F. Reinert's research while affiliated with University of Wuerzburg and other places
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Publications (197)
We study the interplay of lattice, spin, and orbital degrees of freedom in a two-dimensional model system: a flat square lattice of Te atoms on a Au(100) surface. The atomic structure of the Te monolayer is determined by scanning tunneling microscopy and quantitative low-energy electron diffraction. Using spin- and angle-resolved photoelectron spec...
We study the interplay of lattice, spin and orbital degrees of freedom in a two-dimensional model system: a flat square lattice of Te atoms on a Au(100) surface. The atomic structure of the Te monolayer is determined by scanning tunneling microscopy (STM) and quantitative low-energy electron diffraction (LEED-IV). Using spin- and angle-resolved pho...
Inducing a magnetic gap at the Dirac point of the topological surface state (TSS) in a three-dimensional (3D) topological insulator (TI) is a route to dissipationless charge and spin currents. Ideally, magnetic order is present only at the surface, as through proximity of a ferromagnetic (FM) layer. However, experimental evidence of such a proximit...
The electronic structure of Weyl semimetals features Berry flux monopoles in the bulk and Fermi arcs at the surface. While angle-resolved photoelectron spectroscopy (ARPES) is successfully used to map the bulk and surface bands, it remains a challenge to explicitly resolve and pinpoint these topological features. Here we combine state-of-the-art ph...
Inducing a magnetic gap at the Dirac point of the topological surface state (TSS) in a 3D topological insulator (TI) is a route to dissipationless charge and spin currents. Ideally, magnetic order is present only at the surface and not in the bulk, e.g. through proximity of a ferromagnetic (FM) layer. However, such a proximity-induced Dirac mass ga...
We report here on the properties of topological crystalline insulator Pb 0.5 Sn 0.5 Te epitaxial films doped with bismuth at levels from 0% (undoped) to 0.15%. The undoped film exhibits a p-type character due to metal vacancies. As the doping level rises, the hole concentration reduces. At a level of 0.06%, the electrical character inverts to n-typ...
The odd parity nature of 4f states characterized by strong spin–orbit coupling and electronic correlations has led to a search for novel topological phases among rare earth compounds, such as Kondo systems, heavy Fermions, and homogeneous mixed-valent materials. Our target system is thulium telluride thin films whose bandgap is expected to be tuned...
Hybridization between f electrons and conduction bands (c-f hybridization) is a driving force for many unusual phenomena. To provide insight into it, systematic studies of CeCoIn5 heavy fermion superconductor have been performed by angle-resolved photoemission spectroscopy (ARPES) in a large angular range at temperature of T=6 K. The used photon en...
Successful applications of a topological insulator (TI) in spintronics require its bandgap to be wider then in a typical TI and the energy position of the Dirac point in the dispersion relations to be away from the valence and conduction bands. In this study we grew Bi1.1Sb0.9Te2S crystals and examined their elemental composition, structural, optic...
Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl semimetals (WSM) exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such mon...
Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi 4 Te 7 and MnBi 6 Te 10 , the n ¼ 1 and 2 members of a modular ðBi 2 Te 3 Þ n ðMnBi 2 Te 4 Þ series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circul...
Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi4Te7 and MnBi6Te10, the n=1 and 2 members of a modular (Bi2Te3)n(MnBi2Te4) series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-r...
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...
Samarium hexaboride is a candidate for the topological Kondo insulator state, in which Kondo coherence is predicted to give rise to an insulating gap spanned by topological surface states. Here we investigate the surface and bulk electronic properties of magnetically alloyed Sm1−xMxB6 (M=Ce, Eu), using angle-resolved photoemission spectroscopy and...
The hybridization between f-electrons and conduction band carriers ($V_{cf}$) is essential for the heavy fermion behavior and may play an important role in other fascinating phenomena such as quantum phase transitions or unconventional superconductivity. In order to provide an insight into $V_{cf}$, systematic studies of CeCoIn$_5$ heavy fermion su...
We present a combined soft x-ray and high-resolution vacuum-ultraviolet angle-resolved photoemission spectroscopy study of the electron-overdoped cuprate Pr1.3−xLa0.7CexCuO4 (PLCCO). Demonstration of its highly two-dimensional band structure enabled precise determination of the in-plane self-energy dominated by electron-electron scattering. Through...
Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl crystals exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such monopoles l...
We have grown the phase-homogeneous quaternary compound Bi1.1Sb0.9Te2S by the addition of Sb to Bi2Te2S which changed the conductivity from n- to p-type. Far-infrared optical and angle resolved photoemission spectra revealed a wide-band-gap three-dimensional topological insulator phase. The surface electronic spectrum is characterized by the topolo...
Rare-earth ions typically exhibit larger magnetic moments than transition-metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of topological insulators. Yet in a recent photoemission study of Eu-doped
Bi2Te3 films, the spectra remained gapless down to T=20 K.
Here we scrutinize whether the conditions for a...
The layered van der Waals compounds (MnBi 2 Te 4)(Bi 2 Te 3) were recently established as the first intrinsic magnetic topological insulators. We report a study on the epitaxial growth of (MnBi 2 Te 4) m (Bi 2 Te 3) n films based on the co-deposition of MnTe and Bi 2 Te 3 on BaF 2 (111) substrates. X-ray diffraction and scanning transmission electr...
Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$, the $n=$ 1 and 2 members of a modular (Bi$_2$Te$_3$)$_n$(MnBi$_2$Te$_4$) series, which were recently established as intrinsic magnetic topological insulators. We obser...
We present a combined soft x-ray and high-resolution vacuum-ultraviolet angle-resolved photoemission spectroscopy study of the electron-overdoped cuprate Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_4$ (PLCCO). Demonstration of its highly two-dimensional band structure enabled precise determination of the in-plane self-energy dominated by electron-electron s...
In recent years, the method of orbital tomography has been a useful tool for the analysis of a variety of molecular systems. However, the underlying plane-wave final state has been largely expected to be applicable to planar molecules only. Here, we demonstrate on photoemission data from the molecule C60 adsorbed on Ag(110) that it can indeed be a...
An insulating ferromagnetic ground state is a fundamental prerequisite for the quantum anomalous Hall (QAH) effect observed in magnetically doped topological insulators such as (Bi,Sb)2Te3. So far, the QAH effect could only be induced by V and Cr doping, with V resulting in ferromagnetism with a higher TC and a more robust QAH state. To better unde...
Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order¹. Unlike their nonmagnetic counterparts, magnetic topological insulators may have some of the surfaces gapped, which enables a number of exotic phenomena that have potential applications in spintronics¹, such as the quant...
The layered van der Waals antiferromagnet MnBi2Te4 has been predicted to combine the band ordering of archetypical topological insulators such as Bi2Te3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of...
We use time-resolved molecular orbital mapping to explore fundamental processes of excited wave packets and charge transfer dynamics in organic films on femtosecond time scales. We investigate a bilayer pentacene film on Ag(110) by optical laser pump and FEL probe experiments. From the angle-resolved photoemission signal, we obtain time-dependent m...
The layered van der Waals compound MnBi$_2$Te$_4$ has been predicted to combine the band ordering of archetypical topological insulators like Bi$_2$Te$_3$ with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure...
Photoemission-intensity distributions I RCP/LCP (E B , k) measured for right- and left-circularly polarized soft x-rays revealed a large circular dichroism in angular distribution (CDAD) in the 4D parameter space (E B binding energy, k momentum vector). Full-field k-imaging combined with time-of-flight energy recording at a high-brilliance soft x-r...
Understanding the molecular geometry at interfaces is not only of importance for optimizing contact properties but also sets the basis in energy alignment in multilayer samples. Here we present a method to determine the tilt angle of molecular compounds by a combination of the orbital imaging technique with photoemission matrix element calculations...
Angle-resolved photoemission spectroscopy (ARPES) has been used to study the electronic structure of InSe and CuIn5Se8 single crystals and determine the general patterns of its transformation after the introduction of copper. The main dispersion curves of valence bands in the studied multicomponent systems and the effective masses of majority carri...
The quantum anomalous Hall effect (QAHE) has recently been reported to emerge in magnetically-doped topological insulators. Although its general phenomenology is well established, the microscopic origin is far from being properly understood and controlled. Here we report on a detailed and systematic investigation of transition-metal (TM)-doped Sb$_...
Samarium hexaboride (SmB$_6$), a Kondo insulator with mixed valence, has recently attracted much attention as a possible host for correlated topological surface states. Here, we use a combination of x-ray absorption and reflectometry techniques, backed up with a theoretical model for the resonant $M_{4,5}$ absorption edge of Sm and photoemission da...
Paramagnetic heavy fermion insulators consist of fully occupied quasiparticle bands inherent to Fermi liquid theory. The gap emergence below a characteristic temperature is the ultimate sign of coherence for a many-body system, which in addition can induce a non-trivial band topology. Here, we demonstrate a simple and efficient method to compare a...
A comprehensive understanding of spin-polarized photoemission is crucial for accessing the electronic structure of spin-orbit coupled materials. Yet, the impact of the final state in the photoemission process on the photoelectron spin has been difficult to assess in these systems. We present experiments for the spin-orbit split states in a Bi-Ag su...
The fabrication of hybrid systems in which specific physical properties of materials are coupled provides a setting to explore alternative functionality in condensed matter physics. Here, we realize the formation of spin-polarized electronic states, arising from a strong spin-orbit interaction, on the prototype ferroelectric BaTiO3(001) by epitaxia...
We performed a full mapping of the bulk electronic structure including the Fermi surface and Fermi-velocity distribution vF(kF) of tungsten. The 4D spectral function ρ(EB; k) in the entire bulk Brillouin zone and 6 eV binding-energy (EB) interval was acquired in ∼3 h thanks to a new multidimensional photoemission data-recording technique (combining...
We report on the growth of a monolayer-thick BiAg 2 surface alloy on thin Ag films grown on Pt(111) and Cu(111). Using low energy electron diffraction (LEED), angle resolved photoemission spectroscopy (ARPES), and scanning tunneling microscopy (STM) we show that the surface structure of the 1 3 ML Bi/x-ML Ag/Pt(111) system (x 2) is strongly affecte...
Angle-resolved photoemission spectroscopy has been developed to a very high accuracy. However, effects that depend sensitively on the state of the emitted photoelectron were so far hard to compute for real molecules. We here show that the real-time propagation approach to time-dependent density functional theory allows us to obtain final-state effe...
Ex situ analyses on topological insulator films require protection against surface contamination during air exposure. This work reports on a technique that combines deposition of protective capping just after epitaxial growth and its mechanical removal inside ultra-high vacuum systems. This method was applied to Bi2Te3 films with thickness varying...
Samarium hexaboride (SmB6), which lies in the mixed valence regime in the Anderson model, has been predicted to possess topologically protected surface states. The intensive investigations on SmB6 have brought up the long standing questions about the discrepancy between the surface and bulk electronic properties in rare-earth compounds in general....
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...
Samarium hexaboride (SmB6), which lies in the mixed valence regime in the Anderson model, has been predicted to possess topologically protected surface states. The intensive investigations on SmB6 have brought up the long standing questions about the discrepancy between the surface and bulk electronic properties in rare-earth compounds in general....
In this paper we discuss how far a single-particle picture can be applied for the description of photoelectron spectroscopy of large π-conjugated molecules on metal substrates measured near a K edge. In this context we focus on particular spectroscopic features, which only exist at the immediate interface and show a constant kinetic energy when the...
Samarium hexaboride (SmB6) has been presumed to show a topological Kondo
insulating state consisting of fully occupied quasiparticle bands in the
concept of a Fermi liquid. This gap emerging below a small coherence
temperature is the ultimate sign of coherence for a many-body system, which in
addition induces a non-trivial topology. Here, we demons...
To resolve the controversial issue of the topological nature of the
electronic structure of YbB$_{6}$, we have made a combined study using density
functional theory (DFT) and angle resolved photoemission spectroscopy (ARPES).
Accurate determination of the low energy band topology in DFT requires the use
of modified Becke-Johnson exchange potential...
We report the discovery of a novel charge transfer (CT) state at a
molecule-metal interface by the application of resonant photoelectron
spectroscopy (ResPES). This interface feature is neither present for molecular
bulk samples nor for the clean substrate. Within a simplified two-step model
this signal is assigned to a particular final state that...
We report on the spectroscopic observation of a quantized electronic fine
structure near the Fermi energy in thin Fe films grown on W(110). The quantum
well states are detected down to binding energies of $\sim$10 meV by
angle-resolved photoelectron spectroscopy. The band dispersion of these states
is found to feature a pronounced anisotropy within...
Epitaxial films of the ternary topological insulator Bi2Te2Se were grown on Si(111) substrates and investigated for their surface electronic properties and morphology. We employ a Se-capping procedure allowing for the preparation of clean films in the surface-analysis experimental setups. Using angle-resolved photoelectron spectroscopy, we determin...
Fascinating pictures that can be interpreted as showing molecular orbitals have been obtained with various imaging techniques. Among these, angle resolved photoemission spectroscopy (ARPES) has emerged as a particularly powerful method. Orbital images have been used to underline the physical credibility of the molecular orbital concept. However, fr...
Spontaneous symmetry breaking in physical systems leads to salient phenomena at all scales, from the Higgs mechanism and the emergence of the mass of the elementary particles, to superconductivity and magnetism in solids. The hidden-order state arising below 17.5 K in URu2Si2 is a puzzling example of one of such phase transitions: its associated br...
Several experimental methods allow measuring the spatial probability density of electrons in atoms, molecules and solids, that is, the absolute square of the respective single-particle wave function. But it is an intrinsic problem of the measurement process that the information about the phase is generally lost during the experiment. The symmetry o...
We report on the electronic structure of a clean, well ordered CePd 7 surface alloy that was grown in a unique (√ 5 × √ 5)R26.6 • structure, as evidenced by low-energy electron diffraction and Auger electron spectroscopy. Resonant photoemission measurements at the 4d → 4f absorption edge evidenced strong Ce 4f derived states having two components n...
We report on the electronic structure of a clean, well ordered CePd7 surface alloy that was grown in a unique (5‾‾√×5‾‾√)R26.6∘ structure, as evidenced by low-energy electron diffraction and Auger electron spectroscopy. Resonant photoemission measurements at the 4d→4f absorption edge evidenced strong Ce 4f derived states having two components near...
Environmentally driven aging effects play a crucial role in thin-film solar cells based on Cu(In,Ga)(S,Se)2, both for long-term stability and short air exposure during production. For a better understanding of such effects, Cu(In,Ga)(S,Se)2 absorber surfaces were investigated by x-ray photoelectron and Auger electron spectroscopy after exposure to...
We report on a combined low-temperature scanning tunneling spectroscopy (STS), angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) investigation of the R30°Pb/Ag(111) surface alloy which provides a giant Rashba-type spin splitting. With STS we observed spectroscopic features that are assigned to two hole-like Rash...
Ruthenium capped multilayer coatings for use in the extreme ultraviolet (EUV) radiation regime have manifold applications in science and industry. Although the Ru cap shall protect the reflecting multilayers, the surface of the heterostructures suffers from contamination issues and surface degradation. In order to get a better understanding of the...
We studied the electronic structure of the Kondo lattice system ${\mathrm{Ce}}_{2}{\mathrm{Co}}_{0.8}{\mathrm{Si}}_{3.2}$ by angle-resolved photoemission spectroscopy. The spectra obtained below the coherence temperature consist of a Kondo resonance, its spin-orbit partner, and a number of dispersing bands. The quasiparticle weight related to the K...
We present a novel gas cell designed to study the electronic structure of gases and gas/solid interfaces using soft x-ray emission and absorption spectroscopies. In this cell, the sample gas is separated from the vacuum of the analysis chamber by a thin window membrane, allowing in situ measurements under atmospheric pressure. The temperature of th...
The discovery of topologically non-trivial states in band insulators has
induced an extensive search for topological phase in strongly correlated
electron systems. In particular, samarium hexaboride (SmB$_6$) has drawn much
attention as it might represent a new class of condensed matter called
topological Kondo insulator. Kondo insulators (KI) can...
We study the Fermi surface and the band structure of Ru(0 0 0 1) and, for the first time, Ru(1 0 (1) over bar 0) by high-resolution angle-resolved photoemission spectroscopy using He I-alpha and He II alpha radiation for excitation. The experimental results, though consistent with published data, show several features that were not observed previou...
We present a comprehensive study of the adsorption behavior of iron
phthalocyanine on the low-index crystal faces of silver. By combining
measurements of the reciprocal space by means of photoelectron momentum mapping
and low energy electron diffraction, the real space adsorption geometries are
reconstructed. At monolayer coverage ordered superstru...
The adsorption of aromatic molecules on metal surfaces leads to a complex reorganization of the molecular and metal wave functions. Various processes such as charge transfer, hybridization between molecular and metallic states, and the formation of dispersing bands within the interface have been demonstrated for organometallic interface systems. Fo...
We study the structure formation of 1,4,5,8-naphthalenetetracarboxylicacid-
dianhydride (NTCDA) multilayer films on Ag(111) surfaces by energy dispersive near-edge x-ray absorption fine-structure spectroscopy (NEXAFS) and photoelectron spectroscopy. The time resolution of seconds of the method allows us to identify several sub-processes, which occu...
We study the core hole-electron correlation in coherently coupled molecules by energy dispersive near edge x-ray absorption fine-structure spectroscopy. In a transient phase, which exists during the transition between two bulk arrangements, 1,4,5,8-naphthalene-tetracarboxylicacid-dianhydride multilayer films exhibit peculiar changes of the line sha...
We summarize our development of instrumentation for the study of liquids, aqueous solutions, and liquid/solid interfaces using resonant inelastic soft X-ray scattering (RIXS) and illustrate the value of the experimental approach with a few instructional examples. Using a high-transmission, high-resolution soft X-ray spectrometer, we are able to mea...
Spatial variations in the chemical composition of the (Zn,Mg)O/CuIn(S,Se)2 thin-film solar cell interface were studied by photoemission electron microscopy (PEEM). Energy filtered PEEM images indicate significant differences in the magnesium and zinc distribution. Local photoemission measurements reveal a relative difference in the derived Mg/(Zn+M...
The electronic structure of superconducting Fe1.03Te0.94S0.06 has been studied by angle-resolved photoemission spectroscopy (ARPES). Experimental band topography is compared to the calculations using the methods of Korringa-Kohn-Rostoker (KKR) with the coherent potential approximation (CPA) and the linearized augmented plane wave with local orbital...
We study, using high-resolution angle-resolved photoemission spectroscopy,
the evolution of the electronic structure in URu2Si2 at the Gamma, Z and X
high-symmetry points from the high-temperature Kondo-screened regime to the
low-temperature `hidden-order' (HO) state. At all temperatures and symmetry
points, we find structures resulting from the in...
