Dynamics of optical excitations in a Fe/MgO(001) heterostructure from time-dependent density functional theory

Gruner, Markus E.

doi:10.1103/physrevb.99.195104

Cited by 11 publications

(29 citation statements)

References 78 publications

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“…The O K edge absorption change represents transfer of excitations from Fe to MgO. The fact that the maximum change in the oxide is reached at later delays compared to the Fe L 3 edge indicates that this effect is not primarily related to the transfer of hot electrons to MgO which should occur on a significantly shorter time scale, as investigated in [48]. This conclusion agrees with our observation in time-resolved UED that the lattice dynamics in the heterostructure is independent of the pump photon energy (see Fig.…”

Section: Discussionmentioning

confidence: 83%

Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure

et al. 2019

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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.

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Section: Discussionmentioning

confidence: 83%

Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure

et al. 2019

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“…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 .…”

Section: Introductionmentioning

confidence: 99%

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

Eggert

Gruner

Ollefs

et al. 2020

Phys. Rev. Materials

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We combine 57 Fe MÃűssbauer spectroscopy and 57 Fe nuclear resonant inelastic x-ray scattering (NRIXS) in nanoscale polycrystalline [bcc-57 Fe/MgO] multilayers with various Fe layer thicknesses and layer-resolved density-functional-theory (DFT) based first-principles calculations of a (001)-oriented [Fe(8 ML)/MgO(8 ML)](001) heterostructure to unravel the interface-related atomic vibrational properties of a multilayer system. In theory and experiment, we observe consistently enhanced hyperfine magnetic fields compared to bulk which is associated with the Fe/MgO interface layers. NRIXS and DFT both reveal a strong reduction of the longitudinal acoustic (LA) phonon peak in combination with an enhancement of the low-energy vibrational density of states (VDOS) suggesting that the presence of interfaces and the associated increase in the layer-resolved magnetic moments results in drastic changes in the Fe-partial VDOS. From the experimental and calculated VDOS, vibrational thermodynamic properties have been determined as a function of Fe thickness and were found to be in excellent agreement. arXiv:1911.05666v2 [cond-mat.mtrl-sci] 30 Jan 2020

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“…The symmetry breaking at the interface leads to splitting of states with in-plane and out-of-plane orbital character. As pointed out earlier, changing the polarization of the incident light wave may result in a substantially different response [42], which could be used to select particular excitations at the interface and differentiate between the pathways of energy propagation into the bulk materials.…”

Section: Introductionmentioning

confidence: 99%

“…The Fe/MgO system was extensively investigated in the context of TMR (tunnel mag- netoresistance) and benefits from the fact that electronic, magnetic and transport properties are well established [40,41,[43][44][45][46][47][48][49][50]. Recently, also optical and lattice excitations have been subject to theoretical and experimental studies [36,42,51,52]. Bulk Fe is a ferromagnetic metal with bcc structure and a magnetic moment of 2.22 µ B /atom [53], showing a substantial density of occupied and unoccupied d-states in the vicinity of the Fermi level.…”

Section: Introductionmentioning

confidence: 99%

Anisotropic carrier dynamics in a laser-excited Fe$_{1}$/(MgO)$_{3}$(001) heterostructure from real-time time-dependent DFT

Shomali,

Gruner,

Pentcheva

2022

Preprint

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The interaction of a femtosecond optical pulse with a Fe1/(MgO)3(001) metal/oxide heterostructure is investigated using time-dependent density functional theory (TDDFT) calculations in the real-time domain. We systematically study electronic excitations as a function of laser frequency, peak power density and polarization direction. While spin-orbit coupling is found to result in only a small time-dependent reduction of magnetization (less than 10 %), we find a marked anisotropy in the response to in-plane and out-of-plane polarized light, which changes its character qualitatively depending on the excitation energy: the Fe-layer is efficiently addressed at low frequencies by in-plane polarized light, whereas for frequencies higher than the MgO band gap, we find a particularly strong response of the central MgO-layer for cross-plane polarized light. For laser excitations between the charge transfer gap and the MgO band gap, the interface plays the most important role, as it mediates concerted transitions from the valence band of MgO into the 3d states of Fe closely above the Fermi level and from the Fe-states below the Fermi level into the conduction band of MgO. As these transitions can occur simultaneously altering charge balance of the layers, they could potentially lead to an efficient transfer of excited carriers into the MgO bulk, where the corresponding electron and hole states can be separated by an energy which is significantly larger than the photon energy.

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Dynamics of optical excitations in a Fe/MgO(001) heterostructure from time-dependent density functional theory

Cited by 11 publications

References 78 publications

Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure

Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure

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

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