Session W46: Excited State VI: Lattice and Spin Dynamics

Molecular Dynamics Simulations of Lattice Thermal Conductivity with Machine-Learning Anharmonic Interaction

The lattice thermal conductivity of crystals with strong anharmonic interaction is of particular interest, which cannot be adequately evaluated by linearized Boltzmann transport equation. In this work, we adopt compressive sensing, a machine learning technique to obtain high-order force constants from a small amount of training data. By considering strong heredity effects, we show that the dominant anharmonic interactions are short ranged. This largely shrinks the number of force constants needed in the expansion. To calculate the lattice thermal conductivity, molecular dynamics simulations that include anharmonic interactions up to the sixth order are performed. Test results for Si and NaCl will be presented. The anharmonic interaction in NaCl is found to be more significant than that in Si, which is consistent with the fact that NaCl exhibits a smaller thermal conductivity at above 100 K. Simulation results for highly anharmonic thermoelectric materials SnSe and GeSe that exhibit very low thermal conductivity will also be discussed.

*In collaboration with Jing Wang, Jie-Cheng Chen, and Martin Callsen   

Finding Predictive Descriptors for Singlet Fission by Machine Learning

Singlet fission (SF) is the conversion of one photo-generated singlet-state exciton into two triplet-state excitons. SF has the potential to significantly enhance the efficiency of solar cells by harvesting two charge carriers from one photon. Molecular crystals that undergo SF in the solid state are scarce. Computational exploration of the chemical space may accelerate the discovery of new SF materials. However, many-body perturbation theory (MBPT) methods that can reliably describe the excitonic properties of molecular crystals are computationally too expensive for large-scale materials screening. We use the sure-independence-screening-and-sparsifying-operator (SISSO) machine-learning algorithm to identify descriptors that are fast to evaluate and can accurately predict the thermodynamic driving force for SF, calculated by MBPT for a dataset of 101 polycyclic aromatic hydrocarbons (PAH101). SISSO generates interpretable models by iteratively combining physically motivated primary features. The best performing models are then selected by linear regression with cross validation. The SISSO-generated models successfully predict the SF driving force with root mean square errors as low as 0.11 eV. Based on considerations of the cost, accuracy, and classification performance of SISSO-generated models, we propose a hierarchical screening workflow for materials discovery. Three new SF candidates are discovered in the PAH101 set, which have not been previously reported.   

Time-resolved photoemission from a field-driven Holstein chain: A semi-classical Monte Carlo approach

We extend a recently developed time-dependent Monte Carlo method [1] which models the dynamics of electron and frozen phonon coupled systems to include the classical phonon motion. The numerical efficiency of the new method allows us to perform a self-consistent time evolution of the two coupled subsystems to time scales on the order of several picoseconds. We apply this novel method to study the dynamics of an excited state of a charge density wave (CDW) material in the context of pump-probe experiments. Our system is a half-filled, one-dimensional Holstein chain that exhibits CDW ordering due to Peierls transition. The chain is subjected to a time-dependent electromagnetic field that excites it out of equilibrium, and then the second pulse is used to probe its behavior. We capture the complete process of lattice excitation and subsequent relaxation to a new equilibrium, as manifested in the computed photoemission spectrum and relevant order parameters. Our method reveals an indirect driving mechanism of the lattice by the pump pulse. We separate two driving regimes, where the pump can either cause small perturbations or completely invert the initial CDW order.

[1] M. Weber, J. K. Freericks, arXiv:2108.05431 (2021).   

Exotic transport phenomena in quantum materials from ab initio

Exotic transport phenomena provide a powerful way to probe excited states in quantum materials. Emergent signatures can arise when the length scales of momentum-relaxation/conservation for the nearly-free electron are comparable. There, the carriers flow collectively and the conventional diffusive transport description from textbook is not sufficient to fully capture the charge conduction in the hydrodynamic regime [1]. 

We use ab initio methods to investigate this exotic transport phenomenon. First, we focus on the electron-phonon interactions and show how they give rise to anomalous behaviors from both electron and phonon specifics in two prototypical semimetals WP₂ and WTe₂ [2-4]. We show that rapid e-ph scattering can lead to indirect electron-electron interactions, which conserves the total electron momentum. By solving the numerical BTE with the ab initio input, we demonstrate that the current density distribution in a thin long channel resembles a parabolic flow pattern [2,5]. Inspired by the recent space-resolved measurements, we characterize the different transport regimes in the parameter space of lmr, lmc, and w. Finally, we present predictions on candidates ZrSiS and TaAs₂ and propose design principles to realize hydrodynamics in anisotropic metals and semimetals [6].    

Simulating THz field-induced ferroeletricity in quantum paraeletric SrTiO3 by solving Schrodinger-Langevin equation

Studies of light-matter interactions reveal that light can unravel the hidden properties of materials. For example, the higher critical temperature for superconductivity is observed by applying the light on K₃C₆₀. Also, light-induced topological phase transitions in ZrTe₅ and WTe₂ are experimentally demonstrated. Recent experiments reported that high intensity terahertz (THz) and near infreared field pulse can induce a ferroelectric transition in SrTiO₃ from its quantum paraeletric ground state [1-2]. Here, we investigate the THz field-induced transient ferroeletricity by solving the time-dependent lattice Schrödinger-Langevin equation. First, the description of quantum paraeletric SrTiO₃ based on the density functional theory calculation is investigated [3]. We found that the quantum description between ferroeletric soft mode and uniaxial lattice strain is essential to reproduce the experientially observed properties in quantum paraeletric SrTiO₃. Based on this study, we investigate the real-time dynamics of lattice wavefunction of ferroeletric soft mode in quantum paraeletric SrTiO₃ by solving time-dependent Schrödinger-Langevin equation [4]. We found that the THz field-induced transient ferroelecticity originates from the light-mixed state between ground and the first excited lattice wavefunctions of ferroeletric soft mode in quantum paraeletric SrTiO₃. In agreement with the experimental observations, our study reveals that the non-oscillatory second harmonic generation signal is main evidence of transient ferroeletricity in SrTiO₃. Our study unravels the microscopic mechanism of light-induced phase transition and provides the understanding of quantum paraeletric phase.   

Ultrafast Spin Dynamics and Photoinduced Insulator-to-Metal Transition in α-RuCl3

Using time-dependent density functional theory, we explore the ultrafast laser-induced dynamics of the electronic and magnetic structures in α-RuCl₃. Our study unveils that laser pulses can introduce ultrafast demagnetizations in α-RuCl₃, accompanied by an out-of-equilibrium insulator-to-metal transition in a few tens of femtoseconds. The spin response significantly depends on the laser wavelength and polarization on account of the electron correlations, band renormalizations and charge redistributions. These findings provide physical insights into the coupling between the electronic and magnetic degrees of freedom in α-RuCl₃ and shed light on suppressing the long-range magnetic orders and reaching a proximate spin liquid phase for two-dimensional magnets on an ultrafast timescale.

    

Ultrafast dynamics of chiral phonons in WSe2 by first-principles lattice dynamics simulations

Chiral phonons are unique lattice excitations found in transition metal dichalcogenides (TMDs). They link the valley degree of freedom with optical excitations, giving rise to a range of unconventional physical phenomena, including enhanced exciton recombination and the valley-phonon Hall effect. Since their first observation in WSe₂, chiral phonons have been studied experimentally in several helicity-preserving optical processes. Yet, the ultrafast dynamics of chiral phonons governing such optical excitations is still not completely understood. In this talk, we will present a first-principles study of the coupled dynamics of electrons and chiral phonons in WSe₂, using a method we recently developed based on the solution of the coupled electron and phonon real-time Boltzmann transport equations^[1]. Our preliminary results, discussed in this talk, reveal microscopic details of the ultrafast dynamics of chiral phonons, shedding light on their relaxation and on the link between valley and phonon dynamics in TMDs.   

Calculating Phonons within the FLAPW Method using Density Functional Perturbation Theory (DFPT)

Computing phonons using density functional perturbation theory (DFPT) within all-electron DFT methods is a real challenge due to the displacement of muffin-tin spheres and sphere-centered basis functions. In this talk, we present our implementation of the DFPT approach within the FLAPW method, encoded in the FLEUR code (www.flapw.de). We show first results, which we compare to the phonon dispersion, calculated with the finite displacement method. We find a very good agreement and discuss what conceptual details are important to ensure the numerical accuracy required for the determination of the phonon dispersion.   

Electron spin decoherence due to phonons: Unified many-body framework and first-principles calculations

Spin-phonon (s-ph) decoherence governs the intrinsic limit of spintronic and quantum devices, so its understanding is a pressing matter in spin-based technologies. First-principles calculations enable precise predictions of s-ph interactions. However, understanding spin decoherence and relaxation due to phonons remains an open challenge. There are two main s-ph decoherence mechanisms – the Elliott-Yafet (EY) and D’yakonov-Perel’ (DP) – with distinct physical origins and theoretical treatments. Here we present a unified many-body approach for s-ph decoherence, which encompasses both the EY and DP mechanisms. Our method combines the ab-initio s-ph interactions with a diagrammatic approach to treat interacting spins. We apply our method to various centrosymmetric and noncentrosymmetric materials, where the EY and DP mechanisms are dominant, respectively. In both cases, our computed spin relaxation times are in excellent agreement with experiments. We provide evidence that our diagrammatic approach captures both the EY and DP spin decoherence, regardless of material symmetry. Our work demonstrates a widely applicable approach for quantitative analysis of spin decoherence for systems of interest in spintronics, quantum materials and quantum technologies.   

First-principles study of quasiparticle energies and optical excitations of 3C-SiC divacancy

Using large-scale GW and Bethe-Salpeter equation (BSE) calculations. we investigate the quasiparticle energies and optical absorption spectrum of the divacancy defect in 3C-SiC, a prototypical defect for quantum information applications. Our calculations provide a quantitative prediction of the defect quasiparticle energy levels and zero-phonon absorption line. Interestingly, despite the presence of localized defect states in the gap, we find that the low-energy excitonic states are made primarily of transitions from occupied defect states to continuum conduction states from 3C-SiC, especially from the X point of the Brillouin zone (BZ). The large hybridization between defect states and bulk states in 3C-SiC is different the NV^- center in diamond and the divacancy in 4H-SiC, where the deep defect levels are well separated from bulk states. Our study highlights the important role of frontier conduction bands in the optical properties and formation of low-energy excitons in 3C-SiC divacancy.   

Memory Function Representation for the Electrical Conductivity of Solids

We derive a formula for the electrical conductivity of solids that includes relaxation and dissipation due to scattering such as by disorder, phonons, or others which do not conserve momentum, while also including quantum coherence. The derivation is based on the Kubo formula, with a Mori memory function approach to include dissipation effects at all orders in the relaxation interaction. It offers a practical method to evaluate the conductivity with electronic-structure codes, avoiding the complications and limitations of both the Kubo formula in the thermodynamic limit and the semiclassical nature of Boltzmann transport. The derivation of our formula provides a method applicable to other transport coefficients and correlation functions. In the present case, it captures interband contributions to the conductivity as well as the metal-insulator phase transition.   

Study of high temperature Be from first principles

We use first-principles simulations to study the optical properties and electron-electron relaxation times of beryllium (Be) at high temperatures to allow for the design of warm dense matter (WDM) optical conductivity experiments. Within the local-density approximation (LDA), we calculate the imaginary part of the macroscopic dielectric function, with intraband transitions accounted for by the Drude model, to find how the optical absorption of Be changes as a function of temperature from 0.5 to 3.0 eV. We find that absorption does not have a strong temperature dependence at low photon energies due to intraband transitions. For photon energies above 0.75 eV, we see that as temperature increases, the strength of absorption decreases, however the absorption occurs over a larger energy range. We also study electron-electron relaxation time through a GW approach, in which we fit the imaginary part of the self-energy to the Landau theory of the Fermi liquid to determine the lifetimes of electrons at energies above the Fermi energy. This work will lead to a better fundamental understanding of the optical and electronic properties of Be at high temperature and subsequent cooling, allowing for better design and interpretation of results of WDM experiments.