Browsing by Person "Backes, Steffen"

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  • Research Data
    Dispersion kinks from electronic correlations in an unconventional iron-based superconductor
    2024-11-17
    Chang, Ming-Hua
    Backes, Steffen
    Lu, Donghui
    Gauthier, Nicolas
    Hashimoto, Makoto
    Chen, Guan-Yu
    Wen, Hai-Hu
    Mo, Sung-Kwan
    Valentí, Roser
    Pfau, Heike
    The attractive interaction in conventional BCS superconductors is provided by a bosonic mode. However, the pairing glue of most unconventional superconductors is unknown. The effect of electron-boson coupling is therefore extensively studied in these materials. A key signature is dispersion kinks that can be observed in the spectral function as abrupt changes in velocity and lifetime of quasiparticles. Here, we show the existence of two kinks in the unconventional iron-based superconductor RbFe2As2 using angle-resolved photoemission spectroscopy (ARPES) and dynamical mean field theory (DMFT). In addition, we observe the formation of a Hubbard band multiplet due to the combination of Coulomb interaction and Hund’s rule coupling in this multiorbital system. We demonstrate that the two dispersion kinks are a consequence of these strong many-body interactions. This interpretation is in line with a growing number of theoretical predictions for kinks in various general models of correlated materials. Our results provide a unifying link between iron-based superconductors and different classes of correlated, unconventional superconductors such as cuprates and heavy-fermion materials.
      5
  • Research Data
    Observation of Two Cascading Screening Processes in an Iron-based Superconductor
    2025-07-22
    Chang, Ming-Hua
    Backes, Steffen
    Lu, Donghui
    Gauthier, Nicolas
    Hashimoto, Makoto
    Chen, Guan-Yu
    Wen, Hai-Hu
    Mo, Sung-Kwan
    Shen, Zhi-Xun
    Valentí, Roser
    Pfau, Heike
    Understanding how renormalized quasiparticles emerge in strongly correlated electron materials provides a challenge for both experiment and theory. It has been predicted that distinctive spin and orbital screening mechanisms drive this process in multiorbital materials with strong Coulomb and Hund’s interactions. Here, we provide the experimental evidence of both mechanisms from angle-resolved photoemission spectroscopy on RbFe2As2. We observe that the emergence of low-energy Fe 3dxy quasiparticles below 90K coincides with spin screening. A second process changes the spectral weight at high energies up to room temperature. Supported by theoretical calculations we attribute it to orbital screening of Fe 3d atomic excitations. These two cascading screening processes drive the temperature evolution from a bad metal to a correlated Fermi liquid.
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