Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D44: Real-Space Methods for the Electronic Structure Problem III |
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Sponsoring Units: DCOMP Chair: Angel Rubio, Max Planck Inst Structure & Dynamics of Matter Room: 704 |
Monday, March 2, 2020 2:30PM - 2:42PM |
D44.00001: Relaxation Effects in the Electronic Structure of Twisted Bilayer Graphene: a Multi-Scale Approach Nicolas Leconte, Srivani Javvaji, Jiaqi An, Jeil Jung We introduce a multi-scale approach to obtain accurate atomic and electronic structures for atomically relaxed twisted bilayer graphene. High-level exact exchange and random phase approximation (EXX+RPA) correlation data provides the foundation to parametrize systematically improved force fields for molecular dynamics simulations that allow relaxing twisted layered graphene systems containing millions of atoms making possible a fine sweeping of twist angles. These relaxed atomic positions are used as input for tight-binding band-structure calculations where the distance & angle-dependent interlayer hopping terms are extracted from ab-initio calculations & subsequent representation with Wannier orbitals. We benchmark our results against published force fields and widely used tight-binding models and discuss their impact in the spectrum around the flat band energies. We find that our relaxation scheme yields a magic angle of twisted bilayer graphene consistent with experiments between 1.0o∼1.1o using Fermi velocities υF≈1.0∼1.1×106 m/s. We present high-resolution spectral function calculations to compare with experimental ARPES. |
Monday, March 2, 2020 2:42PM - 2:54PM |
D44.00002: Dielectric response of aligned SWCNT films: A theoretical versus experimental study Chandra Adhikari, Igor Bondarev Finite-thickness films of aligned single-wall carbon nanotubes (SWCNT) are demonstrated to have a tunable negative dielectric response in a wide photon energy range[1]. We explain this theoretically using the Maxwell-Garnett (MG) mixing[2] for the dielectric responses of planar periodic arrays of identical SWCNTs[3]. We start with the calculation of the conductivity of an individual SWCNT using the k.p method[4]. Then we derive a semi-analytical expression for the dielectric tensor of an ultrathin finite-thickness periodically aligned SWCNT array. The dielectric responses of about a dozen of SWCNT arrays with different chiralities and very close diameters are then mixed using the MG method to obtain the dielectric response of the SWCNT film which explains the experimental ellipsometry measurements. We also note that the inhomogeneously broadened interband plasmon resonance overlaps with the broadened exciton resonance, making the exciton-plasmon coupling and associated photon bandgap formation possible in these systems[5]. – [1]J.A.Roberts et al, NL 19, 3131 (2019); [2]V.A.Markel, JOSA A33, 1244 (2016); [3] I.V.Bondarev, Opt.Mater.Expr. 9, 285 (2019); [4]T.Ando, J.Phys.Soc.Jpn.74, 777 (2005); [5]I.V.Bondarev, PRB 85, 035448 (2012). |
Monday, March 2, 2020 2:54PM - 3:06PM |
D44.00003: Real-Space Calculations of Dielectric Screening in Large Silicon Nanocrystals Timothy Liao, Kai-Hsin Liou, James Chelikowsky The nature of dielectric screening in semiconductor nanocrystals (NCs) is a outstanding topic. We examine the screening of a point charge in hydrogen-terminated Si NCs. We consider NC’s containing up to 5,400 atoms using pseudopotential-density-functional theory. We solve the Kohn-Sham equation in real-space using the PARSEC code. We compute the dielectric properties of a Si NC by replacing a Si nucleus at the center of the NC with a P nucleus while maintaining the electron number. We consider NCs of sufficient size to converge the dielectric properties to the bulk limit. We find the resulting screening functions are consistent with previous studies on smaller NCs. The ability to calculate dielectric properties of a large confined system allows us to consider charged defects in bulk Si in a straight forward manner without invoking a compensating background. |
Monday, March 2, 2020 3:06PM - 3:18PM |
D44.00004: Superconductivity in heavily boron-doped carbon materials Yuki Sakai, James Chelikowsky, Marvin L Cohen We investigate the superconducting properties of heavily boron-doped carbon materials composed of sp3 hybridized atoms: cubic diamond, hexagonal diamond, and body centered tetragonal C4 (bct C4). We find that a high density of states (DOS) at the Fermi energy result in a high superconducting transition temperature (Tc) in all three materials. The high DOS is particularly realized when boron dopants are not placed at nearest neighbor sites. A Tc above 60 K can be obtained for cubic diamond, although other allotropes can exhibit a Tc of 40-50 K at 25 % boron-doping. We also discuss superconductivity in amorphous carbon. |
Monday, March 2, 2020 3:18PM - 3:30PM |
D44.00005: Discriminating functional groups, atomic species and molecular geometries in organic molecules using real-space simulations of non-contact atomic force microscopy Dingxin Fan, Yuki Sakai, James Chelikowsky Noncontact atomic force microscopy (nc-AFM) with a CO functionalized probe tip is a powerful tool for molecular structure characterizations. In many organic molecules, the visualization of individual atoms is a real possibility, save for the complexity of interpreting the nc-AFM images. In order to gain a better understanding of such experimental images, we employ a real-space pseudopotential constructed within density functional theory code, PARSEC, to simulate nc-AFM images. We are able to discriminate functional groups (such as -C≡C-, -CH2 and -C=O groups) and heteroatoms (such as O, N and S atoms) in organic molecules by mapping our simulated images to experimental images. Also, we find that nc-AFM is capable of directly visualizing the orientation of organic molecules at varies adsorption sites on metal substrates. This can be very useful for characterizing large long-chain organic molecules in heavy oils and asphaltenes. |
Monday, March 2, 2020 3:30PM - 3:42PM |
D44.00006: Time-dependent magnons from first principles Nicolas Tancogne-Dejean, Florian G Eich, Angel Rubio We propose an efficient and not perturbative scheme to compute magnetic excitations for extended systems employing the framework of time-dependent density-functional theory. Within our approach we drive the system out of equilibrium using an ultra-short magnetic kick perpendicular to ground-state magnetization of the material. The dynamical properties of the system are obtained by propagating the time-dependent Kohn-Sham equations in real time and the analysis of the time-dependent magnetization reveals the transverse magnetic excitation spectrum of the magnet. We illustrate the performance of the method by computing the magnetization dynamics, obtained from a real-time propagation, for iron, cobalt and nickel and compare them to known results obtained using the linear-response formulation of time-dependent density-functional theory. Moreover, we point out that our time-dependent approach is not limited to the linear-response regime, and we present first results for non-linear magnetic excitations from first-principles in iron. |
Monday, March 2, 2020 3:42PM - 3:54PM |
D44.00007: A real-space pseudopotential method for magnetocrystalline anisotropy energies and the search for magnets without rare-earth metals Masahiro Sakurai, James Chelikowsky We present a real-space pseudopotential formalism for calculating magnetocrystalline anisotropy energies within relativistic density-functional theory (DFT). Our method is implemented in our real-space pseudopotential DFT code, PARSEC, which is designed to run efficiently on massively parallel computing platforms [1]. We show that our formalism works well for prototypical transition-metal compounds, such as YCo5 and Mn2Ga, yielding an accurate magnetization and a magnetocrystalline anisotropy constant consistent with other density-functional methods. We illustrate how our methods can be applied to the search for permanent magnets without rare-earth metals [2,3,4]. In particular, we identified several ZrCo5 and Fe–Ni–B compounds as possible candidate materials that can provide high magnetocrystalline anisotropy energies along with sufficient saturation magnetization. |
Monday, March 2, 2020 3:54PM - 4:06PM |
D44.00008: Ab initio calculations of electron and nuclear spin interactions in molecules and solids using a mixed pseudopotential-all electron approach Krishnendu Ghosh, He Ma, Vikram Gavini, Giulia Galli The interaction between electronic and nuclear spins in the presence of external magnetic fields can be described by a spin Hamiltonian (SH), with parameters obtained from first principles electronic structure calculations. We previously developed an approach [1] to compute these parameters, applicable to both molecules and solids, which is based on real space density functional theory (DFT) based all-electron calculations using finite elements. Here we improve the efficiency of our approach for the calculations of spin-defects in semiconductors by combining all-electron and pseudopotential calculations: for a small region around the defect we treat explicitly all the electrons, while the rest of the crystal is described using pseudopotential calculations. We present results for the nitrogen vacancy in diamond and for divacancies in silicon carbide, including hyperfine and zero-field splitting tensors, and we show that the results of the mixed approach are in an excellent agreement with those of all-electron calculations for the full crystals. Our strategy opens the way to accurate large-scale calculations of SH parameters for the prediction of spin defect qubits in complex systems. |
Monday, March 2, 2020 4:06PM - 4:18PM |
D44.00009: Nonlocal Density Embedding Theory Wenhui Mi, Michele Pavanello By invoking a divide-and-conquer strategy, density embedding methods dramatically reduce the computational cost of large-scale, ab-initio electronic structure simulations of molecules and materials. The central ingredient setting density embedding apart from Kohn-Sham DFT is the non-additive kinetic energy functional (NAKE). Currently employed NAKEs are at most semi-local (i.e., they only depend on the electron density and its gradient), and as a result of this approximation, only systems composed of weakly interacting subsystems can be successfully tackled. The limitation of semi-local NAKEs originate from the natural nonlocality of the underlying KEDF. Recently, we advance the state-of-the-art by introducing fully nonlocal NAKEs in density embedding simulations for the first time. Benchmark analysis based on the S22-5 set shows that nonlocal NAKEs considerably improve the computed interaction energies and electron density compared to commonly employed GGA NAKEs, especially when the inter-subsystem electron density overlap is high. Most importantly, we resolve the long standing problem of too attractive interaction energy curves typically resulting from the use of GGA NAKEs. |
Monday, March 2, 2020 4:18PM - 4:30PM |
D44.00010: Modeling device-level semiconductors and their interfaces by orbital-free DFT Xuecheng Shao, Kaili Jiang, Michele Pavanello Orbital-Free Density Functional Theory (OFDFT) is one of most promising methods for large-scale quantum mechanical simulations as it offers a good balance of accuracy and computational cost. Million atom simulations are nowadays achievable with OFDFT because the noninteracting kinetic energy functional is approximated by an explicit functional of the electron density removing the need to employ orbitals and diagonalizations. The commonplace belief is that because of the underlying approximations, OFDFT can only approach metallic systems. However, newly developed functionals allow the quantitative description of semiconductors and semiconducting quantum dots. We present an implementation of OFDFT entirely in Python providing some useful abstractions to deal with molecules and materials. The simulations are extremely computationally efficient, delivering converged electronic structures for million-atom system sizes realizing experimentally observed devices. We present calculations of work functions and Schottky barriers for an array of realistically sized systems that are still completely out of reach of commonly employed Kohn-Sham DFT methods. |
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