TY  - JOUR
A1  - Min, Chul-Hee
A1  - Goth, F.
A1  - Lutz, P.
A1  - Bentmann, H.
A1  - Kang, B.Y.
A1  - Cho, B.K.
A1  - Werner, J.
A1  - Chen, K.-S.
A1  - Assaad, F.
A1  - Reinert, F.
T1  - Matching DMFT calculations with photoemission spectra of heavy fermion insulators: universal properties of the near-gap spectra of SmB\(_{6}\)
JF  - Scientific Reports
N2  - 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 model study and an experimental result for heavy fermion insulators. The temperature dependence of the gap formation in both local moment and mixed valence regimes is captured within the dynamical mean field (DMFT) approximation to the periodic Anderson model (PAM). Using the topological coherence temperature as the scaling factor and choosing the input parameter set within the mixed valence regime, we can unambiguously link the theoretical energy scales to the experimental ones. As a particularly important result, we find improved consistency between the scaled DMFT density of states and the photoemission near-gap spectra of samarium hexaboride (SmB\(_{6}\)).
KW  - SmB\(_{6}\)
KW  - heavy fermion insulators
KW  - dynamical mean field
KW  - samarium hexaboride
KW  - near-gap spectra
Y1  - 2017
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-170328
VL  - 7
IS  - 11980
ER  - 
TY  - JOUR
A1  - Wiessner, M.
A1  - Rodriguez Lastra, N. S.
A1  - Ziroff, J.
A1  - Forster, F.
A1  - Puschnig, P.
A1  - Dössel, L.
A1  - Müllen, K.
A1  - Schöll, A.
A1  - Reinert, F.
T1  - Different views on the electronic structure of nanoscale graphene: aromatic molecule versus quantum dot
JF  - New Journal of Physics
N2  - Graphene's peculiar electronic band structure makes it of interest for new electronic and spintronic approaches. However, potential applications suffer from quantization effects when the spatial extension reaches the nanoscale. We show by photoelectron spectroscopy on nanoscaled model systems (disc-shaped, planar polyacenes) that the two-dimensional band structure is transformed into discrete states which follow the momentum dependence of the graphene Bloch states. Based on a simple model of quantum wells, we show how the band structure of graphene emerges from localized states, and we compare this result with ab initio calculations which describe the orbital structure.
KW  - well
KW  - confinement
KW  - states
KW  - Ag(111)
KW  - photoemission
KW  - vicinal surfaces
KW  - coronene
KW  - energy
KW  - films
KW  - nanographenes
Y1  - 2012
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-130184
VL  - 14
IS  - 113008
ER  - 
TY  - JOUR
A1  - Müller, S.
A1  - Spriestersbach, F.
A1  - Min, C.-H.
A1  - Fornari, C. I.
A1  - Reinert, F.
T1  - Molecular beam epitaxy of TmTe thin films on SrF\(_{2}\) (111)
JF  - AIP Advances
N2  - 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 as a function of lattice parameter. We systematically investigate the growth conditions of TmxTey thin films on SrF\(_{2}\) (111) substrates by molecular beam epitaxy. The ratio between Te and Tm supply was precisely tuned, resulting in two different crystalline phases, which were confirmed by x-ray diffraction and x-ray photoemission spectroscopy. By investigating the crystalline quality as a function of the substrate temperature, the optimal growth conditions were identified for the desired Tm1Te1 phase. Additional low energy electron diffraction and reflective high energy electron diffraction measurements confirm the epitaxial growth of TmTe layers. X-ray reflectivity measurements demonstrate that homogeneous samples with sharp interfaces can be obtained for varied thicknesses. Our results provide a reliable guidance to prepare homogeneous high-quality TmTe thin films and thus serve as a basis for further electronic investigations.
KW  - thulium telluride
KW  - molecular beam epitaxy
KW  - thin films
Y1  - 2022
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-300876
VL  - 12
IS  - 2
ER  - 
TY  - JOUR
A1  - Ünzelmann, M.
A1  - Bentmann, H.
A1  - Figgemeier, T.
A1  - Eck, P.
A1  - Neu, J. N.
A1  - Geldiyev, B.
A1  - Diekmann, F.
A1  - Rohlf, S.
A1  - Buck, J.
A1  - Hoesch, M.
A1  - Kalläne, M.
A1  - Rossnagel, K.
A1  - Thomale, R.
A1  - Siegrist, T.
A1  - Sangiovanni, G.
A1  - Di Sante, D.
A1  - Reinert, F.
T1  - Momentum-space signatures of Berry flux monopoles in the Weyl semimetal TaAs
JF  - Nature Communications
N2  - 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 monopoles locate at the crossing point of spin-polarized bands forming the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of Berry flux monopoles in TaAs as a paradigmatic WSM. We carried out angle-resolved photoelectron spectroscopy at bulk-sensitive soft X-ray energies (SX-ARPES) combined with photoelectron spin detection and circular dichroism. The experiments reveal large spin- and orbital-angular-momentum (SAM and OAM) polarizations of the Weyl-fermion states, resulting from the broken crystalline inversion symmetry in TaAs. Supported by first-principles calculations, our measurements image signatures of a topologically non-trivial winding of the OAM at the Weyl nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results provide directly bulk-sensitive spectroscopic support for the non-trivial band topology in the WSM TaAs, promising to have profound implications for the study of quantum-geometric effects in solids. Weyl semimetals exhibit Berry flux monopoles in momentum-space, but direct experimental evidence has remained elusive. Here, the authors reveal topologically non-trivial winding of the orbital-angular-momentum at the Weyl nodes and a chirality-dependent spin-angular-momentum of the Weyl bands, as a direct signature of the Berry flux monopoles in TaAs.
KW  - electronic properties and materials
KW  - topological insulators
Y1  - 2021
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-260719
VL  - 12
IS  - 1
ER  - 
TY  - JOUR
A1  - Fornari, C. I.
A1  - Rappl, P. H. O.
A1  - Morelhao, S. L.
A1  - Peixoto, T. R. F.
A1  - Bentmann, H.
A1  - Reinert, F.
A1  - Abramof, E.
T1  - Preservation of pristine Bi\(_2\)Te\(_3\) thin film topological insulator surface after ex situ mechanical removal of Te capping layer
JF  - APL Materials
N2  - 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 from 8 to 170 nm. Contrarily to other methods, this technique does not require any sputtering or thermal annealing setups installed inside the analyzing system and preserves both film thickness and surface characteristics. These results suggest that the technique presented here can be expanded to other topological insulator materials.
KW  - Insulator surfaces
KW  - Atomic force microscopy
KW  - Insulating thin films
KW  - Molecular beam epitaxy
KW  - Surface states
KW  - Vacuum chambers
KW  - Thin film growth
KW  - Sputter deposition
KW  - Epitaxy
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-164468
VL  - 4
ER  - 
TY  - JOUR
A1  - Scholz, M.
A1  - Sauer, C.
A1  - Wiessner, M.
A1  - Nguyen, N.
A1  - Scholl, A.
A1  - Reinert, F.
T1  - Structure formation in organic thin films observed in real time by energy dispersive near-edge x-ray absorption fine-structure spectroscopy
JF  - New Journal of Physics
N2  - 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 occur during the post-growth three-dimensional structural ordering, as well as their characteristic time scales. After deposition at low temperature the NTCDA molecules are preferentially flat lying and the films exhibit no long-range order. Upon annealing the molecules flip into an upright orientation followed by an aggregation in a transient phase which exists for several minutes. Finally, threedimensional islands are established with bulk-crystalline structure involving substantial mass transport on the surface and morphological roughening. By applying the Kolmogorov–Johnson–Mehl–Avrami model the activation energies of the temperature-driven sub-processes can be derived from the time evolution of the NEXAFS signal.
KW  - physics
Y1  - 2013
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-129749
VL  - 15
IS  - 083052
ER  - 
TY  - JOUR
A1  - Dauth, M.
A1  - Wiessner, M.
A1  - Feyer, V.
A1  - Schöll, A.
A1  - Puschnig, P.
A1  - Reinert, F.
A1  - Kuemmel, S.
T1  - Angle resolved photoemission from organic semiconductors: orbital imaging beyond the molecular orbital interpretation
JF  - New Journal of Physics
N2  - 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, from the theory of the photoemission process it is evident that imaging experiments do not show molecular orbitals, but Dyson orbitals. The latter are not eigenstates of a single-particle Hamiltonian and thus do not fit into the usual simple interpretation of electronic structure in terms of molecular orbitals. In a combined theoretical and experimental study we thus check whether a Dyson-orbital and a molecular-orbital based interpretation of ARPES lead to differences that are relevant on the experimentally observable scale. We discuss a scheme that allows for approximately calculating Dyson orbitals with moderate computational effort. Electronic relaxation is taken into account explicitly. The comparison reveals that while molecular orbitals are frequently good approximations to Dyson orbitals, a detailed understanding of photoemission intensities may require one to go beyond the molecular orbital picture. In particular we clearly observe signatures of the Dyson-orbital character for an adsorbed semiconductor molecule in ARPES spectra when these are recorded over a larger momentum range than in earlier experiments.
KW  - Dyson orbitals
KW  - electronic structure
KW  - PTCDA
KW  - AG(110)
KW  - density-functional theory
KW  - approximation
KW  - energies
KW  - monolayers
KW  - spectroscopy
KW  - NTCDA
KW  - ARPES
KW  - orbital imaging
KW  - photoemission spectroscopy
Y1  - 2014
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-115180
SN  - 1367-2630
VL  - 16
ER  - 
TY  - JOUR
A1  - Peixoto, Thiago R. F.
A1  - Bentmann, Hendrik
A1  - Rüßmann, Philipp
A1  - Tcakaev, Abdul-Vakhab
A1  - Winnerlein, Martin
A1  - Schreyeck, Steffen
A1  - Schatz, Sonja
A1  - Vidal, Raphael Crespo
A1  - Stier, Fabian
A1  - Zabolotnyy, Volodymyr
A1  - Green, Robert J.
A1  - Min, Chul Hee
A1  - Fornari, Celso I.
A1  - Maaß, Henriette
A1  - Vasili, Hari Babu
A1  - Gargiani, Pierluigi
A1  - Valvidares, Manuel
A1  - Barla, Alessandro
A1  - Buck, Jens
A1  - Hoesch, Moritz
A1  - Diekmann, Florian
A1  - Rohlf, Sebastian
A1  - Kalläne, Matthias
A1  - Rossnagel, Kai
A1  - Gould, Charles
A1  - Brunner, Karl
A1  - Blügel, Stefan
A1  - Hinkov, Vladimir
A1  - Molenkamp, Laurens W.
A1  - Friedrich, Reinert
T1  - Non-local effect of impurity states on the exchange coupling mechanism in magnetic topological insulators
JF  - NPJ Quantum Materials
N2  - Since the discovery of the quantum anomalous Hall (QAH) effect in the magnetically doped topological insulators (MTI) Cr:(Bi,Sb)\(_2\)Te\(_3\) and V:(Bi,Sb)\(_2\)Te\(_3\), the search for the magnetic coupling mechanisms underlying the onset of ferromagnetism has been a central issue, and a variety of different scenarios have been put forward. By combining resonant photoemission, X-ray magnetic circular dichroism and density functional theory, we determine the local electronic and magnetic configurations of V and Cr impurities in (Bi,Sb)\(_2\)Te\(_3\). State-of-the-art first-principles calculations find pronounced differences in their 3d densities of states, and show how these impurity states mediate characteristic short-range pd exchange interactions, whose strength sensitively varies with the position of the 3d states relative to the Fermi level. Measurements on films with varying host stoichiometry support this trend. Our results explain, in an unified picture, the origins of the observed magnetic properties, and establish the essential role of impurity-state-mediated exchange interactions in the magnetism of MTI.
KW  - shape-truncation functions
KW  - semiconductors
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-230686
VL  - 5
ER  - 
TY  - JOUR
A1  - Sauer, C
A1  - Wießner, M
A1  - Schöll, A
A1  - Reinert, F
T1  - Observation of a molecule-metal interface charge transfer related feature by resonant photoelectron spectroscopy
JF  - New Journal of Physics
N2  - We report the discovery of a charge transfer (CT) related low binding energy feature at a molecule-metal interface by the application of resonant photoelectron spectroscopy (RPES). This interface feature is neither present for molecular bulk samples nor for the clean substrate. A detailed analysis of the spectroscopic signature of the low binding energy feature shows characteristics of electronic interaction not found in other electron spectroscopic techniques. Within a cluster model description this feature is assigned to a particular eigenstate of the photoionized system that is invisible in direct photoelectron spectroscopy but revealed in RPES through a relative resonant enhancement. Interpretations based on considering only the predominant character of the eigenstates explain the low binding energy feature by an occupied lowest unoccupied molecular orbital, which is either realized through CT in the ground or in the intermediate state. This reveals that molecule-metal CT is responsible for this feature. Consequently, our study demonstrates the sensitivity of RPES to electronic interactions and constitutes a new way to investigate CT at molecule-metal interfaces.
KW  - transfer dynamics
KW  - photoemission
KW  - states
KW  - interface
KW  - charge transfer
KW  - organic thin films
KW  - resonant photoelectron spectroscopy
KW  - energy
KW  - model calculation
KW  - NEXAFS spectroscopy
KW  - ce compounds
KW  - absorption
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-148672
VL  - 17
IS  - 043016
ER  - 
TY  - JOUR
A1  - Maaß, Henriette
A1  - Bentmann, Hendrik
A1  - Seibel, Christoph
A1  - Tusche, Christian
A1  - Eremeev, Sergey V.
A1  - Peixoto, Thiago R.F.
A1  - Tereshchenko, Oleg E.
A1  - Kokh, Konstantin A.
A1  - Chulkov, Evgueni V.
A1  - Kirschner, Jürgen
A1  - Reinert, Friedrich
T1  - Spin-texture inversion in the giant Rashba semiconductor BiTeI
JF  - Nature Communications
N2  - 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 experimentally challenging and yet essential for exploiting spin–orbit effects for spin manipulation. Here we employ a state-of-the-art photoelectron momentum microscope with a multichannel spin filter to directly image the spin texture of the layered polar semiconductor BiTeI within the full two-dimensional momentum plane. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the valence and conduction band electrons in BiTeI have spin textures of opposite chirality and of pronounced orbital dependence beyond the standard Rashba model, the latter giving rise to strong optical selection-rule effects on the photoelectron spin polarization. These observations open avenues for spin-texture manipulation by atomic-layer and charge carrier control in polar semiconductors.
KW  - applied physics
KW  - spintronics
KW  - semiconductors
Y1  - 2016
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-173769
VL  - 7
ER  -