Concerted Mechanism of Carrier Dynamics in Laser‐Excited Fen/(MgO)m(001) Heterostructures from Real‐Time Time‐Dependent DFT

Abstract

AbstractUsing real‐time time‐dependent density functional theory (RT‐TDDFT), the electronic response of a Fen/(MgO)m(001) (n=1,3,5 and m=3,5,7) metal/insulator heterostructure to an optical excitation is calculated, considering laser frequencies below, near, and above the bandgap of the insulator and two directions of polarization. The spatial redistribution of electronic charge after illumination shows a strong dependence on the frequency and polarization direction of the laser pulse with a similar pattern for all thicknesses. The comparison of the layer‐resolved changes in occupation of the ground‐state orbitals after optical excitation obtained for Fen/(MgO)m(001) and bulk Fe reveals the origin of excited carriers in the heterostructures: In the central and interface Fe layers carriers are excited from states in the vicinity of the Fermi‐level to the conduction band of MgO. Simultaneously, excitations take place from the valence band of MgO to Fe states above the Fermi‐level. This concerted mechanism allows for an effective bidirectional relocation of excited carriers between the metallic and insulating subsystems in heterostructures with a thickness of several nanometers, providing an effective accumulation of hot carriers in the insulating layers, even at photon energies in the vicinity and below the bandgap of bulk MgO.