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Register a new userDynamics of optical excitations in a Fe/MgO(001) heterostructure from time-dependent density functional theory
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In the framework of real-time time-dependent density functional theory (RT-TDDFT) we unravel the layer-resolved dynamics of the electronic structure of a (Fe)$_1$/(MgO)$_3$(001) multilayer system after an optical excitation with a frequency below the band gap of bulk MgO. Substantial transient changes to the electronic structure, which persist after the duration of the pulse, are mainly observed for in-plane polarized electric fields, corresponding to a laser pulse arriving perpendicular to the interface. While the strongest charge redistribution takes place in the Fe layer, a time-dependent change in the occupation numbers is visible in all layers, mediated by the presence of interface states. The time evolution of the layer-resolved time-dependent occupation numbers indicates a strong orbital dependence with the depletion from in-plane orbitals (e. g., $d_{x^2-y^2}$ of Fe) and accumulation in out-of-plane orbitals ($d_{3z^2-r^2}$ of Fe and $p_z$ of apical oxygen). We also observe a small net charge transfer away from oxygen towards the Mg sites even for MgO layers which are not directly in contact with the metallic Fe.
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The optical spectra of two-dimensional (2D) periodic systems provide a challenge for time-dependent density-functional theory (TDDFT) because of the large excitonic effects in these materials. In this work we explore how accurately these spectra can be described within a pure Kohn-Sham time-dependent density-functional framework, i.e., a framework in which no theory beyond Kohn-Sham density-functional theory, such as $GW$, is required to correct the Kohn-Sham gap. To achieve this goal we adapted a recent approach we developed for the optical spectra of 3D systems [Cavo, Berger, Romaniello, Phys. Rev. B 101, 115109 (2020)] to those of 2D systems. Our approach relies on the link between the exchange-correlation kernel of TDDFT and the derivative discontinuity of ground-state density-functional theory, which guarantees a correct quasi-particle gap, and on a generalization of the polarization functional [Berger, Phys. Rev. Lett., 115, 137402 (2015)], which describes the excitonic effects. We applied our approach to two prototypical 2D monolayers, $h$-BN and MoS$_2$. We find that our protocol gives a qualitative good description of the optical spectrum of $h$-BN, whereas improvements are needed for MoS$_2$ to describe the intensity of the excitonic peaks.
Linear-response time-dependent density-functional theory (TDDFT) can describe excitonic features in the optical spectra of insulators and semiconductors, using exchange-correlation (xc) kernels behaving as $-1/k^{2}$ to leading order. We show how excitons can be modeled in real-time TDDFT, using an xc vector potential constructed from approximate, long-range corrected xc kernels. We demonstrate for various materials that this real-time approach is consistent with frequency-dependent linear response, gives access to femtosecond exciton dynamics following short-pulse excitations, and can be extended with some caution into the nonlinear regime.
We present accurate optical spectra of semiconductors and insulators within a pure Kohn-Sham time-dependent density-functional approach. In particular, we show that the onset of the absorption is well reproduced when comparing to experiment. No empirical information nor a theory beyond Kohn-Sham density-functional theory, such as $GW$, is invoked to correct the Kohn-Sham gap. Our approach relies on the link between the exchange-correlation kernel of time-dependent density functional theory and the derivative discontinuity of ground-state density-functional theory. We show explicitly how to relate these two quantities. We illustrate the accuracy and simplicity of our approach by applying it to various semiconductors (Si, GaP, GaAs) and wide-gap insulators (C, LiF, Ar).
The origin of large perpendicular magneto-crystalline anisotropy (PMCA) in Fe/MgO (001) is revealed by comparing Fe layers with and without the MgO. Although Fe-O $p$-$d$ hybridization is weakly present, it cannot be the main origin of the large PMCA as claimed in previous study. Instead, perfect epitaxy of Fe on the MgO is more important to achieve such large PMCA. As an evidence, we show that the surface layer in a clean free-standing Fe (001) dominantly contributes to $E_{MCA}$, while in the Fe/MgO, those by the surface and the interface Fe layers contribute almost equally. The presence of MgO does not change positive contribution from $langle xz|ell_Z|yzrangle$, whereas it reduces negative contribution from $langle z^2|ell_X|yzrangle$ and $langle xy|ell_X|xz,yzrangle$.
Time-dependent density-functional theory (TDDFT) is a computationally efficient first-principles approach for calculating optical spectra in insulators and semiconductors, including excitonic effects. We show how exciton wave functions can be obtained from TDDFT via the Kohn-Sham transition density matrix, both in the frequency-dependent linear-response regime and in real-time propagation. The method is illustrated using one-dimensional model solids. In particular, we show that our approach provides insight into the formation and dissociation of excitons in real time. This opens the door to time-resolved studies of exciton dynamics in materials by means of real-time TDDFT.
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