surface state in Bi 2 (Te,Se) 3 (0001) across full momentum space using spin-and angle-resolved photoelectron spectroscopy and relativistic one-step photoemission theory. For an approximately isotropic Fermi surface in Bi 2 Te 2 Se, the measured intensity and spin momentum dist...

Quantum well states formed by d electrons in metallic thin films are responsible for many fundamental phenomena that oscillate with layer thickness, such as magnetic anisotropy or magnetoresistance. Using momentum microscopy and angle-resolved photoemission, we mapped in unprecedented detail the quantized electronic states of Fe(001) in a broad pho...

This book presents photoelectron spectroscopy as a valuable method for studying the electronic structures of various solid materials in the bulk state, on surfaces, and at buried interfaces. This second edition introduces the advanced technique of high-resolution and high-efficiency spin- and momentum-resolved photoelectron spectroscopy using a nov...

Spin-momentum locking in topological insulators and materials with Rashba-type interactions is an extremely attractive feature for novel spintronic devices and is therefore under intense investigation. Significant efforts are underway to identify new material systems with spin-momentum locking, but also to create heterostructures with new spintroni...

Magnetic anisotropy (MA) is a material preference that involves magnetization aligned along a specific direction and provides a basis for spintronic devices. Here we report the first observation of a strong MA in a cobalt-molybdenum disulfide (Co/MoS2) heterojunction. Element-specific magnetic images recorded with an X-ray photoemission electron mi...

Photoelectron spectroscopy is our main tool to explore the electronic structure of novel material systems, the properties of which are often determined by an intricate interplay of competing interactions. Elucidating the role of this interactions requires studies over an extensive range of energy, momentum, length, and time scales. We show that imm...

JuSPARC, the Jülich Short-Pulsed Particle and Radiation Center, is a laser-driven facility to enable research with short-pulsed photon and particle beams to be performed at the Forschungszentrum Jülich. The conceptual design of JuSPARC is determined by a set of state-of-the-art time-resolved instruments, which are designed to address the electronic...

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-...

Two‐dimensional oxide quasicrystals (OQCs) are recently discovered aperiodic, but well‐ordered oxide interfaces. In this topical review, an introduction to these new thin film systems will be given. The concept of quasicrystals and their approximants is explained for BaTiO3 and SrTiO3‐derived OQCs and related periodic structures in these two‐dimens...

The valence band electronic structure of a BaTiO3-derived oxide quasicrystal (OQC) is studied by photoemission using momentum microscopy. An upward-dispersive O2p band is identified, and it can be assigned to a combination of in-plane orbitals according to the symmetry and the overlap of the wave functions. In addition, the signature of Ti3d states...

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 cath...

Time-resolved potoemission with femtosecond pump and probe pulses is an emerging technique with a large potential. Real-time recording of ultrafast electronic processes, transient states in chemical reactions or the interplay of electronic and structural dynamics bears fascinating opportunities for future research. Combining valence-band and core-l...

A stable BiI3 monosheet has been grown for the first time on the (0001) surface of the topological insulator Bi2Se3 as confirmed by scanning tunnelling microscopy, surface X-ray diffraction, and X-ray photoemision spectroscopy. BiI3 is deposited by molecular beam epitaxy from the crystalline BiTeI precursor that undergoes decomposition sublimation....

Many properties of real materials can be modeled using ab initio methods within a single-particle picture. However, for an accurate theoretical treatment of excited states, it is necessary to describe electron-electron correlations including interactions with bosons: phonons, plasmons, or magnons. In this work, by comparing spin- and momentum-resol...

Bismuth selenide is a prototype 3D topological insulator characterized by a Dirac surface state, which exhibits a pronounced chirality due to spin‐momentum locking. One key property is that the topological protection can be lifted by a magnetic field oriented perpendicular to the surface involving an opening of a gap at the Dirac point which is dec...

In the Comment on our publication [Phys. Rev. B 95, 180202(R) (2017)], R. A. Gordon claims that our main conclusion is not valid, namely that gold atoms deposited in situ on the (0001) surface of single-crystalline Bi2Se3 reside in substitutional sites, i.e., replacing bismuth atoms within the topmost quintuple layer (QL). Based on x-ray absorption...

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 qua...

The thermal excitation of electrons to higher, unoccupied states leads in certain cases to the paradox situation that the chemical potential needs to be shifted to lower energies. Here, a manipulation of Dirac fermions through the temperature dependence of the chemical potential is analyzed while maintaining sufficient insulating character in the b...

Many properties of real materials can be modeled by ab initio methods within a single-particle picture. However, for an accurate theoretical treatment of excited states, it is necessary to describe electron-electron correlations including interactions with bosons: phonons, polarons, or magnons. Electron dispersion anomalies, such as kinks, are rega...

We review recent progress in the characterization of the atomic and electronic structure of pristine and adsorbate‐covered Bi2Se3(0001) using state‐of‐the‐art analysis tools like surface X‐ray diffraction, X‐ray absorption fine structure, and spin‐ and momentum‐resolved photoemission spectroscopy. On the basis of several examples we show how the to...

Structural and electronic properties of the SmB6(001) single-crystal surface prepared by Ar⁺ ion sputtering and controlled annealing are investigated by scanning tunneling microscopy. In contrast to the cases of cleaved surfaces, we observe a single phase surface with a non-reconstructed p(1 × 1) lattice on the entire surface at an optimized anneal...

Time-of-flight momentum microscopy reveals sixfold symmetric sharp features of decreased intensity (dark lines) in constant-energy maps for clean Ir(111) and graphene/Ir(111). The dark lines have been observed for p- and s-polarized light in the photon-energy range of 20–27 eV and result from scattering of photoelectrons at the surface potential ba...

The combination of momentum microscopy (high resolution imaging of the Fourier plane) with an imaging spin filter has recently set a benchmark in k-resolution and spin-detection efficiency. Here we show that the degree of parallelization can be further increased by time-of-flight energy recording. On the quest towards maximum information (in earlie...

Spin-momentum locking of surface states has attracted great interest in recent years due to envisioned technological applications in the field of spintronics. Normal metal surfaces like W(1 1 0) and Ir(1 1 1) show surface states with energy dispersions and spin-polarization textures, which are reminiscent of topologically non-trivial surface states...

Momentum-resolved photoemission spectroscopy indicates the instability of the Dirac surface state upon deposition of gold on the (0001) surface of the topological insulator Bi2Se3. Based on the structure model derived from extended x-ray absorption fine structure experiments showing that gold atoms substitute bismuth atoms, first-principles calcula...

The neutralization of a single He2+ ion near a Ir surface leads to the emission of an electron pair. Via coincidence spectroscopy we give evidence that a sizable amount of these electron pairs originate from a correlated single step neutralization of the ion involving a total of four electrons from the metal. These correlated electron pairs cannot...

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...

We find in the case of W(110) previously overlooked anomalous surface states having their spin locked at right angle to their momentum using spin-resolved momentum microscopy. In addition to the well known Dirac-like surface state with Rashba spin texture near the -point, we observe a tilted Dirac cone with circularly shaped cross section and a Dir...

In the quest for detailed spectroscopic insight into the electronic structure at solid surfaces in a large momentum range, we have developed an advanced experimental approach. It combines the 3D detection scheme of a time-of-flight momentum microscope with an optimized filling pattern of the BESSY II storage ring. Here, comprehensive data sets cove...

We report the in situ preparation of surfaces of the proposed topological Kondo insulator SmB by controlled cycles of Ar ion sputtering and annealing. The procedure provides a reproducible way for the preparation of Sm- or B-rich surface terminations by low (1080 C) or high (1200 C) temperature annealing. The surface quality and termination were ch...

Semiconductors with strong spin-orbit interaction as the underlying mechanism for the generation of spin-polarized electrons are showing potential for applications in spintronic devices. Unveiling the full spin texture in momentum space for such materials and its relation to the microscopic structure of the electronic wave functions is experimental...

We have imaged the magnetic domains on magnetite (001) through the Verwey transition by means of spin-resolved photoemission electron microscopy. A He laboratory source is used for illumination. The magnetic domains walls above the Verwey transition are aligned with 〈110〉 in-plane directions. Below the Verwey transition, the domain structure is int...

Electron-phonon coupling is one of the most fundamental effects in condensed matter physics. We here demonstrate that photoelectron momentum mapping can reveal and visualize the coupling between specific vibrational modes and electronic excitations. When imaging molecular orbitals with high energy resolution, the intensity patterns of photoelectron...

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-resolve...

A comprehensive mapping of the spin polarization of the electronic bands in ferroelectric a-GeTe(111) films has been performed using a time-of-flight momentum microscope equipped with an imaging spin filter that enables a simultaneous measurement of more than 10.000 data points (voxels). A Rashba type splitting of both surface and bulk bands with o...

Procedures to prepare clean Bi2Se3(0001) surfaces from bulk samples and epitaxial FeSe nanocrystals on Bi2Se3(0001) are reported. Bi2Se3(0001) substrates are prepared by in vacuo cleavage of bulk samples, followed by ion bombardment and annealing cycles. FeSe is prepared by Fe deposition onto Bi2Se3 at 303 K, followed by annealing at T ≈ 623 K. We...

The detailed electronic structure of a layered semiconductor 1T-TiS2 and its modification in Ni-intercalated Ni1/3TiS2 were studied beyond the full surface Brillouin zone by use of a momentum microscope and He-I light source on their in-situ cleaved surfaces. Clear dispersions associated with the electron Fermi surface (FS) pockets induced by the s...

The resonant capture of electrons from a metallic surface into the outer shell of a helium ion creates doubly excited states in a spin singlet (↑↓) or triplet (↑↑) configuration. Here it is shown that the capture of one or two electrons can be described in a simple quantitative model, and the capture of two electrons by He^{++} proceeds in a single...

Ultrahigh spectral brightness femtosecond XUV and X-ray sources like free electron lasers (FEL) and table-top high harmonics sources (HHG) offer fascinating experimental possibilities for analysis of transient states and ultrafast electron dynamics. For electron spectroscopy experiments using illumination from such sources, the ultrashort high-char...

High resolution photoelectron spectroscopy is recognized to be a very powerful approach to study surface and bulk electronic structures of various solids by employing different photon energies (hν). In particular, angle resolved photoelectron spectroscopy (ARPES) has progressed dramatically in the last few decades providing useful information on Fe...

We present a spin resolving "momentum microscope" for the high resolution imaging of the momentum distribution of photoelectrons. Measurements of the band structure of a Au(111) single crystal surface demonstrate an energy resolution of ΔE=12meV and a momentum resolution of Δk∥=0.0049A˚(-1), measured at the line-width of the spin-orbit split Shockl...

Extending previous work on the imaging spin-filter technique based on electron diffraction from W(001) in the specular (00)-LEED (low-energy electron diffraction) spot, we studied the scattering-energy and angle-of-incidence landscape of spin sensitivity $S$ and reflectivity ${I/I}_{0}$. The setup includes a spin-polarized GaAs electron source and...