Session L22: First-Principles Modeling of Excited-State Phenomena in Materials IV: TDDFT and Dynamics

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Quantum states of matter are tied to the interplay of interactions in the Hamiltonian and novel states can emerge in materials depending on the relative coupling strength, e.g. between electronic and lattice degrees of freedom as well as coupling to external fields. Fabricating and processing novel materials for electronic devices with nanoscale dimensions requires extremely precise techniques with control at the atomic level. In addition, characterizing and probing properties, e.g. via electronic and optical excitations, require knowledge about a material on ultrafast time scales. In this talk I will present recent quantum-mechanical first-principles predictions for electron dynamics and the subsequent ionic motion that follows after an initial excitation of the electronic system in semiconductors and metals. In particular, we showed that long-lived electronic excitations in proton irradiated MgO can facilitate diffusion of oxygen vacancies. For silicon material under swift heavy ion irradiation, we analyzed the charge state dynamics of the projectile ion and used it to explain electronic stopping behavior. Finally, for proton and laser irradiated aluminum surfaces, we quantify electron emission, projectile charge capture, and pre-equilibrium electronic stopping behavior, that is unique to thin films and two-dimensional materials. Limitations and possible extensions of the theoretical description, including the bridging of multiple time or length scales, will be included in the discussion.   

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The Bethe-Salpeter equation (BSE) is the standard computational method for optical excitations in solids, including excitonic effects. We show that time-dependent density-functional theory is a promising alternative to the BSE, using simple global hybrid functionals where the admixture of nonlocal exchange is controlled by the dielectric constant. Optical absorption spectra are presented for semiconductors and wide-gap insulators, as well as several types of inorganic perovskites. The hybrid approach produces excellent agreement with the BSE while reducing the computational cost significantly.   

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Electronic excitation of defect states has been proven to reduce the migration barrier which indicates an opportunity to enhance ion diffusion via manipulating the explicit electron distribution. Ultrashort laser pulses have the potential to achieve such manipulation with little physical damage to the material, contrary to other excitation sources, such as proton irradiation. Hence, a detailed understanding of laser-materials interaction is essential for manufacturing with nanoscale precision but still remains unclear, due to its non-linear and non-equilibrium character. Here, we apply real-time TDDFT, which can accurately describe such nonlinear effects, to reveal underlying ultrafast electron dynamics in laser-irradiated MgO. To further understand the influence of excited electrons on the oxygen diffusion, we calculate the time evolution of the occupation number, under different laser frequencies and intensities. Comparison to the distribution of hot electrons following proton irradiation can provide insights into how diffusion enhancement can be achieved by the transient localized electron dynamics and how it depends on specific laser parameters.   

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The behavior of electronic stopping power, the drag force induced on an incident ion by electrons, is rife with material-specific effects at low ion velocities. For example, semiconductors and insulators have a threshold velocity below which an ion cannot excite electrons across the band gap and electronic stopping vanishes. Also, directional bonding in these materials makes electronic stopping sensitive to the ion's trajectory even for slow ions, when core electrons are negligible. Graphene presents a highly interesting case with directional bonding but no band gap. Our real-time time-dependent density functional theory simulations of proton-irradiated graphene reveal a shoulder in the low-velocity stopping of channeling protons which does not occur for protons traversing a centroid path. From analyzing the post-impact band occupations and projectile charge state, we infer that resonant charge capture from certain valence bands by channeling protons is responsible for this feature. This prediction of a new form of anomalous low-velocity stopping has implications for ion beam imaging, where such trajectory-dependent behavior could be exploited to achieve high resolution.   

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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 excitons can be obtained from real-time TDDFT by propagating the time-dependent Kohn-Sham equation using an exchange-correlation potential with long-range electron-hole interactions. Using an implementation in the Qb@ll code, we demonstrate for various small- and large-gap materials that this approach is not only consistent with frequency-dependent linear response, but also gives access to excitonic effects in the short-pulse and nonlinear regime.   

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The behavior of electrons in solids in the presence of strong external electric fields is central to various electronic applications. However, simulations of this high-field regime are difficult due to competing timescales for collision and field-driven electron dynamics. The dependence of the carrier drift velocity on the applied electric field (so-called velocity-field curve) is particularly important, but it can only be computed with obsolete semi-empirical Monte-Carlo methods requiring many fitting parameters. Therefore, computing velocity-field curves entirely from first principles remains an open challenge. Here we show a novel ab initio approach to investigate ultrafast electron dynamics in the presence of high electric fields in the time-dependent Boltzmann transport equation (BTE) framework. This method, implemented in our Perturbo open source code, enables explicit time-stepping of the electron populations in high electric fields until a steady state is reached, from which we obtain velocity-field curves. We demonstrate calculations of the velocity-field curves in bulk semiconductors (Si and GaAs) and in a 2D material, graphene. Our analysis reveals the dominant electron-phonon scattering mechanisms as a function of applied electric field up to velocity saturation.   

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Understanding exciton decay processes in functional materials is of extreme interest due to the crucial role of time-resolved excited-state phenomena in energy conversion and storage and in photophysics applications. Nevertheless, theoretical predictions of the underlying interaction mechanisms involved are difficult to achieve. Many-body perturbation theory, within the GW approximation and the Bethe-Salpeter equation (GW-BSE), provides a reliable approach to examine structure-sensitive excitonic properties. In this work, we use recent GW-BSE development to account for excitonic bandstructures in the quasi 1D-systems of single-walled carbon nanotubes (SWCNTs). We study the relation between exciton dispersion and radiative lifetime, and explore excited-state propagation as a function of the tube structure and symmetry.   

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When a material is exposed to a perturbation such as light or an electron beam, charge is redistributed dynamically. This process is particularly important for many applications such as photovoltaics or photocatalysis, where migration of charge due to irradiation with light is a key phenomenon. In linear response, the induced charge is given by the polarizability. The macroscopic response is measured in spectroscopies such as optical absorption, but to describe the charge dynamics, all microscopic components of the polarizability are needed. In the present work, we calculate the full microscopic polarizability of silver chloride [1] including crystal local field and electron-hole interaction effects and, from this, the charge dynamics. In particular, we demonstrate the dramatic impact of bound excitons.

[1] A.Lorin, M.Gatti, L.Reining, F.Sottile, "First-principles study of excitons in the optical spectra of silver chloride" arXiv:2009.08699 [cond-mat.mtrl-sci]   

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Perylene diimides (PDI) are promising materials for optoelectronics. We study the photo-induced excited-state dynamics of a PDI derivative using the NEXMD software that goes beyond the Born-Oppenheimer description of electron-nuclear interactions. By comparing the internal conversion processes of the monomer and dimer, we determine the role of inter-molecular interactions on the time-scales associated with energy transfer and exciton localization dynamics. We predict that energy conversion in the dimer happens significantly faster than in the monomer. For the dimer, analysis of the electronic transition density during photo-dynamics reveals the transient trapping of the exciton, suggesting that the strength of thermal fluctuations exceeds electronic coupling between the two molecules. Lastly, vibrational normal mode analysis during the dynamics allows us to identify the intra- and iner-molecular modes that assist the electronic relaxation.   

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Ions shooting through solids are slowed down by electronic excitation in the so-called electronic stopping process. This century-old, far from equilibrium problem is of fundamental and applied interest in various contexts, including nuclear, space and biomedical industries. The electronic stopping problem is approached theoretically from two prevalent paradigms, one based on linear response for weak effective interactions, the other on the homogeneous electron liquid, a successful model for simple metals. A more general theory for crystalline solids has recently been introduced, which exploits the Floquet theory for modelling electronic stopping [1]. Here we utilise this general theory to present a solution for the unconventional scattering problem that arises from the electronic stopping process using a tight-binding approach.
[1] Forcellini, N. and Artacho, E.,arXiv:1908.09783 (2019)   

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Metal nanoparticles support strongly absorbing localized surface plasmon resonances, which render them attractive for plasmon-enhanced hot-carrier generation. Detailed understanding of plasmonic hot-carrier generation at the atomic scale is highly important for plasmonic catalysis as chemical reactions take place at this size scale. In this presentation, we discuss, based on time-dependent density-functional theory modeling, the femtosecond dynamics of plasmon formation and dephasing into hot carriers in metal nanoparticles. Our results suggest that the distribution of hot electrons varies significantly between different atomic sites. In particular, catalytically-relevant surface sites can exhibit enhanced hot-electron generation in comparison to the nanoparticle as a whole [1]. These results indicate possibilities for tailoring hot-carrier distributions by careful design of atomic-scale features in nanoscale systems.

[1] T. P. Rossi et al., ACS Nano 14, 9963 (2020).   

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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 Fe₁/(MgO)₃(001) multilayer after optical excitations. We compare short optical pulses with two polarization directions of the light and frequencies up to the band gap of bulk MgO. We observe substantial transient changes to the electronic structure, which persist after the duration of the pulse. The response at particular frequencies and polarizations of the electric field reflects the strong anisotropy of the carrier dynamics of the metal/insulator system. Time-dependent changes in the layer-resolved occupation numbers are visible in all layers even for small excitation frequencies, due to the presence of interface states close to the Fermi level. The time evolution suggests that the presence of such states and their orbital character are decisive for the propagation of optically induced excitations into and across the interface region. Furthermore, we also see 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 [1].

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