Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C20: FirstPrinciples Modeling of ExcitedState Phenomena in Materials II: Complex Materials and DefectsFocus Live

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Sponsoring Units: DCOMP DCP DMP Chair: Vojtech Vlcek, University of California, Santa Barbara 
Monday, March 15, 2021 3:00PM  3:36PM Live 
C20.00001: Ab Initio Quantum Dynamics in Nanoscale Materials Invited Speaker: Oleg Prezhdo Excited state dynamics play key roles in numerous condensed phase and molecular materials designed for solar energy, optoelectronics, spintronics and other applications. Controlling these farfromequilibrium processes and steering them in desired directions require understanding of material’s response on the nanometer scale and with fine time resolution. We couple, in a unique way, realtime timedependent density functional theory for the evolution of electrons with nonadiabatic molecular dynamics for atomic motions to model such nonequilibrium response in the timedomain and at the atomistic level. The talk will describe the basics of the simulation methodology and will discuss several recent applications, such as metal halide perovskites, metallic and semiconducting quantum dots, and transition metal dichalcogenides, among the broad variety of systems studied in our group. Interplay of photoinduced charge and energy transfer, Augertype processes, charge trapping and recombination, and energy losses creates many challenges due to large differences between molecular and periodic, and organic and inorganic matter. Our simulations provide a unifying description of quantum dynamics on the nanoscale, characterize the timescales and branching ratios of competing processes, resolve debated issues, and generate theoretical guidelines for development of novel systems. 
Monday, March 15, 2021 3:36PM  3:48PM Live 
C20.00002: The impact of point defects and temperature on the excitonic properties of monolayer germanium selenide Tianlun Huang, Sahar Sharifzadeh We utilize firstprinciples density functional theory and manybody perturbation theory to study the optoelectronic properties of monolayer germanium selenide (GeSe), emphasizing the role of point defects and electronphonon interactions; two phenomena that will be present in and can dominate the properties of real materials. First, we study the impact of a single Se vacancy on the optoelectronic properties of the monolayer, demonstrating the presence of localized excitons trapped by the defect. Additionally, by approximating the role of electronphonon interactions, we study the role of vibrations on exciton photophysics. We determine that the optical absorption spectrum is redshifted by ~0.1 eV at roomtemperature phonons with both acoustic and optical phonons coupling to the excitonic state. Overall, we determine that the excitonic properties of GeSe are significantly affected by the presence of defects and phonons. 
Monday, March 15, 2021 3:48PM  4:00PM Live 
C20.00003: The SpinFlip BetheSalpeter Equation approach to transition metal dimers and defects in solids Bradford Barker, David A Strubbe The electronic structure of transition metal dimers is difficult to calculate for many computational methods due to strong correlation effects and openshell configurations. The “spin flip” approach allows a singlereference approach to describe openshell states as an excitation  up or down in energy  from a related singlereference highspin state. The SpinFlip Bethe Salpeter Equation (“SFBSE”) approach provides an ab initio interaction kernel to describe these excitations. We show vertical excitation energies calculated in the SFBSE approach for the diamond NV^{} center, in good agreement with configuration interactionbased methods and experiment. The spinflip approach even allows for an improved treatment of electronic correlation from the relevant electronic configurations. This allows for the calculation of ground and excitedstates of ferromagnetic ions of Mn_{2} and Cr_{2} dimers. Finally, SFBSE can also be used to parameterize a simple Heisenberg model [Mayhall, et al. J Chem Phys, (2014)] for the antiferromagnetic Cr_{2} dimer, which demonstrates promise for the method to be applied to more complex magnetic molecules. 
Monday, March 15, 2021 4:00PM  4:12PM Live 
C20.00004: Study of optical properties of CuI from first principles Brian Robinson, Andre Schleife Copper(I) Iodide (CuI) is a wide band gap semiconductor that is a good candidate to be used as a transparent conductive material. In this study, we used firstprinciples simulations to explore the optical properties of CuI. The PerdewBurkeErnzerhof (PBE) functional was used to determine the influence of spinorbit coupling on the electronic band structure and optical properties of CuI. From density functional theory (DFT) simulations of the band structure and optical spectra, we conclude that the effect of spinorbit coupling is small, but not insignificant. To account for the excitonic effects in CuI, we solved the BetheSalpeter Equation (BSE) calculation for the optical polarization function. The resulting optical spectrum agrees very well with experiment across a photon energy range up to 4 eV. However, at energies greater than 4 eV, differences on the order of 0.5 eV arise. We believe this can be explained by spinorbit coupling, screening effects, and quasiparticle effects. The future of this study is to optically pump the system and explore the real time dynamics to understand the fundamental aspects of relaxation. 
Monday, March 15, 2021 4:12PM  4:24PM Live 
C20.00005: Firstprinciples Studies of Tl activated Scintillator Phosphor Materials: Towards an understanding of the Scintillation mechanism Andrew Canning, Mauro Del Ben, Shivani Srivastava, Jaroslaw Glodo Tl doped halide scintillator phosphors are amongst the most commonly used gamma ray detector materials for medical imaging, high energy physics and nuclear materials detection applications (e.g. CsI:Tl, NaI:Tl). Even so the complete scintillation process in these materials is poorly understood. Recently there has been interest in codoping these materials to try and improve their detection performance. We have performed firstprinciples studies based on GGA, hybrid functionals and the GW/BSE method in tandem with experiments to understand the scintillation mechanism in these materials and how it could be improved by codoping. In particular we have looked at the Tl exciton optical emission states and energy transfer mechanisms from the gamma ray to the Tl. Recently there has also been interest in new Tl bulk scintillators such as TLYC (Tl_{2}LiYCl_{6}) which we have also studied. 
Monday, March 15, 2021 4:24PM  4:36PM Live 
C20.00006: An energetics perspective on why there are so few triplet–triplet annihilation emitters Xiaopeng Wang, Rithwik Tom, Xingyu Alfred Liu, Daniel congreve, Noa Marom The efficiency of solar cells may be increased by utilizing photons with energies below the band gap of the absorber. This may be enabled by upconversion of low energy photons into high energy photons via triplettriplet annihilation (TTA) in organic chromophores. The quantum yield of TTA is often low due to competing processes. The singlet pathway, where a high energy photon is emitted, is one of three possible outcomes of an encounter between two triplet excitons. The quintet pathway is often too high in energy to be accessible, leaving the triplet pathway as the main competing process. Using manybody perturbation theory in the GW approximation and the BetheSalpeter equation, we calculate the energy release in both the singlet and triplet pathways for 59 chromophores of different chemical families. We find that in most cases the triplet pathway is open and has a larger energy release than the singlet pathway. Thus, the energetics perspective explains why there are so few TTA emitters and why the quantum yield of TTA is typically low. Our results also indicate that the performance of emitters from known chemical families may be improved by chemical modifications and new chemical families could be explored to discover more TTA emitters [JMCC 8, 10816 (2020)]. 
Monday, March 15, 2021 4:36PM  4:48PM Live 
C20.00007: Optical properties of qubits from manybody perturbation theory: the boron vacancy in 2D hBN Francesco Libbi, Pedro Melo, Zeila Zanolli, Matthieu Verstraete, Nicola Marzari Singlephoton emission from defect centers in semiconductors plays a crucial role for their application in quantum technologies. These phenomena have been investigated mostly using phenomenological models or constrainedDFT calculations, but indepth studies based on manybody perturbation theory are required for predictive accuracy on the absorption and emission mechanisms. In this work, we use nonequilibrium Green’s functions to study the absorption and emission of negativelycharged boron vacancies in 2D hexagonal boron nitride, which currently stands out among defect centers in 2D materials for its promise for quantum information and quantum sensing applications [1,2]. We calculate first the absorption spectrum by solving the equilibrium BetheSaltpeter equation (BSE); furthermore, we solve the nonequilibrium BSE to study the radiative recombination of the thermalized excitons and to compute the photoluminescence spectrum. 
Monday, March 15, 2021 4:48PM  5:00PM Live 
C20.00008: Assessing Zethrene Derivatives as Singlet Fission Candidates Based on Multiple Descriptors Xingyu Alfred Liu, Rithwik Tom, Siyu Gao, Noa Marom Singlet fission (SF) is a process where one singlet exciton splits into two triplet excitons. Utilizing SF may potentially increase the efficiency of solar cells. To discover new SF materials, predictive descriptors for SF performance are needed. We consider multiple descriptors to assess several zethrene derivatives as candidate SF materials. The descriptors include single molecule multiradical characters, manybody perturbation theory calculations of the thermodynamic driving force for SF and the singlet exciton charge transfer character in crystals, and a kinetic model based on molecular dimers extracted from the crystal structures. We find that all zethrene derivatives studied here may exhibit SF. In particular, 7,14bis(2,4,6trimethylphenyl)dibenzo[\textit{de,mn}]naphthacene (ZT) emerges as a promising candidate. Its SF driving force is higher than tetracene, whose fission process is slightly endoergic, but lower than pentacene; Its singlet exciton charge transfer character is close to pentacene; and its crystal packing leads to a higher SF rate than other zethrene derivatives. Therefore, it may undergo fast SF with high energy efficiency. The approach of considering multiple descriptors may be useful for evaluating additional candidate materials for SF. 
Monday, March 15, 2021 5:00PM  5:12PM Live 
C20.00009: Timedependent density functional theory study of nitrogenvacancy centers in diamond under particle irradiation Tatiane Pereira Dos Santos, Andre Schleife NitrogenVacancy (NV) centers in diamond are promising candidates for quantum information applications. In particular, that is because of the possibility to manipulate individual quantum states under relatively high stability. However, the scalable fabrication and design of such sensitive schemes rely on high precision of techniques such as irradiation of ions and electrons to induce defects. Therefore, a better understanding of the electronic and ionic dynamics during particle irradiation in diamond crystals with energetic particles is of fundamental importance to improve control of the defect production. To this end, using timedependent abinitio calculations, we perform accurate nonadiabatic dynamical simulations of ion and electron projectiles propagating near NV centers in diamond and estimate the electronic stopping and occupation number under the projectile impact at femtosecond timescales to profile the radiation damageinduced excitations. This study can help improve both control and stability during the fabrication and preparation of the defect states and provide information at temporal and spatial resolutions typically challenging to experimental measurements. 
Monday, March 15, 2021 5:12PM  5:24PM Live 
C20.00010: Electronic structure of (organic)inorganic metal halide perovskites: the dilemma of choosing the right functional Cecilia Vona, Dmitrii Nabok, Claudia Draxl Organicinorganic metal halide perovskites are materials widely studied for their lightharvesting properties. Owing to the interplay between strong electronelectron interaction and spinorbit coupling (SOC), their theoretical investigation is still a challenge. Here, we evaluate methodologies to compute the electronic structure of APbI_{3}, where A can be organic like MA or FA, or inorganic like Cs, and their precursor PbI_{2}. To this extent, we investigate several approaches within density functional theory (DFT) and manybody perturbation theory (MBPT), taking into account SOC effects. Hybrid functionals, such as PBE0 and HSE, are at the center of the investigation, since they can provide results as accurate as MBPT at a lower computational cost. Hence, we explore several nonempirical methods to tune their parameters. Additionally, at the MBPT level, we investigate the dependence of the calculations from the DFT starting point. All the calculations are performed with the fullpotential allelectron computer package exciting. We observe that properly tuned hybrid functionals are the most suitable functionals to compute the electronic structure of APbI_{3} and PbI_{2}. 
Monday, March 15, 2021 5:24PM  5:36PM Live 
C20.00011: Radiationinduced effects in solar cells for future space missions: a combined Monte Carlo and abinitio study of proton impact, electronic stopping and threshold displacement energy Fabiana Da Pieve, Natalia E. Koval, Daniel MuñozSantiburcio, Jos Teunissen, Emilio Artacho Solar cells in space are degraded mainly because of atomic displacements induced by radiation (protons, electrons …), in particular in the regime where the Non Ionizing Energy Loss (NIEL) [1] of the impacting particle is transferred to such displacements via Coulomb interaction. The NIEL is generally calculated via the Monte Carlo binary collision approximation, without considering crystalline order and the coupling between ionic and the electronic degrees of freedom. Here, after analyzing the energies of protons passing through triplejunction solar cells in a realistic space scenario via Monte Carlo particle transport, we present an abinitio study on a) the real time dynamics of the electronic stopping for protons passing through the thre layers of the cell [2]; b) the sensitivity of the minimum energy to create a stable defect to electronic excitations. The results suggest that the current NIEL model used to estimate the degradation of solar cells has to be revisited. 
Monday, March 15, 2021 5:36PM  5:48PM On Demand 
C20.00012: Engineering Optical Excitations of 2D materials with Defects and Molecules Dan Wang, Diana Qiu Twodimensional (2D) materials are the subject of significant ongoing research for exploring exciton physics and device applications. One of the most promising classes of 2D materials, monolayer transition metal dichalcogenides (TMDs), features strong excitonic emission and the locking of the valley and spin degrees of freedom, leading to the selective excitation of states in different valleys by left and righthand circularly polarized light. These unique properties make such materials desirable for optical manipulation and enable their application in valleytronics. Here, we explore the modulation of the excitons and valleyselective circular dichroism in monolayer TMDs by proximity effects from the adsorption of chiral molecules and induction of point defects, using ab initio GW and BetheSalpeter equation calculations of quasiparticle energy levels and the optical spectra. We also study the charge and energy transfer at the molecule/TMD (perfect or defective with point defects) heterointerfaces. Our results suggest a pathway for manipulating the valley degree of freedom in 2D materials for valleytronics via defect engineering and molecular functionalization. 
Monday, March 15, 2021 5:48PM  6:00PM Not Participating 
C20.00013: Spinwave dispersion of Co_{2}Mn_{1x}Fe_{x}Si based on the quasiparticle selfconsistent GW calculation Haruki Okumura, Kazunori Sato, Katsuhiro Suzuki, Takao Kotani Cobased full Heusler alloys with halfmetallicity are capable of spincurrent resources or materials for giant magnetoresistance (GMR). It is important to reveal the electronic structures of these materials and the magnetic properties. In this study, we calculate the electronic structure and spinwave dispersion of Co_{2}Mn_{1x}Fe_{x}Si (x=0.00, 0.25, 0.50, 0.75, 1.00) within the quasiparticle selfconsistent GW (QSGW) method. In ecalj package, we implemented the QSGW calculation for electronic structures. The dynamical transverse spin susceptibility are calculated based on the linear response theory. The spinwave dispersions are described as the imaginary part of the susceptibility. The calculated density of states at Fermi energy decrease as the Fe concentration increases. It differs the electronic structures experimentally predicted from the Gilbert damping constant. The calculated stiffness constant decreases from 294 meV Å^{2} (Co_{2}MnSi) to 59 meV Å^{2} (Co_{2}FeSi). It is same tendency as the experimental stiffness constant by the temperature dependence of magnetization. 
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