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

TL;DR

This study uses real-time TDDFT to explore how femtosecond laser pulses induce anisotropic electronic excitations in Fe/MgO heterostructures, revealing polarization-dependent responses and interface-mediated charge transfer mechanisms.

Contribution

It provides a detailed analysis of anisotropic carrier dynamics and identifies the role of interface transitions in laser-excited Fe/MgO heterostructures using real-time TDDFT.

Findings

In-plane polarized light excites the Fe layer at low frequencies.

Out-of-plane polarized light strongly excites the MgO layer at high frequencies.

Interface transitions facilitate charge transfer across the heterostructure.

Abstract

The interaction of a femtosecond optical pulse with a Fe$_{1}$/(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.