Effect of lattice excitations on transient near-edge x-ray absorption spectroscopy

N. Rothenbach, M. E. Gruner, K. Ollefs, C. Schmitz-Antoniak, S. Salamon, P. Zhou, R. Li, M. Mo, S. Park, X. Shen, S. Weathersby, J. Yang, X. J. Wang, O. Šipr, H. Ebert, K. Sokolowski-Tinten, R. Pentcheva, U. Bovensiepen, A. Eschenlohr, and H. Wende

Phys. Rev. B 104, 144302 – Published 8 October 2021

Abstract

Time-dependent and constituent-specific spectral changes in soft near-edge x-ray absorption spectroscopy (XAS) of an ${[Fe / MgO]}_{8}$ metal/insulator heterostructure after laser excitation are analyzed at the O K-edge with picosecond time resolution. The oxygen absorption edge of the insulator features a uniform intensity decrease of the fine structure at elevated phononic temperatures, which can be quantified by a simple simulation and fitting procedure presented here. Combining XAS with ultrafast electron diffraction measurements and ab initio calculations demonstrates that the transient intensity changes in XAS can be assigned to a transient lattice temperature. Thus, the sensitivity of transient near-edge XAS to phonons is demonstrated.

Received 30 April 2021
Revised 28 July 2021
Accepted 20 August 2021

DOI:https://doi.org/10.1103/PhysRevB.104.144302

Physics Subject Headings (PhySH)

Lattice dynamics

Solid-solid interfaces

Density functional theory Electron diffraction Ultrafast pump-probe spectroscopy X-ray absorption spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

N. Rothenbach^1,*, M. E. Gruner ¹, K. Ollefs¹, C. Schmitz-Antoniak², S. Salamon¹, P. Zhou¹, R. Li^3,†, M. Mo ³, S. Park^3,‡, X. Shen³, S. Weathersby³, J. Yang^3,§, X. J. Wang ³, O. Šipr ⁴, H. Ebert⁵, K. Sokolowski-Tinten¹, R. Pentcheva¹, U. Bovensiepen ¹, A. Eschenlohr ¹, and H. Wende ^1,∥

¹Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstr. 1, 47057 Duisburg, Germany
²Peter-Grünberg-Institut (PGI-6), Forschungszentrum Jülich, 52425 Jülich, Germany
³SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, California 94025, USA
⁴FZU Institute of Physics of the Czech Academy of Sciences, Cukrovarnicka 10, CZ-162 53 Prague, Czech Republic
⁵Universität München, Department Chemie, Physikalische Chemie, Butenandtstr. 5-13, 81377 München, Germany

^*nico.rothenbach@uni-due.de
^†Present address: Department of Engineering Physics, Tsinghua University, Beijing 100084, China.
^‡Present address: Brookhaven National Laboratory, Center for Functional Nanomaterials, Upton, NY 11973-5000, USA.
^§Present address: Center of Basic Molecular Science, Department of Chemistry, Tsinghua University, Beijing 100084, China.
^∥heiko.wende@uni-due.de

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Vol. 104, Iss. 14 — 1 October 2021

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Images

Figure 1
Schematic presentations of the x-ray absorption process including the effect of structural thermal disorder (phonons) by analyzing: (a) The wave functions of the initial state $| i 〉$ and the final state $| f 〉$ without (solid line) and with structural thermal disorder (dashed line), (b) the density of the initial and final states, and (c) a real-space representation of the outgoing photoelectron wave from the absorbing atom (marked blue) and the backscattered photoelectron wave from the structurally disordered surrounding atoms. The atomic equilibrium positions are depicted by full circles, whereas the thermally disordered positions are shown as open circles. For illustration purposes, the displacement in the initial states $| i 〉$ is intentionally exaggerated and not to scale.
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Figure 2
Schematic of the UV femtosecond laser pump and picosecond soft x-ray absorption (XAS) or UED probe experiment and the investigated [Fe/MgO] heterostructure sample. For more details, see text.
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Figure 3
Top: X-ray absorption coefficient $μ (E)$ at the oxygen K-edge before and after the UV pump-pulse excitation (pump-probe delay time $90 ps$ , pump fluence 25 $mJ / {cm}^{2}$ ). Bottom: Pump fluence dependence of the relative pump-induced change { $[μ_{pumped} (E) - μ_{unpumped} (E)] / μ_{unpumped} (E)$ }. The inset shows a linear dependence of the size of the pump-induced change, defined as half of the peak-to-peak value, on the pump fluence.
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Figure 4
Depiction of the simulation and fitting procedure of the intensity decrease for the case of the O K-edge. Top: Measured XAS at the oxygen K-edge before and after optical excitation (pump-probe delay time $90 ps$ , pump fluence $25 mJ / {cm}^{2}$ ). Measured spectra are shown together with a linear base line (blue solid line) and an artificially generated pumped spectrum (green dashed line) by an amplitude reduction factor. Middle: Spectra of the top half after subtracting the base line. The vertical lines indicate the crossing points of the unpumped and pumped spectra. The crossing points define distinct energy windows where the pumped spectrum has either lower or higher intensity than the unpumped spectrum in this region, indicated by the arrows. Bottom: Simulation of the pump-induced signal by an amplitude reduction factor compared with the experimental results. For details of the simulation, see the main text.
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Figure 5
Time-resolved UED to determine the temperature in the Fe/MgO multilayer systems after laser excitation. Left: Time dependence of the temperature in the Fe and MgO layers for symmetric [Fe(2 nm)/MgO(2 nm) labeled as (2/2)] and asymmetric multilayers [Fe(2 nm)/MgO(5 nm) labeled as (2/5)]. The wavelength in nanometer of the pump pulse is given additionally. The labels “Fe” or “MgO” refer to the component-specific diffraction pattern of the heterostructure. Right: Correlation of the asymptotic temperature $T_{asy}$ on long time scales with the maximum temperature in the Fe layers $T_{Fe, \max}$ .
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Figure 6
Top: DFT calculations of the O K-edge XAS of bulk MgO at four different temperatures modeled with a corresponding set of thermal displacements in the framework of the alloy analogy model [28, 38] (solid lines). With increasing temperature, the calculations show an overall intensity suppression of the fine structures. There are well-defined energy regions where the XAS at elevated temperature has lower or higher energy than the spectrum at lower temperature, indicated by the arrows. Additionally, spectra generated from the $300 K$ spectrum (dotted lines) are shown (modeling according to Fig. 4). These match the corresponding finite temperature KKR-CPA spectra. Bottom: Comparison of the experimental pump-induced signal (Fig. 3, pump fluence 25 $mJ / {cm}^{2}$ ) with the relative difference of the calculated XAS at 300 and 750 K. For this comparison, relative changes after subtraction of a base line (see Fig. 4) are considered.
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