Microscopic nonequilibrium 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 L3 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 L3 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.

Electron thermalization and relaxation in laser-heated nickel by few-femtosecond core-level transient absorption spectroscopy

“…It does not imply that core-level absorption spectroscopy is insensitive to phonon dynamics overall and depending on the system measured, lattice dynamics can contribute to core-level TA signals (e.g., Ref. [85]). related to the electron temperature change by a power law (ω s ∝ T α ), it is expected that the decay dynamics of spectral shift follow the relation ω s (t ) ∝ T (t ) α ∝ e −αt/τ σ , with α = τ σ /τ s .…”

“…The spectral shift in the core-level absorption spectra can be interpreted as an overall change in the energy of the coreexcited state in laser-heated nickel. This can be caused by electron-phonon interactions [40], and in particular, lattice displacement and heating due to optical excitations [52,85]. However, as the experimental results suggest that the direct influence of phonon excitations on the TA profile is negligible, we restrict the discussion within the electronic domain.…”

Interface-related magnetic and vibrational properties in Fe/MgO heterostructures from nuclear resonant spectroscopy and first-principles calculations

“…We expect the hybridization of Fe and Mg modes to play an important role for the energy transfer (thermalization) between the Fe and MgO the subsystem, when the Fe slab is selectively heated up. This would, for instance, occur as a consequence of an optical excitation with an energy below the band gap of bulk MgO 47 . In contrast, the sharp peak in the interface Mg-VDOS at 28.2 meV can be traced back to a rather extended, particularly flat mode around M , which does not exhibit significant Fe-character at the zone boundary.…”

“…45 and 47, the modifications of g(E) in nanoscale multilayers are anticipated to influence the phonon transport and the vibrational thermodynamic properties in these systems. Therefore, 57 Fe NRIXS measurements and DFT-based computations of the phononic properties of [Fe/MgO] multilayers are highly desirable, in particular in view of the fact that Fe/MgO is of interest as a prototype system for the study of time-resolved phenomena on the femtosecond scale 47,48 .…”

Using the photoinduced L3 resonance shift in Fe and Ni as time reference for ultrafast experiments at low flux soft x-ray sources

“…Resonant X-ray absorption in 3d-transition metals can change immediately upon photo-excitation by an ultrashort laser pulse -even on a sub-fs scale -as recently shown in Ti [34] and Ni [35] in the EUV spectral range. In the soft X-ray spectral range, transient changes especially of the L 3 absorption resonance have been observed and studied [32,[36][37][38]. As shown in the inset of Fig.…”