Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z24: Quantum Many-Body Systems and Methods II |
Hide Abstracts |
Sponsoring Units: DCOMP Chair: Gopal Priya Room: 203AB |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z24.00001: Ten-fold speed up of DFT: Improving k-point integration Gus Hart, Derek C. Thomas, Jeremy J. Jorgensen, Matthew M. Burbidge, Bret C. Hess, Conrad W. Rosenbrock, Ian H. Sloan, Rodney W. Forcade, Stefano Curtarolo The amount of recent cpu time ($>100$ mega cpu hours) spent in our group on high-throughput materials prediction led us to re-examine convergence issues in standard DFT calculations. For total energy calculations, k-point convergence can be increased two-fold and ten-fold for semiconductor and metals, respectively. For semiconductors, the popular ``rectangle approximation method'' using Monkhorst-Pack grids convergences much faster than expected (for reasons that will be explained), which explains why it gained popularity in the early development of DFT codes. (Its simplicity is also a likely factor.) However, it is not possible to adapt the method to the case of partially-occupied bands in metals while preserving the rapid convergence of semiconductors. Using a rectangle rule for metals, irrespective of any smearing method, leads to the well-known problem that convergence rates are 100s times worse than for semiconductors. Revisiting the k-point integration issue in light of modern DFT practice, we demonstrate that this ``metal deficit'' can be reduced to only a factor of 5--10 worse than semiconductors. The further complication of integrating inside the Fermi surface for metals is solved with our approach without the need for smearing and its associated ad hoc parameters. [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z24.00002: Strongly Interacting Molecular Subsystems Using DFT-in-DFT Embedding Theory with External Orbital Orthogonality Patrick Tamukong, Mark Hoffmann, Yuriy Khait Since most ab initio methods for molecular electronic structure are limited in applicability by polynomially increasing computational costs with system size, localization and embedding techniques are among the leading research efforts in the development of methods that can well describe large systems. Since embedding schemes naturally use a ``divide and conquer'' approach, they are particularly attractive. We recently introduced a new variant of DFT-in-DFT embedding theory that enforces orbital orthogonality between subsystems, thereby completely obviating the use of error-prone kinetic energy functionals; moreover, no calculation of the total system is required at any stage. Here, we present density difference maps and potential energy curves for selected strongly interacting subsystems, including complete dissociation of covalent bonds, obtained with our new embedding protocol. The electron density difference maps presented here compare densities obtained with the new embedding method, and with conventional DFT-in-DFT, to Kohn-Sham (KS)-DFT densities. It is shown that whereas conventional DFT-in-DFT leads to large density deviations, particularly at the interfaces between subsystems, the new method accurately represents the density at all points in space and leads to only negligible density deviations ($\approx $ 10$^{-5}$ e/a$^3_{\circ}$. To our knowledge, this new embedding method is the first variant of DFT-in-DFT to accurately dissociate actual covalent bonds. [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 11:51AM |
Z24.00003: Effect of uniaxial and biaxial strain on the electronic and dynamical properties of CdO: ACBN0 functional study Jacob Gallaway, Priya Gopal, Marco Fornari, Stefano Curtarolo, Marco Buongiorno Nardelli We have investigated the influence of uniaxial and biaxial strain on the electronic and vibrational properties of CdO in the rocksalt structure using Density functional theory calculations with our newly developed ACBN0 pseudo--hybrid Hubbard density functional.\footnote{Agapito, L \textit{et.al.}, arXiv:1406.3259} ACBN0 is a fast, accurate and parameter-free extension of traditional DFT+$U$ proved to correct both the band gap and the relative position of the different bands in transition metal compounds. CdO is a technologically important materials with a direct band-gap of 2.2 eV, which makes it a potential candidate for applications as transparent conductor (TCO) in optoelectronic devices. In such devices, CdO is often grown as a thin film on a substrate and this substrate induces strain that alters the electronic properties. It is clearly very important to understand this phenomenon to properly predict the performance of these devices. All earlier first principles making the system semi-metallic. In this talk, we show that the newly developed ACBN0 functional opens up the gap in closer agreement with the experiments, thus making possible to study band-gap modulation. We will discuss in detail the variation of the electronic and vibrational properties under epitaxial strain. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z24.00004: Bulk and surface properties of rutile TiO$_2$: an ACBN0 case study Laalitha Liyanage, Priya Gopal, Luis Agapito, Marco Fornari, Stefano Curtarolo, Marco Buongiorno Nardelli Using the newly developed \underline{A}gapito--\underline{C}urtarolo--\underline{B}uongiorno--\underline{N}ardelli (ACBN0) functional\footnote{L. Agapito, S. Curtarolo, M. Buongiorno Nardelli, arXiv:1406.3259} we investigate bulk and surface properties of rutile TiO$_2$. ACBN0 is a pseudo-hybrid Hubbard density functional that is a fast, accurate and parameter-free extension of traditional DFT+$U$ that has been proved to correct both the band gap and the relative position of the different bands in transition metal compounds. Within ACBN0, the values of U and J are functionals of the electron density and depend directly on the chemical environment and crystalline field, thus providing a direct way of computing the Hubbard corrections for any individual atom in any local environment. With rutile TiO$_2$ as a stringent test-bed, we have applied ACBN0 to the evaluation of a broad range of physical and electronic properties of the bulk and surfaces ((100), (110), and (001)), including electronic structure, vacancy formation energy, surface formation energy and water adsorption energy. Our results compare favorably with existing GGA, traditional GGA+$U$ and hybrid functional calculations, demonstrating the versatility and accuracy of the ACBN0 approach. [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z24.00005: Improved predictions of the electronic and structural properties of Zn- and Cd- based compounds. An ACBN0 study Priya Gopal, Marco Fornari, Stefano Curtarolo, Luis Agapito, Laalitha Liyanage, Marco Buongiorno Nardelli In this talk, we will present our results of the performance of the recently developed ACBN0 pseudo-hybrid Hubbard density functional in predicting the electronic and structural properties of the Zn- and Cd- based semiconductors. ACBN0 is a fast, accurate and parameter-free extension of traditional DFT+$U$ proved to correct the band gap in transition metal compounds. Within ACBN0, the values of U and J are functionals of the electron density and depend directly on the chemical environment and crystalline field.\footnote{L. Agapito, S. Curtarolo and M. Buongiorno Nardelli, arXiv:1406.3259}) We will compare the structural and electronic properties of ZnX and CdX (X=0,S,Se,Te) semiconductors calculated in \textit{rs},\textit{wz} and \textit{zb} phases using ACBN0 with the results obtained by semi-local PBE, hybrid HSE06 functionals and experiments whenever available. Our results demonstrate that the lattice constants, bulk modulii and band-gaps are more accurately described by ACBN0 compared to the PBE functionals. Overall, we show that ACBN0 is a powerful tool which preserves the accuracy of the HSE calculations with higher computational efficiency. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z24.00006: First-principles Evidence for Intermediate Hole Polarons in ZnO Honghui Shang, Christian Carbogno, Patrick Rinke, Matthias Scheffler, Hikmet Sezen, Fabian Bebensee, Chengwu Yang, Maria Buchholz, Alexei Nefedov, Stefan Heissler, Christof W\"oll We performed density functional theory calculations at the hybrid-functional level (HSE06) to investigate the nature of the polaronic states in ZnO. Our calculations confirm that neither small~(i.e., strong coupling) electron nor hole polarons are stable in ZnO, in agreement with previous studies [1]. The binding energy of large polarons~(i.e., weak coupling) was determined by evaluating the renormalization of the band edges due to the zero-point motion of the atoms [2]. However, for intermediate polarons at intermediate coupling strength, the harmonic approximation breaks down, and there is currently no first-principle theory. We use the HSE06 effective masses to calculate the Fr\"ohlich coupling constants~$\alpha$. Feynman's path integral technique then yields an intermediate hole polaron, whose binding energy of 245~meV and associated peaks in the optical absorption spectrum are consistent with infrared reflection absorption spectroscopy.\\[4pt] [1] J. B. Varley {\it et al.}, Phys. Rev. B \textbf{85}, 081109(R)(2012).\\[0pt] [2] G. Antonius {\it et al.}, Phys. Rev. Lett. {\bf 112}, 215501 (2014) [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z24.00007: Numerical detection of symmetry enriched topological phases with space group symmetry Ling Wang, Andrew Essin, Michael Hermele, Olexei Motrunich Topologically ordered phases of matter, in particular so-called symmetry enriched topological (SET) phases, can exhibit quantum number fractionalization in the presence of global symmetry. In $Z_2$ topologically ordered states in two dimensions, fundamental translations $T_x$ and $T_y$ acting on anyons can either commute or anticommute. This property, crystal momentum fractionalization, can be seen in a periodicity of the excited-state spectrum in the Brillouin zone. We present a numerical method to detect the presence of this form of symmetry enrichment given a projected entangled pair state (PEPS); we study the minima of spectrum of correlation lengths of the transfer matrix for a cylinder. As a benchmark, we demonstrate our method using a modified toric code model with perturbation. An enhanced periodicity in momentum clearly reveals the nontrivial anticommutation relation $\{T_x,T_y\}=0$ for the corresponding quasiparticles in the system. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z24.00008: Chiral d-Wave Superconductivity in coupled ladders Jean Paul Latyr Faye, Syed R. Hassan, P.V Sriluckshmy, Ganapathy Baskaran, David S\'en\'echal We study the Hubbard model on the trellis lattice, a two-dimensional frustrated lattice of coupled two-leg ladders, with hopping amplitude $t$ within ladders and $t'$ between ladders. For large $U/t$ this is a model for the cuprate Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$. We use the variational cluster approximation (VCA), with clusters of sizes 8 to 12. We investigate the phase diagram as a function of doping, $U/t$ and $t'/t$ and find a superconducting dome ending at roughly 20\% doping. Repulsion-induced spin singlet correlations within ladders block inter-ladder single electron tunneling, but allow pair tunneling and help establish 2D superconductivity. However, the nature of the order parameter depends on doping. At small doping ($\delta<3\%$ for $t'=0.15t$ and $U=10t$), the order parameter is real and its interladder component grows steeply with $\delta$. Beyond that value, the order parameter becomes complex for a finite range of doping and gives the bulk chiral, PT violating, two-dimensional superconductivity. In all cases, the ladder component of the order parameter has d-wave character. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z24.00009: Density matrix of disjoint regions as a way of determining dominant correlations in interacting systems Hitesh Changlani, Olabode Sule, Shinsei Ryu In the context of strongly correlated systems, studying the ground state reduced density matrix (or derived quantities, such as the entanglement entropy and spectrum) of a local region has turned out to be useful for characterizing a wide variety of phases. However, to make definitive quantitative mappings of lattice simulations to field theories one needs to go beyond the density matrix of a single region. We use critical spin chains to demonstrate how information from the density matrix of disjoint regions (obtained from the density matrix renormalization group) [1,2] can be used to calculate the low-lying scaling dimensions (and operators) of the corresponding conformal field theory. In a related context, we will also discuss the use of density matrices that involve more than just the ground state, as a way of detecting order in the system [3]. [1] W. Muender, A. Weichselbaum, A. Holzner, J. von Delft, C. L. Henley, New. J. Phys., 12, 075027 (2010) [2] H.J. Changlani, O. Sule, S. Ryu (in preparation) [3] C. L. Henley and H.J. Changlani, J. Stat. Mech. 2014(11), 11002 (2014) [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z24.00010: Real-time decay of a highly excited charge carrier in the one-dimensional Holstein model Lev Vidmar, Florian Dorfner, Fabian Heidrich-Meisner, Christoph Brockt, Eric Jeckelmann We study the real-time dynamics of a highly excited charge carrier coupled to quantum phonons via a Holstein-type electron-phonon coupling [1]. This is a prototypical example for the non-equilibrium dynamics in an interacting many-body system where excess energy is transferred from electronic to phononic degrees of freedom. We use an efficient numerical method [2,3], i.e., diagonalization in a limited functional space, to study the non-equilibrium dynamics on a finite one-dimensional chain. We perform a comprehensive analysis of the time evolution in different parameter regimes by calculating the electron, phonon and electron-phonon coupling energies, and the electronic momentum distribution function. For example, we demonstrate that in the weak coupling regime, the relaxation dynamics obtained from the Boltzmann equation agrees very well with the numerical data. We also study the time dependence of the eigenstates of the single-site reduced density matrix, the so-called optimal phonon modes, unveiling that their structure in non-equilibrium contains very useful information for the interpretation of the numerical data. [1] Dorfner et al, submitted (2014) [2] Vidmar et al, PRB 83, 134301 (2011) [3] Golez et al, PRL 109, 236402 (2012) [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z24.00011: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z24.00012: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z24.00013: Basic Variables of Quantum Mechanics for Electrons in Electrostatic and Magnetostatic Fields Xiao-Yin Pan, Viraht Sahni We consider a system of $N$ electrons in an external electrostatic {\boldmath $\cal{E}$ } = - {\boldmath $\nabla$ } $v ({\bf{r}})$ and magnetostatic ${\bf{B}} ({\bf{r}}) = \nabla \times {\bf{A}} ({\bf{r}})$ fields, and the Hamiltonian to include the interaction of the latter with both the orbital and spin angular momentum. We prove the one-to-one relationship $\{ v ({\bf{r}}), {\bf{A}} ({\bf{r}}) \} \leftrightarrow \{ \rho ({\bf{r}}), {\bf{j}} ({\bf{r}}) \}$, where $\rho ({\bf{r}})$ and ${\bf{j}} ({\bf{r}})$ are the nondegenerate ground state density and physical current density. The proof accounts for the many-to-one relationship between the $\{ v ({\bf{r}}), {\bf{A}} ({\bf{r}}) \}$ and the ground state $\Psi$. In parallel with the Hohenberg-Kohn theorem proof in which the wave function $\Psi$ of the different physical systems considered is constrained\footnote{X.-Y. Pan and V. Sahni, J. Chem. Phys. \textbf{132}, 164116 (2010)} to a fixed electron number $N$, the corresponding $\Psi$ in our proof is constrained to having the same total orbital ${\bf{L}}$ and spin ${\bf{S}}$ angular momentum. Thus, $\{ \rho ({\bf{r}}), {\bf{j}} ({\bf{r}}) \}$ constitute the basic variables in the rigorous HK sense. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z24.00014: Corner Contributions to the Entanglement Entropy of Strongly Interacting Systems in 2+1 Dimensions Edwin Miles Stoudenmire, Peter Gustainis, Ravi Johal, Stefan Wessel, Roger Melko In D=2+1 quantum critical systems, the entanglement entropy of a region with a sharp corner in its boundary contains a subleading logarithmic scaling term with a universal coefficient. In certain cases it is known that this coefficient captures the number of low-energy degrees of freedom in the associated field theory. Using a combination of density matrix renormalization group and numerical linked cluster calculations to isolate the corner coefficient for critical systems in the O(N) Wilson-Fisher universality class, we observe a striking confirmation of the unversality of this quantity and find that, to leading order, the corner coefficient is proportional to the number of field components N. [Preview Abstract] |
Friday, March 6, 2015 2:03PM - 2:15PM |
Z24.00015: Spin-coherent states and instanton calculus on a Riemann surface Tobias Gulden, Michael Janas, Alex Kamenev Semiclassical instanton calculations require solutions to the classical equations of motion, however in complexified phase space of spin-coherent states these are rarely attainable. But identification of the constant energy submanifold of the phase space with a Riemann surface allows to evaluate the semiclassical actions without explicitely knowing the actual classical paths. Furthermore we show that such actions may be solely derived from monodromy properties of the corresponding Riemann surface. Among other results, we prove that the period of quenched tunneling in an external magnetic field is semiclassically exact. [Preview Abstract] |
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