Electronic origin of x-ray absorption peak shifts

Letter
Open Access

Electronic origin of x-ray absorption peak shifts

S. Shallcross, C. v. Korff Schmising, P. Elliott, S. Eisebitt, J. K. Dewhurst, and S. Sharma

Phys. Rev. B 106, L060302 – Published 15 August 2022

Abstract

Encoded in the transient x-ray absorption (XAS) and magnetic circular (MCD) response functions resides a wealth of information of the microscopic processes of ultrafast demagnetization. Employing state-of-the-art first-principles dynamical simulations we show that the experimentally observed energy shift of the $L_{3}$ XAS peak in Ni, and the absence of a corresponding shift in the dichroic MCD response, can be explained in terms of laser-induced changes in band occupation. Strikingly, we predict that for the same ultrashort pump pulse applied to Co the opposite effect will occur: a substantial shift upward in energy of the MCD peaks will be accompanied by very small change in the position of XAS peaks, a fact we relate to the reduced $d$ -band filling of Co that allows a greater energetic range above the Fermi energy into which charge can be excited. We also carefully elucidate the dependence of this effect on pump pulse parameters. These findings (i) establish an electronic origin for early-time peak shifts in transient XAS and MCD spectroscopy and (ii) illustrate the rich information that may be extracted from transient response functions of the underlying dynamical system.

Received 22 May 2022
Revised 14 July 2022
Accepted 3 August 2022

DOI:https://doi.org/10.1103/PhysRevB.106.L060302

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Electronic structure Magnetism Magnetization dynamics Spin dynamics Spin-orbit coupling

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. Shallcross¹, C. v. Korff Schmising¹, P. Elliott¹, S. Eisebitt ¹, J. K. Dewhurst², and S. Sharma ^1,*

¹Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Straße 2A, 12489 Berlin, Germany
²Max-Planck-Institut fur Mikrostrukturphysik Weinberg 2, 06120 Halle, Germany

^*sharma@mbi-berlin.de

Article Text

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Issue

Vol. 106, Iss. 6 — 1 August 2022

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Images

Figure 1
XAS, MCD, and $L$ edge peak shifts for Ni and Co. Shown in panels (a) and (b) respectively are the XAS and MCD spectra before and after ( $t = 60$ fs) laser pumping for Ni, with the difference shown in green. The energy shift of the XAS and MCD $L_{3}$ peak (on meV) as a function of time is shown in panel (c), revealing a substantial shift down in energy of the XAS peak but almost no shift in the MCD $L_{3}$ peak. Panels (d)–(f): Corresponding data for Co. In dramatic contrast to Ni, for this material it is the MCD that shows a substantial shift (upward) in energy. In both cases the same 1.55 eV linearly polarized pump pulse (with polarization parallel to the spin-quantization axis) was used with fluence 6.7 $mJ / {cm}^{2}$ , full width at half maximum 24.5 fs, and intensity $7.2 \times 10^{12}$ W/ ${cm}^{2}$ ; the A field of this pump pulse is shown in gray in panels (c) and (f).
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Figure 2
Difference in the spin and $d$ -state projected DOS (in states/eV/spin) before and 60 fs after pumping for (a) Ni and (c) Co. The energy-resolved change in moment (in $μ_{B}$ ) from the ground state to 60 fs after pumping for (b) Ni and (d) Co. The pump pulse is linearly polarized with polarization direction parallel to the spin-quantization axis. In both cases the same 1.55 eV pump pulse was used with fluence 6.7 $mJ / {cm}^{2}$ , full width at half maximum 24.5 fs, and intensity $7.2 \times 10^{12}$ W/ ${cm}^{2}$ .
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Figure 3
Dynamics of $L_{3}$ peak position upon laser pumping: XAS $L_{3}$ peak for (a) Ni and (b) Co. MCD $L_{3}$ peak for (c) Ni and (d) Co. The linearly polarized pump laser pulses (with polarization parallel to the spin-quantization axis) have a central frequency of 1.55 eV, with the duration and incident fluence as shown in the legend. The Ni XAS and Co MCD $L_{3}$ peaks show a shift independent of the pulse parameters; however the amount of the shift depends strongly upon the pump pulse. In contrast, the Ni MCD and Co XAS peaks display almost no shift, a fact that holds for all pulse parameters.
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Figure 4
Normalized change in moment as a function of time in laser pumped (a) Ni and (b) Co. The central frequency of all the pulses is 1.55 eV. The duration (in fs), the incident fluence (in $mJ / {cm}^{2}$ ), and the absorbed energy per unit volume (in eV) are quoted in the legend. For all cases these pump pulses are linearly polarized with polarization parallel to the spin-quantization axis. As in experiments, we find that for a given absorbed energy, Ni demagnetizes more that Co.
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