Session A39: First-Principles Modeling of Excited-State Phenomena in Materials I: Plasmons, Phonons, Photons, and Spins

Plasmon-induced excited-state catalysis understood via embedded correlated wavefunction theory

Metallic nanoparticles (MNPs) with nearly-free-electron-like valence electrons have enhanced ability to scatter/absorb light by means of local surface plasmon resonances (LSPRs). Such LSPRs produce amplified electric fields that can excite molecules or other materials coupled to the MNPs. Photocatalysis mediated by MNPs exploits this unique optical phenomenon. First-principles quantum mechanics can aid in understanding such light-driven chemistry but the methods used must properly account for both electronic excitations and surface reactions. Embedded correlated wavefunction (ECW) theory is ideally suited for this purpose, wherein the extended surface is described by an embedding potential derived from density functional embedding theory (DFET). The chemical reaction then is treated with CW theory subject to this DFET-derived potential. ECW calculations of a variety of endoergic reactions on pure and doped surface-plasmon-active metals reveal that enhanced kinetics can occur on excited-state reactive potential energy surfaces accessed via plasmon-enhanced light absorption or resonance energy transfer. Our calculations explain experimentally observed plasmon-driven enhanced rates and suggest candidate MNPs for photocatalytic nanoplasmonics.   

Ultrafast dynamics of plasmons and strong plasmon-molecule coupling at the nanoscale: Insights from first-principles modeling

Localized surface plasmons render metal nanoparticles efficient light absorbers at their resonance frequencies. After light absorption by the plasmon mode, the system can display different femtosecond-scale processes: The plasmon can decay into incoherent electrons and holes or a coherent energy exchange can take place between plasmon and other strongly-coupled electronic excitations. In this presentation, we employ time-dependent density-functional theory (TDDFT) for providing first-principles insights on these ultrafast processes at the nanoscale. We analyze the electron-hole transitions involved in photoabsorption and in the subsequent dynamics of the electronic system, which enables us to scrutinize the plasmonic character [1], follow the plasmon decay into hot electrons and holes [2], and dissect the symmetric and antisymmetric hybrid modes caused by strong coupling between plasmon and molecular excitation [3]. Our work paves the way for addressing spatiotemporal dynamics of plasmon-enhanced processes down to the atomic-scale details.

[1] T. P. Rossi et al., J. Chem. Theory Comput. 13, 4779 (2017).
[2] T. P. Rossi et al. (unpublished).
[3] T. P. Rossi et al., Nat. Commun. 10, 3336 (2019).   

Decoherence in First-Principles Simulation of Excited Electron Dynamics in Nanomaterials and at Interfaces

Understanding hot carrier dynamics in nanomaterials and at molecule-material interfaces is central to gain scientific insights into the operation of various future optoelectronic technologies. We investigate the excited electron dynamics at molecule-material interfaces using first-principles simulation based on the fewest switches surface hopping (FSSH) method. Within the FSSH method, identification of trivial crossings poses a major numerical challenge in practice as it can influence simulation results in a few different ways. In particular, calculation of the decoherence rate from the energy autocorrelation function is found to be rather sensitive to the trivial crossings. In addition to discussing how decoherence influences both hot electron relaxation and transfer at interfaces, we propose a numerical method to treat trivial crossings and discuss the extent to which trivial crossings affect the calculated decoherence rate.   

First-principles calculations of the ultrafast dynamics of coupled electrons and phonons

Progress on experiments probing the ultrafast electron dynamics calls for the development of accurate simulations of materials in the time domain. However, widely employed approaches to model the coupled nonequilibrium dynamics of electrons and atomic vibrations (phonons) employ empirical or approximate interactions, or are limited to femtosecond timescales. Here we show a numerical approach to evolve in time the coupled Boltzmann transport equations (BTEs) of electrons and phonons, using ab initio electron-phonon and phonon-phonon interactions together with a parallel algorithm to explicitly time step the BTEs. Our approach can simulate the dynamics up to hundreds of picoseconds (with a femtosecond time resolution) and provide microscopic insight into the scattering mechanisms. The accuracy of the interactions used in the calculations can be validated by computing transport properties. We show example calculations on graphene and semiconductors, for which we compute carrier cooling rates, mode-resolved phonon dynamics, transient absorption, structural snapshots and diffuse X-ray scattering. Possible extensions to include static electric or electromagnetic fields will also be discussed. Our
findings open new avenues for quantitative simulations of ultrafast dynamics in materials.   

Spin-phonon relaxation in diverse materials from a universal ab initio density matrix approach

We present a new, universal first-principles methodology based on Lindbladian dynamics of density matrices to calculate the spin-phonon relaxation time of solids with arbitrary spin mixing and crystal symmetry. In particular, this method describes contributions of the Elliott-Yafet and D'yakonov-Perel' (DP) mechanisms to spin relaxation, corresponding to systems with and without inversion symmetry, on an equal footing. Our \emph{ab initio} predictions are in excellent agreement with experiment data for a broad range of materials, including metals and semiconductors with inversion symmetry (silicon and iron), and materials without inversion symmetry (MoS$_2$ and MoSe$_2$). We find strong magnetic field dependence of electron and hole spin relaxation in MoS$_2$ and MoSe$_2$. As a function of temperature, we find the spin relaxation time to be proportional to carrier/momentum relaxation time in all cases, consistent with experiments but distinct from the commonly-quoted inverse relation for simplified models of the DP mechanism. We emphasize that first-principles spin-orbit coupling and electron-phonon scattering is crucial for general, accurate prediction of spin relaxation in solids.   

Predicting weak-to-strong light-matter coupling in cavities from first principles

Quantum electrodynamical density functional theory (QEDFT) is a first principles, non-perturbative framework for interactions of quantum matter with quantized electromagnetic fields. QEDFT studies have usually considered lossless cavities. Experimentally accessible cavities, however, exhibit finite photon lifetimes leading to incoherences that dominate dynamics of the light-matter excitations. Here we extend QEDFT by considering lossy cavity modes. This allows us to study ab initio correlated optical interactions in matter ranging from the weak-coupling to strong-coupling regime. As an example, we study excited-state dynamics and spectral responses of benzene and chlorobenzene under weak-to-strong light-matter coupling. By tuning the coupling we achieve cavity-mediated energy transfer between electronic excited states. We interpret the first principles results using a Fano-like model parametrized with the (QE)DFT data. This extension to QEDFT moves toward closing the loop between first principles calculations employed in electronic structure theory and parametric models of the quantum optics community.   

Local basis formulation for the Floquet theory of electronic stopping of ion projectiles

Ions shooting through solids are slowed down by electronic excitation in the so-called electronic stopping process. This century-old 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 been introduced recently based on exploiting a discrete translational invariance along a space-time diagonal, the Floquet theory of electronic stopping [1]. Here we will present how to formalise this theory using a local basis for the practical solution of the unconventional scattering problem that arises from the theory.

[1] Forcellini, N. and Artacho, E., arXiv:1908.09783 (2019).   

The effect of pressure (P) on the intrinsic optical dynamics of nitrogen-vacancy NV- colored centers in diamond: reexamining the intersystem crossing (ISC) with ΔSCF calculations within density functional theory (DFT) combined to the extended Hubbard model

The sensitivity of the NV center to its surrounding magnetic field is expected to be a powerful tool to detect the onset of superconductivity at ultrahigh P in diamond anvil cells. In this work, we propose a combined approach to study the many-body excited states of the NV^-center under P: the extended Hubbard model with all of the parameters fit on ΔSCF-DFT calculations of the differences of total energies with constrained occupations, thereby including the electron-hole interaction for any P.
We study the behavior of all of the many body states under P. In particular, we compute for the first time the pressure coefficients of the singlet-singlet emission consecutive to the ISC for the two models reported in the literature [1,2], and show that they are widely different, the application of hydrostatic pressure being an effective tool to discriminate between them.
[1] Doherty, Manson, Delaney, Jelezko, Wrachtrup, Hollenberg, Phys. Rep. 528,1 (2013).
[2] Ma, Rohlfing, Gali, Phys. Rev. B 81, 041204 (2010).   

Magnetocrystalline Anisotropy and Potential Switching in Antiferromagnetic Fe2As

Antiferromagnetic Fe₂As attracts interest due to its tetragonal chemical structure with easy-plane magnetism and metallic electrical conductivity. To understand the electrically-activated switching process of the Néel vector in this material, anisotropy energy becomes an important parameter as an energy barrier. Anisotropy energy originates from spin-orbit interaction (SOI) and magnetic dipole-dipole interaction (MDD). We use density functional theory and classical modeling to investigate SOI and MDD, and find that in-plane anisotropy is dominated by SOI, which is two times larger than the MDD contribution to out-of-plane anisotropy. The in-plane anisotropy presents four-fold symmetry and is 276.5 J/m³ which is confirmed by torque magnetometry, and total out-of-plane anistoropy energy is 829772 J/m³ with two-folded symmetry. Based on anisotropy energy and magnetic susceptibility, the lowest frequency spin wave is predicted to have a frequency of 0.64 THz and out-of-plane anisotropy dominates. This suggests that easy-plane antiferromagnetic materials might exhibit fast dynamics and high speed of switching similar to uniaxial materials. However, these materials might suffer from low thermal stability, due to the extremely small in-plane anisotropy.   

First-principles study of the phonons and crystal-field excitations in the magnetodielectric regime of Ce2O3

The mechanism associated with the giant magnetodielectric effect that was recently reported in A-type Ce2O3 remains an open question [1]. Recent Raman measurements indicate that there are vibronic excitations in its magnetodielectric regime, which in turn suggest a strong coupling between its spin, lattice and electronic degrees of freedom [2]. In this study, we apply density functional theory with LDA+U+SOC functional to disentangle the multiple coupling channels. The results show that strong coupling exists between the phonons and crystal-field excitations. This study helps to elucidate the underlying mechanism of the strong magnetodielectric effect in Ce2O3.

[1] T. Kolodiazhnyi, H. Sakurai, M. Avdeev, T. Charoonsuk, K. V. Lamonova, Y. G. Pashkevich, and B. J. Kennedy, Phys. Rev. B 98, 054423 (2018).
[2] A. Sethi, J. E. Slimak, T. Kolodiazhnyi, and S. L. Cooper, Phys. Rev. Lett. 122, 177601.   

Matter-Beam Interactions: Photochemistry with Virtual Photons?

It has recently been appreciated that the convergent electron beams used in scanning transmission electron microscopes (STEMs) can not only be employed for purposes of structural determination, but also for inducing local chemical transformations. STEMs have already been used to build heterostructures from substitutional defects in two-dimensional materials with atomic precision; a feat with transformative technological implications. However, the mechanisms underlying the chemical processes initiated by the matter-beam interaction are not yet well-understood. I will present our recent progress towards first principles methods for describing the electronic response of materials to high energy electron irradiation. I will also discuss the selection rules for electronic excitations that can occur in the materials, which depend explicitly on the incident electron's point of impact. Finally, I will give an overview of a study in which these methods are applied to reveal defect-centered excited states of silicon-doped graphene nanostructures, and the effects of beam-induced population of these exited states on the reactivity of the silicon defect.   

Discretized Diagonalization for Efficient Berry Curvature Integration: Application to Electric Polarization

An important property in characterizing the response of a material to an electric field is the induced change in electric polarization. For periodic systems, the modern theory of polarization relates this change to a change in the Berry phase, raising the question of the correct choice of branch. In this talk, I present a new method for predicting the electric-field-induced change in polarization using only the wavefunctions of only the initial and final states, based on finely subdividing the relevant phase change into gauge-invariant pieces. The underlying assumptions are automatically checked within the method and are valid for most known ferroelectrics, allowing the computation of switching polarization without the need to identify an explicit path or to perform calculations for intermediate states. The extension of this approach to the computation of other quantities expressed in terms of Berry curvature, notably topological invariants, will be discussed.   

Study of the near-edge optical properties of monoclinic HfO$_2$ from first-principles

HfO$_2$ is a wide band gap dielectric material, that has applications in optical coatings, rendering the importance of a precise description of its optical properties. The full understanding, however, has been limited by the lack of a consistent explanation of a shoulder-like feature near the absorption onset, reported in multiple experimental studies. In this work, by solving the Bethe-Salpeter equation (BSE) for the optical polarization function, we compute the optical spectra of monoclinic HfO$_2$ from first-principles. From these results we show that the shoulder-like effect is intrinsic to the crystal and is related to excitonic effects. Further, we show that since HfO$_2$ is a polar material, lattice screening, especially beyond the current static screening approximation, can potentially be important in describing dielectric screening of the electron-hole Coulomb interaction. In this work, we explored the effect of \emph{dynamical} lattice screening in HfO$_2$, and aim at describing the influence of lattice screening on the near-edge optical spectra.