@article{MuellerSpriestersbachMinetal.2022, author = {M{\"u}ller, S. and Spriestersbach, F. and Min, C.-H. and Fornari, C. I. and Reinert, F.}, title = {Molecular beam epitaxy of TmTe thin films on SrF\(_{2}\) (111)}, series = {AIP Advances}, volume = {12}, journal = {AIP Advances}, number = {2}, doi = {10.1063/5.0083276}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-300876}, year = {2022}, abstract = {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.}, language = {en} } @article{UenzelmannBentmannFiggemeieretal.2021, author = {{\"U}nzelmann, M. and Bentmann, H. and Figgemeier, T. and Eck, P. and Neu, J. N. and Geldiyev, B. and Diekmann, F. and Rohlf, S. and Buck, J. and Hoesch, M. and Kall{\"a}ne, M. and Rossnagel, K. and Thomale, R. and Siegrist, T. and Sangiovanni, G. and Di Sante, D. and Reinert, F.}, title = {Momentum-space signatures of Berry flux monopoles in the Weyl semimetal TaAs}, series = {Nature Communications}, volume = {12}, journal = {Nature Communications}, number = {1}, doi = {10.1038/s41467-021-23727-3}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-260719}, year = {2021}, abstract = {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.}, language = {en} } @article{FornariRapplMorelhaoetal.2016, author = {Fornari, C. I. and Rappl, P. H. O. and Morelhao, S. L. and Peixoto, T. R. F. and Bentmann, H. and Reinert, F. and Abramof, E.}, title = {Preservation of pristine Bi\(_2\)Te\(_3\) thin film topological insulator surface after ex situ mechanical removal of Te capping layer}, series = {APL Materials}, volume = {4}, journal = {APL Materials}, doi = {10.1063/1.4964610}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-164468}, pages = {106107}, year = {2016}, abstract = {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.}, language = {en} } @article{MinGothLutzetal.2017, author = {Min, Chul-Hee and Goth, F. and Lutz, P. and Bentmann, H. and Kang, B.Y. and Cho, B.K. and Werner, J. and Chen, K.-S. and Assaad, F. and Reinert, F.}, title = {Matching DMFT calculations with photoemission spectra of heavy fermion insulators: universal properties of the near-gap spectra of SmB\(_{6}\)}, series = {Scientific Reports}, volume = {7}, journal = {Scientific Reports}, number = {11980}, doi = {10.1038/s41598-017-12080-5}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-170328}, year = {2017}, abstract = {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\(_{6}\)).}, language = {en} } @article{WiessnerRodriguezLastraZiroffetal.2012, author = {Wiessner, M. and Rodriguez Lastra, N. S. and Ziroff, J. and Forster, F. and Puschnig, P. and D{\"o}ssel, L. and M{\"u}llen, K. and Sch{\"o}ll, A. and Reinert, F.}, title = {Different views on the electronic structure of nanoscale graphene: aromatic molecule versus quantum dot}, series = {New Journal of Physics}, volume = {14}, journal = {New Journal of Physics}, number = {113008}, doi = {10.1088/1367-2630/14/11/113008}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-130184}, pages = {12}, year = {2012}, abstract = {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.}, language = {en} } @article{ScholzSauerWiessneretal.2013, author = {Scholz, M. and Sauer, C. and Wiessner, M. and Nguyen, N. and Scholl, A. and Reinert, F.}, title = {Structure formation in organic thin films observed in real time by energy dispersive near-edge x-ray absorption fine-structure spectroscopy}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {083052}, doi = {10.1088/1367-2630/15/8/083052}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129749}, year = {2013}, abstract = {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.}, language = {en} } @article{DauthWiessnerFeyeretal.2014, author = {Dauth, M. and Wiessner, M. and Feyer, V. and Sch{\"o}ll, A. and Puschnig, P. and Reinert, F. and Kuemmel, S.}, title = {Angle resolved photoemission from organic semiconductors: orbital imaging beyond the molecular orbital interpretation}, series = {New Journal of Physics}, volume = {16}, journal = {New Journal of Physics}, issn = {1367-2630}, doi = {10.1088/1367-2630/16/10/103005}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-115180}, pages = {103005}, year = {2014}, abstract = {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.}, language = {en} }