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  - 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  - 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  -