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Femtosecond X-ray induced changes of the electronic and magnetic response of solids from electron redistribution.

  • Author(s): Higley, Daniel J
  • Reid, Alex H
  • Chen, Zhao
  • Guyader, Loïc Le
  • Hellwig, Olav
  • Lutman, Alberto A
  • Liu, Tianmin
  • Shafer, Padraic
  • Chase, Tyler
  • Dakovski, Georgi L
  • Mitra, Ankush
  • Yuan, Edwin
  • Schlappa, Justine
  • Dürr, Hermann A
  • Schlotter, William F
  • Stöhr, Joachim
  • et al.
Abstract

Resonant X-ray absorption, where an X-ray photon excites a core electron into an unoccupied valence state, is an essential process in many standard X-ray spectroscopies. With increasing X-ray intensity, the X-ray absorption strength is expected to become nonlinear. Here, we report the onset of such a nonlinearity in the resonant X-ray absorption of magnetic Co/Pd multilayers near the Co L[Formula: see text] edge. The nonlinearity is directly observed through the change of the absorption spectrum, which is modified in less than 40 fs within 2 eV of its threshold. This is interpreted as a redistribution of valence electrons near the Fermi level. For our magnetic sample this also involves mixing of majority and minority spins, due to sample demagnetization. Our findings reveal that nonlinear X-ray responses of materials may already occur at relatively low intensities, where the macroscopic sample is not destroyed, providing insight into ultrafast charge and spin dynamics.

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