Microscopic non-equilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure

Abstract

The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]${}_n$ as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or non-local relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L${}_3$ absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, non-thermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L${}_3$ edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with non-equilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these non-local, non-equilibrium dynamics.

arXiv comment12 pages, 10 figures

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Cited by in corpus (13)

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• Theoretical description of optical and X-ray absorption spectra of MgO including many-body effects
• The interplay of local electron correlations and ultrafast spin dynamics in fcc Ni
• Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL
• Intrinsic energy flow in laser-excited 3$d$ ferromagnets
• Photo-induced charge-transfer renormalization in NiO
• Electron Dynamics at High-Energy Densities in Nickel from Non-linear Resonant X-ray Absorption Spectra
• A Rate Model of Electron Populations for Non-linear High-Fluence X-ray Absorption Near-Edge Spectra
• Interface-related magnetic and vibrational properties in Fe/MgO heterostructures from nuclear resonant spectroscopy and first-principles calculations
• Anisotropic carrier dynamics in a laser-excited Fe${}_1$/(MgO)${}_3$(001) heterostructure from real-time time-dependent DFT
• 4$f$ electron temperature driven ultrafast electron localization
• Using the photo-induced $L_3$ resonance shift in Fe and Ni as time reference for ultrafast experiments at low flux soft X-ray sources
• Effect of lattice excitations on transient near edge X-ray absorption spectroscopy