Session F59: First Principles Modeling of Excited-State Phenomena in Materials: Electron-Phonon

Ultrafast phonon dynamics in two-dimensional and layered materials.

Recent advancements in pump-probe spectroscopy and scattering techniques have unlocked unprecedented capabilities to directly probe phonons, electron-phonon interactions, and their non-equilibrium dynamics. Achieving a theoretical description of these phenomena from first principles is of paramount importance for understanding and harnessing the emergent behaviour of light-driven solids.

In this talk, I will discuss the recent progress and open challenges in the theoretical description of the ultrafast phonon dynamics based on ab-initio electronic-structure approaches. Particular emphasis will be placed on the problems of (i) lattice thermalization in photoexcited semiconductors, and (ii) coherent lattice dynamics in semimetals.  In both cases, the systematic and seamless modeling of electron-phonon and phonon-phonon interactions stands as an essential prerequisite for the attainment of realistic insights into pump-probe experiments.  I will exemplify these concepts within the context of 2D and layered materials, where ultrafast diffuse scattering measurements have recently enabled to directly probe of the non-equilibrium phonon dynamics.   

Density Matrix Dynamics for Electron-Phonon Interactions

The time evolution of the electron density matrix in the presence of electron-phonon (e-ph) interactions gives access to nonequilibrium electronic populations and coherences. Here, we derive the dynamics of the density matrix of a coupled electron-phonon system using both the Heisenberg and Lindblad equations. Rather than using the Lindblad equation with the Redfield procedure [1], we focus on solving the equations of motion for the density matrix in the Heisenberg picture, which allows us to explicitly compute the damping factor that appears in the Lindblad equation, as well as derive higher-order e-ph terms. We discuss the implementation of a simple multi-band model as a prototype toward the full first-principles implementation in the PERTURBO code developed by our group.

[1] R. Rosati, F. Dolcini, and F. Rossi, Electron-phonon coupling in metallic carbon nanotubes: Dispersionless electron propagation despite dissipation. Phys. Rev. B 92 (2015)   

Nanoplasmonic hot carriers: from excitation to catalysis

Localized surface plasmons in metallic nanoparticles give rise to very strong light absorption. The decay of these excitations results in the generation of energetic or “hot” electrons and holes which can be harvested and harnessed for applications in photovoltaics, photocatalysis and light sensing. To optimize hot carrier production in devices, a detailed theoretical understanding of the relevant microscopic processes, including light-matter interactions, plasmon decay and hot electron thermalization, is needed. In my talk, I will describe a material-specific theory of hot-carrier generation and relaxation in metallic nanoparticles which combines a classical description of the electromagnetic radiation with large-scale atomistic quantum-mechanical simulations. I will present results for hot carrier distributions in spherical nanoparticles of gold, silver and copper and discuss the relative importance of interband and intraband transitions as function of nanoparticle size. Next, I will describe how CO2 reduction performance of gold nanoparticles can be enhanced by changing the nanoparticle shape. Finally, I will present results for bimetallic Au-Pd photocatalysts and demonstrate a large enhancement in hydrogen production can be achieved in antenna-reactor architectures.   

Auger-Meitner Recombination in Semiconductors from First Principles

Auger-Meitner recombination (AMR) is an intrinsic, non-radiative recombination process that affects device performance in applications such as light emitting diodes and solar cells. Unfortunately, studying the contributions from direct and phonon-assisted mechanisms is challenging experimentally. To this end we have developed a computational methodology and software to calculate the AMR coefficients from first principles, enabling the focused study and analysis of both direct and phonon-assisted AMR in bulk semiconductor materials. We apply different parallelization schemes for the various AMR mechanisms, enabling efficient AMR calculations for any bulk semiconductor. We have demonstrated the utility of our methodology in recent studies of silicon,[1,2] indium phosphide, and AlGaN, providing previously inaccessible insights into the AMR mechanisms in these important semiconducting materials.

 

[1] Phys. Rev. Lett. 131, 076902 (2023)

[2] arXiv:2309:09927 (2023)   

Auger-Meitner recombination in InP from first principles

The direct-bandgap semiconductor indium phosphide (InP) is a popular material for optoelectronics. InP has been used in photodiodes, solar cells, transistors, and, most recently, quantum dot LEDs. The efficiency of InP as a light emitter is limited by non-radiative recombination processes such as Auger-Meitner recombination, in which the energy from an electron-hole recombination is transferred to another free carrier rather than being emitted as light. We apply first-principles methods to calculate the Auger-Meitner recombination rate in bulk InP, examining both the direct and the indirect, or phonon-assisted, recombination processes. We find that the hhe process, where the energy is transmitted to a hole, is stronger than the eeh process, where the energy is transmitted to an electron, in both the direct and phonon-assisted cases. We find the direct process is stronger than the phonon-assisted process for hhe AMR. However, we do find that the phonon-assisted mechanism plays a significant role in eeh AMR. This work provides an atomistic understanding of an important non-radiative loss mechanism in the common light-emitting material InP.   

Title:Oral: Polarons in the thermoelectric compound SnSe

A charge carrier propagating through ionic crystals induces lattice distortions via electron-phonon interaction and forms a localized quasiparticle, which is dubbed polaron. Recent ultrafast electron diffraction (UES) experiment [PNAS 119.3 (2022): e2113967119] observed both large and small polarons in the thermoelectric material SnSe, and identified the timescales for the formation of these quasiparticles. In this work, we investigate polarons in SnSe using a first-principles approach, based on a recently-developed methodology [PRB 99, 235139 (2019)]. We elucidate the driving force for the formation of polarons in this compound, and make contact with experimental data.   

Polarons in the emerging ultra-wide-band-gap semiconductor rutile GeO2

Polarons are quasiparticles that form when carriers spontaneously self-trap by deforming the lattice through the electron-phonon interaction. The formation of polarons can adversely affect the carrier-transport properites of semiconductors. Compared to their narrower-gap counterparts, ultra-wide-band-gap semiconductors typically exhibit heavier effective masses and reduced electronic screening, factors that can increase the propensity for self-trapping. Here, we investigate the polaronic properites of rutile GeO₂, a semiconductor with an ultra-wide band gap of 4.5 eV that has recently been synthesized n-type and predicted to be p-type dopable. We comment on the impact of polaron formation on the transport properites of rutile GeO₂, and examine its suitability for power-electronics applications.   

First-principles many-body calculations of polarons in materials

A polaron is a particle together with the induced polarization of the surrounding lattice. The description of a polaron in first principles started with Sio et. al. [Phys. Rev. B 99, 235139 (2019); Phys. Rev. Lett. 122, 246403 (2019)], and recently Lafuente-Bartolome et. al. developed the theory of first-principles many-body calculations of polarons at all couplings and applied it to LiF [Phys. Rev. B 106, 075119 (2022); Phys. Rev. Lett. 129, 076402 (2022)]. Based on these we can systematically calculate polaronic corrections to phonon-induced band energy renormalization beyond perturbation theory since we can apply the method to various different groups of materials with different strengths of electron-phonon interactions, calculating Fan-Migdal, Debye-Waller, polaron contributions to polaron formation energies. We also identify localized polaron wavefunctions and project them into Bloch wavefunction and normal modes.   

Ab initio Bethe-Salpeter equation calculations of light absorption and emission in the metal-organic framework Zn-MFU-4l

Zn-MFU-4l is a metal-organic framework whose light emission can be tuned via anion substitution and guest molecule intercalation. While photoluminescence experiments report light emission that is consistent with self-trapped excitons, the nature and character of the excitons is still far from clear. Here, we compute electronic structure, phonon spectrum, electron-phonon coupling, and optical properties with density functional theory and the ab initio GW-Bethe-Salpeter equation approach for a range of anions. From our calculation of the excitons and phonon spectrum, we assess what part of the framework is involved with exciton self-trapping, and evaluate which modifications to the metal-organic framework structure can change the emission properties of the self-trapped excitons.   

Static and time-dependent optical properties of CuI

We aim to use first-principles electronic-structure theory to explain the static and time-dependent optical properties of the wide-band gap semiconductor CuI, which is a promising candidate for a transparent conducting material. In particular, we aim to clarify the importance of the spin-orbit interaction for the appearance of an above-gap spectral feature that is reported in multiple experiments. Measurements of the linear optical properties that have been reported in the literature agree in attributing that peak in the spectrum to a spin-orbit split-off valence-band state. However, the significant oscillator strength of the peak might raise doubts about its origin due to the spin-orbit interaction. We use a combination of density-functional and many-body perturbation theory to simulate the electronic structure and optical properties including excitonic effects. Our simulations do not reproduce the experimentally reported peak structure and we show that the optical dipole transitions from the corresponding spin-orbit split-off electronic state do not show a polarization dependence. We interpret this as an indication that more direct experimental evidence is needed to support the spin-orbit origin of any feature in this spectral range. We also solve the Boltzmann transport equation to account for electron-phonon scattering as a relaxation mechanism in pump-probe experiments and implement the resulting time-dependent occupation numbers as constraint in simulations of the spectrum. Our predicted pump-probe spectra for this material show that increasing the intensity of the excitation only significantly changes the magnitude of the spectrum values, while increasing the excitation energy leads to changes in the presence of peaks, peak location, and peak width.