@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{WagnerCrippaAmariccietal.2023,
  author    = {Wagner, N. and Crippa, L. and Amaricci, A. and Hansmann, P. and Klett, M. and K{\"o}nig, E. J. and Sch{\"a}fer, T. and Di Sante, D. and Cano, J. and Millis, A. J. and Georges, A. and Sangiovanni, G.},
  title     = {Mott insulators with boundary zeros},
  series = {Nature Communications},
  volume    = {14},
  journal   = {Nature Communications},
  doi       = {10.1038/s41467-023-42773-7},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-358150},
  year      = {2023},
  abstract  = {The topological classification of electronic band structures is based on symmetry properties of Bloch eigenstates of single-particle Hamiltonians. In parallel, topological field theory has opened the doors to the formulation and characterization of non-trivial phases of matter driven by strong electron-electron interaction. Even though important examples of topological Mott insulators have been constructed, the relevance of the underlying non-interacting band topology to the physics of the Mott phase has remained unexplored. Here, we show that the momentum structure of the Green's function zeros defining the "Luttinger surface" provides a topological characterization of the Mott phase related, in the simplest description, to the one of the single-particle electronic dispersion. Considerations on the zeros lead to the prediction of new phenomena: a topological Mott insulator with an inverted gap for the bulk zeros must possess gapless zeros at the boundary, which behave as a form of "topological antimatter" annihilating conventional edge states. Placing band and Mott topological insulators in contact produces distinctive observable signatures at the interface, revealing the otherwise spectroscopically elusive Green's function zeros.},
  language  = {en}
}
@article{HausoelKarolakŞaşιoğluetal.2017,
  author    = {Hausoel, A. and Karolak, M. and Şa{\c{s}}ιoğlu, E. and Lichtenstein, A. and Held, K. and Katanin, A. and Toschi, A. and Sangiovanni, G.},
  title     = {Local magnetic moments in iron and nickel at ambient and Earth's core conditions},
  series = {Nature Communications},
  volume    = {8},
  journal   = {Nature Communications},
  number    = {16062},
  doi       = {10.1038/ncomms16062},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-170681},
  year      = {2017},
  abstract  = {Some Bravais lattices have a particular geometry that can slow down the motion of Bloch electrons by pre-localization due to the band-structure properties. Another known source of electronic localization in solids is the Coulomb repulsion in partially filled d or f orbitals, which leads to the formation of local magnetic moments. The combination of these two effects is usually considered of little relevance to strongly correlated materials. Here we show that it represents, instead, the underlying physical mechanism in two of the most important ferromagnets: nickel and iron. In nickel, the van Hove singularity has an unexpected impact on the magnetism. As a result, the electron-electron scattering rate is linear in temperature, in violation of the conventional Landau theory of metals. This is true even at Earth's core pressures, at which iron is instead a good Fermi liquid. The importance of nickel in models of geomagnetism may have therefore to be reconsidered.},
  language  = {en}
}
@article{HansmannParraghToschietal.2014,
  author    = {Hansmann, P. and Parragh, N. and Toschi, A. and Sangiovanni, G. and Held, K.},
  title     = {Importance of d-p Coulomb interaction for high T-C cuprates and other oxides},
  series = {New Journal of Physics},
  volume    = {16},
  journal   = {New Journal of Physics},
  number    = {33009},
  issn      = {1367-2630},
  doi       = {10.1088/1367-2630/16/3/033009},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-117165},
  year      = {2014},
  abstract  = {Current theoretical studies of electronic correlations in transition metal oxides typically only account for the local repulsion between d-electrons even if oxygen ligand p-states are an explicit part of the effective Hamiltonian. Interatomic interactions such as U-pd between d- and (ligand) p-electrons, as well as the local interaction between p-electrons, are neglected. Often, the relative d-p orbital splitting has to be adjusted 'ad hoc' on the basis of the experimental evidence. By applying the merger of local density approximation and dynamical mean field theory to the prototypical case of the three-band Emery dp model for the cuprates, we demonstrate that, without any 'ad hoc' adjustment of the orbital splitting, the charge transfer insulating state is stabilized by the interatomic interaction U-pd. Our study hence shows how to improve realistic material calculations that explicitly include the p-orbitals.},
  language  = {en}
}
@article{BistiRogalevKarolaketal.2017,
  author    = {Bisti, F. and Rogalev, V. A. and Karolak, M. and Paul, S. and Gupta, A. and Schmitt, T. and G{\"u}ntherodt, G. and Eyert, V. and Sangiovanni, G. and Profeta, G. and Strocov, V. N.},
  title     = {Weakly-correlated nature of ferromagnetism in nonsymmorphic CrO\(_2\) revealed by bulk-sensitive soft-X-ray ARPES},
  series = {Physical Review X},
  volume    = {7},
  journal   = {Physical Review X},
  number    = {4},
  doi       = {10.1103/PhysRevX.7.041067},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172251},
  year      = {2017},
  abstract  = {Chromium dioxide CrO\(_2\) belongs to a class of materials called ferromagnetic half-metals, whose peculiar aspect is that they act as a metal in one spin orientation and as a semiconductor or insulator in the opposite one. Despite numerous experimental and theoretical studies motivated by technologically important applications of this material in spintronics, its fundamental properties such as momentumresolved electron dispersions and the Fermi surface have so far remained experimentally inaccessible because of metastability of its surface, which instantly reduces to amorphous Cr\(_2\)O\(_3\). In this work, we demonstrate that direct access to the native electronic structure of CrO\(_2\) can be achieved with soft-x-ray angle-resolved photoemission spectroscopy whose large probing depth penetrates through the Cr\(_2\)O\(_3\) layer. For the first time, the electronic dispersions and Fermi surface of CrO\(_2\) are measured, which are fundamental prerequisites to solve the long debate on the nature of electronic correlations in this material. Since density functional theory augmented by a relatively weak local Coulomb repulsion gives an exhaustive description of our spectroscopic data, we rule out strong-coupling theories of CrO\(_2\). Crucial for the correct interpretation of our experimental data in terms of the valence-band dispersions is the understanding of a nontrivial spectral response of CrO\(_2\) caused by interference effects in the photoemission process originating from the nonsymmorphic space group of the rutile crystal structure of CrO\(_2\).},
  language  = {en}
}