Anisotropic carrier dynamics in a laser-excited Fe/(MgO)(001) heterostructure from real-time time-dependent DFT

The interaction of a femtosecond optical pulse with a metal/oxide interface is addressed in the framework of time-dependent density functional theory (TDDFT) in the real-time domain using the Elk code. In particular, we systematically investigate the layer-resolved dynamics of the electronic excitations in a Fe₁/(MgO)₃(001) heterostructure as a function of laser frequency, peak power density and polarization direction [1,2]. We find a marked anisotropy in the response to in- 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 large response of the MgO-layers for cross-plane polarized light. This points towards a path to selectively manipulate the excitation dynamics pattern in anisotropic systems. Moreover, the hybridized states at the interface play an essential role, as they mediate concernted transitions from the valence band of MgO into the 3d states of Fe closely above the Fermi-level and transitions from the Fe-states below the Fermi level into the conduction band of MgO. As these transitions occur simultaneously without altering the charge balance of the layers, they could potentially lead to an efficient transfer of excited carriers into the MgO bulk, with the corresponding electron and hole states separated by a significantly larger energy than the photon energy.

[1] M. E. Gruner and R. Pentcheva, Phys. Rev. B 99, 195104 (2019)

[2] E. Shomali, M. E. Gruner and R. Pentcheva, Phys. Rev. B 105, 245103 (2022)