Session W60: Ultrafast Dynamics from Electron-Phonon Interactions II

Ab Initio Investigations on the Quantum Dynamics of Excited Carriers in Condensed Matter Systems

The ultrafast dynamics of charge carriers in condensed matter systems plays an important role in charge transport, optoelectronics and solar energy conversion. Although ab initio calculation is widely applied to understand the electronic structure of different materials, it is challenging to track the quantum dynamics of charge carriers in solide materials in mult-dimensions including time and energy domains, as well as real and momentum spaces. Our research goal is develop ab initio simulation approach to achieve a state-of-the-art understanding of multi-dimensional carrier dynamics in solid materials. We have developed Hefei-NAMD code, which can be applied to study i) the excited electron or hole dynamics based on the single-particle picture in real space and momentum space; ii) Spin-orbital Coupling (SOC) induced spin dynamics; and iii) exciton dynamics using GW + real-time BSE. In this talk, I will simply review the theoretical framework of Hefei-NAMD and introduce the GW + real-time BSE and the newly developed ab initio real-time quantum dynamics of charge carriers in momentum space (NAMD_k approach).   

Implementation of velocity-gauge RT-TDDFTB Ehrenfest dynamics

Length-gauge real-time time-dependent density functional tight-binding (LG-RT-TDDFTB) has been widely used to probe light-matter interactions for non-periodic systems. However, the translational symmetry of the Hamiltonian is broken by the external electric field in this gauge and cannot be used for periodic systems. Recently, we derived a new velocity-gauge real-time time-dependent density functional tight-binding (VG-RT-TDDFTB) formalism for probing electron dynamics in large, condensed matter systems within periodic boundary conditions. Furthermore, we extended VG-RT-TDDFTB to the semiclassical Ehrenfest method, making our approach accessible for probing ultrafast electron-nuclear dynamics for complex systems. Our implementation uses a hybrid MPI/OpenMP parallelization scheme for massive parallelization to treat large systems on multi-core supercomputers. Our calculations demonstrate that the approach enables large electron dynamics for periodic systems containing thousands of atoms on a modest computer cluster.   

AB-G0W0: An efficient G0W0 method without frequency integration based on an auxiliary boson expansion

Common G₀W₀ implementations rely on numerical or analytical frequency integration for the determination of the G₀W₀ self-energy.

This frequency integration results in a variety of practical complications, potentially limiting the area of application, or necessitating additional approximations.

Recently, we have demonstrated the exact connection between G₀W₀ and equation-of-motion (EOM) approaches [1].

Based on this connection, we propose a new and efficient method for the determination of G₀W₀ quasiparticle energies which avoids frequency integration and consequently commonly faced problems in G₀W₀ calculations.

To achieve this, we make use of an auxiliary boson (AB) expansion which scales linearly with the system size.

1) J. Tölle, and Garnet K.-L. Chan, J. Chem. Phys. 158, 124123 (2023)   

Creating Bloch-Floquet states with partially coherent driving fields

In the field of Floquet engineering, one seeks to control material properties on ultrafast timescales by driving the system with a time-periodic external electromagnetic field. Such periodic perturbations break the continuous time symmetry and lead to the formation of replicas of energy bands – analogous to the broken spatial translation symmetry in crystals. Through manipulating the frequency and intensity of the driving field, it is possible to engineer unusual properties of matter, ranging from photo-driven magnetism and superconductivity to topological phase transitions. A promising new strategy to engineer Floquet states which circumvents the problem of heating, typically associated with photo-driven systems, is to utilize an internal time-dependent oscillation of the system, such as phonons or excitons. In this talk we examine to what extent an oscillating coupling field with finite temporal coherence, like that produced by a thermal bath of bosons, can drive Floquet phenomena. Through a combination of model and ab initio real-time simulations, we demonstrate the conditions over which Floquet physics can be observed with partially coherent driving fields, an important step towards engineering long-lived Floquet phenomena in real materials.   

First principles calculation using multiple scattering theory at the exascale.

Current high performance computer systems, such as Frontier at the Oak Ridge Leadership Computing Facility, are providing unprecedented opportunities for the quantitative exploration of complex materials.  Here, I will present our implementation of multiple scattering theory for first principles density functional calculations. This approach directly obtains the single particle Green’s function of the Kohn-Sham equation, either in reciprocal space (Korringa-Kohn-Rostocker i.e. KKR) or real space (Localy-Selfconsistent Multiple Scattering i.e. LSMS). The KKR method allows an efficient description of random solid solution alloys using the Coherent Potential Approximation (CPA), while our LSMS code allows for scalable large scale first principles density functional calculations of materials. A fundamental science driver for scalable, large scale, first principles calculations of materials is the need to understand states beyond periodic crystalline lattices. For large simulation cells, needed to describe extended electronic and magnetic orderings, defect states or disorder in alloys, the cubic scaling of traditional first principles methods have prevented direct calculations. The linear scaling nature of the LSMS ab initio code enables the treatment of extremely large disordered systems from the first principles using the largest parallel supercomputers available, such ascalculations for O(10,000 - 1,000,000) atoms on current high performance computing architectures. For exascale systems, we have extended the use of accelerators to enable the efficient calculation for embedding methods and forces. Currently ongoing work focuses on the calculation of electric conductivity in the presence of disorder and defects. We will present scaling results of our LSMS code from single node calculations to the full Frontier system up to O(1,000,000) atom calculations.

These computational capabilities are available in our Multiple Scattering Theory suite (MuST) [https://github.com/mstsuite]   

Coherent-to-incoherent crossover in the ultrafast dynamics of electrons and phonons in photoexcited 2D materials

First-principles simulations of photoexcited materials are extremely challenging due to the simultaneous interplay of light−matter, electron−electron, and electron−nuclear interactions. We here present a novel non-equilibrium Green's function method [1] based on the simultaneous inclusion of the GW [2], Ehrenfest, and Fan-Migdal [3] self-energies. The method accounts for quantum coherence and non-Markovian effects while treating electrons and nuclei on equal footing, thereby preserving fundamental conservation laws like the total energy. The impact of this advancement is demonstrated through real-time simulations of the complex multivalley dynamics in a molybdenum disulfide monolayer [4]. At high carrier density the energy exchange between electrons and phonons is very efficient, leading to a sizable increase of the lattice temperature within one picosecond. In this process, electronic coherence is lost, whereas lattice coherence endures for a significantly longer period. In this regime we also lay down the microscopic theory of coherent phonons interacting with excitons and provide the ab initio expression of the corresponding coupling [5]. The simulated transient absorption spectrum well reproduces the coherent oscillations of the exciton energies observed in recent experiments [6].

[1] G. Stefanucci, R. van Leeuwen, and E. Perfetto Phys. Rev. X, 13, 031026 (2023).

[2] E. Perfetto, Y. Pavlyukh, G. Stefanucci, Phys. Rev. Lett. 128, 016801 (2022).

[3] D. Karlsson, R. van Leeuwen, Y. Pavlyukh, E. Perfetto, and G. Stefanucci, Phys. Rev. Lett. 127, 036402 (2021).

[4] E. Perfetto and G. Stefanucci, Nano Letters 23, 7029 (2023).

[5] E. Perfetto, K. Wu and G. Stefanucci, submitted.

[6] C. Trovatello et al., ACS Nano 14, 5700 (2020).   

Quantum MASALA: Quantum MAterialS Ab initio eLectronic-structure pAckage

We present QuantumMASALA, a compact package that implements different electronic-structure methods in Python. Within just 9000 lines of pure Python code, we have implemented Density Functional Theory (DFT), Time-dependent Density Functional Theory (TDDFT) and the GW Method. The program can run across multiple process cores and in Graphical Processing Units with the help of easily-accessible Python libraries. The package does not compromise on the speed and can easily be used for small to medium scale calculations. With QuantumESPRESSO and BerkeleyGW I/O interfaces implemented, it is also a perfect tool for learning ab intio methods. The package is aimed to provide a framework with its modular and simple code design to rapidly build and test new methods for first-principles calculation.   

An Expressway towards Heaven: High-Throughput Hybrid Density Functional Theory for Complex Condensed-Phase Systems Containing Thousands of Atoms using SeA

By climbing the five rungs of Jacob's ladder, the Density Functional Theory (DFT) hierarchy approaches the "heaven" of chemical accuracy needed for accurate and reliable modeling of complex materials. In particular, fourth-rung hybrid functionals can provide semi-quantitative accuracy, and have therefore been used to gain a fundamental understanding of important gas-phase systems and reactive processes. However, such hybrid DFT based applications remain rare for large-scale condensed-phase systems due to the prohibitive computational cost associated with evaluating the exact-exchange (EXX) interaction in periodic systems. In this work, we provide an accurate, efficient, and robust algorithmic framework for performing high-throughput hybrid DFT calculations for large-scale finite-gap condensed-phase systems containing thousands of atoms. The resulting SeA approach (SeA = SCDM+exx+ACE) seamlessly integrates three recent theoretical developments, including orbital localization via the non-iterative selected columns of the density matrix (SCDM) method, a black-box linear-scaling EXX solver (exx), and the adaptively compressed exchange (ACE) operator. By harnessing three levels of computational savings, SeA performs hybrid DFT based calculations at an overall time-to-solution comparable to second-rung GGA functionals (i.e., the computational workhorse for condensed-phase systems). We showcase the capabilities of SeA to treat a wide range of condensed-phase systems—without the need for system-dependent parameters—including molecular crystals, aqueous solutions, interfaces, and highly porous materials.   

Ab initio prediciton of vacuum propagating electronic states: a truncated Green's function approach for use with plane-wave density-functional theory software

Plane-wave density functional theory (DFT) has proven to be a powerful tool in modern computational physics for the ab initio calculation of the electronic structure of solids, surfaces, and molecules. A substantial drawback for systems lacking periodicity in at least one direction is the interaction of long-range functions with periodic images. While Coulomb truncation methods have been developed to limit simple charge-image effects, Kohn-Sham orbital-mediated electronic interactions between images become a major source of error when calculating scattering, vacuum-propagating, states. We will present a new, truncated Green's Function approach to break periodicity along one or more dimensions in such a way that is compatible with standard plane-wave DFT pseudopotential codes, yet allows accurate calculation of scattering states at arbitrary energies. Finally, as an application, we will present transmission, reflection, and photoemission predictions for a variety of 2D materials including transition-metal dichalcogenides (TMDs).   

Direct Computation of Berry Phase and Polarizations in Solids by Auxiliary Field Quantum Monte Carlo

We present a direct, ab initio computation of Berry phases with auxiliary field quantum Monte Carlo (AFQMC). AFQMC has shown to be an excellent total energy method, and gradients such as forces and stress can now be computed in solids [1]. With an improved correlated sampling algorithm [2], we propose a method that allows the calculation of Berry phases and polarizations. We demonstrate the accuracy of the method in the Hubbard dimer model system and real solids. This development paves the path for an ab initio many-body approach to directly compute electric and topological properties in periodic systems.

[1] S. Chen and S. Zhang, Phys. Rev. B 107, 195150 (2023)

[2] S. Chen, Y. Yang, M. A. Morales, and S. Zhang, arXiv: 2307.15203   

Quantitative characterization of flat band states in a molecular orbital model via hyperuniformity

The localization of electron states due to disorder, known as Anderson localization, has been actively studied. In recent years, there has been an interest on studying single-particle states with unique characteristics. Flat band systems are one of such examples, and indeed, there have been several reports of their distinctive behaviors to disorder [1, 2]. In our previous study [3], it also was suggested that flat band states in a model called molecular-orbital (MO) model [4] exhibit some distinct features. However, we find that the characterization methods used for single-particle wave functions, such as multifractal analysis, are ineffective for the flat band state due to the macroscopic degeneracy of the flat band.

In this work, to characterize the flat band state of the MO model, we employ a concept of hyperuniformity [5]. Hyperuniformity is a concept that quantifies the spatial fluctuation of distributions, and has recently been used for condensed matter systems, such as quasiperiodic systems [6]. In this presentation, we will show the results for quantitatively characterizing the features of the flat band states in a MO model with randomness using hyperuniformity [7].

 

[1] M. Goda, et al., PRL 96, 126401 (2006).

[2] J. T. Chalker, et al., PRB 82, 104209 (2010).

[3] T. Kuroda, et al., JPSJ 91, 044703 (2022).

[4] Y. Hatsugai, Annals of Physics 168453 (2021).

[5] S. Torquato, et al., PRE 68, 041113 (2003).

[6] S. Sakai, et al. PRB 105, 054202 (2022).

[7] T. Kuroda, T. Mizoguchi, Y. Hatsugai, in preparation.