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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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