@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{PeixotoBentmannRuessmannetal.2020,
  author    = {Peixoto, Thiago R. F. and Bentmann, Hendrik and R{\"u}ßmann, Philipp and Tcakaev, Abdul-Vakhab and Winnerlein, Martin and Schreyeck, Steffen and Schatz, Sonja and Vidal, Raphael Crespo and Stier, Fabian and Zabolotnyy, Volodymyr and Green, Robert J. and Min, Chul Hee and Fornari, Celso I. and Maaß, Henriette and Vasili, Hari Babu and Gargiani, Pierluigi and Valvidares, Manuel and Barla, Alessandro and Buck, Jens and Hoesch, Moritz and Diekmann, Florian and Rohlf, Sebastian and Kall{\"a}ne, Matthias and Rossnagel, Kai and Gould, Charles and Brunner, Karl and Bl{\"u}gel, Stefan and Hinkov, Vladimir and Molenkamp, Laurens W. and Friedrich, Reinert},
  title     = {Non-local effect of impurity states on the exchange coupling mechanism in magnetic topological insulators},
  series = {NPJ Quantum Materials},
  volume    = {5},
  journal   = {NPJ Quantum Materials},
  doi       = {10.1038/s41535-020-00288-0},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-230686},
  year      = {2020},
  abstract  = {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.},
  language  = {en}
}