Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z23: General Physics |
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Sponsoring Units: APS Room: 202B |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z23.00001: Classifying symmetry-protected topological phases through the anomalous action of the symmetry on the edge Dominic Else, Chetan Nayak It is well known that ($1+1$)-D bosonic symmetry-protected topological (SPT) phases with symmetry group $G$ can be identified by the projective representation of the symmetry at the edge. Here, we generalize this result to higher dimensions. We assume that the representation of the symmetry on the spatial edge of a ($d+1$)-D SPT is local but not necessarily on-site, such that there is an obstruction to its implementation on a region with boundary. We show that such obstructions are classified by the cohomology group $H^{d+1}(G, U(1))$, in agreement with the classification of bosonic SPT phases proposed by Chen et al [Science 338, 1604 (2012)]. Our analysis allows for a straightforward calculation of the element of $H^{d+1}(G, U(1))$ corresponding to physically meaningful models such as non-linear sigma models with a theta term in the action. SPT phases outside the classification of Chen et al are those in which the symmetry cannot be represented locally on the edge. With some modifications, our framework can also be applied to fermionic systems in (2+1)-D. [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z23.00002: Topological phase transition in a bilaer toric code model Hong-Chen Jiang, Yuan-Ming Lu, Ashvin Vishwanath We study a bilayer toric-code model in two spatial dimensions by density matrix renormalization group approach. We show that as the interlayer coupling is increased, the system goes through a continuous phase transition from two decoupled copies of $Z_2$ topological orders (bilayer limit) to a single $Z_2$ topological order (monolayer limit). This phase transition is revealed by a jump of topological entanglement entropy. Moreover, the two phases are featured by distinct topological properties: in the bilayer limit the system supports symmetry protected gapless edge states, while the edge states are fully gapped in the monolayer limit. The nature of this continuous topological phase transitions is also investigated. [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 11:51AM |
Z23.00003: Predicting Heat Transport across Multiple Tokamaks with Neural Networks Christopher Luna, Robert Budny, Orso Meneghini, Sterling Smith, James Penna Three multi-layer, feed-forward, back-propagation neural networks have been built and trained on heat transport data from DIII-D, TFTR, and JET respectively. A comparative analysis shows that previous success of neural networks in predicting heat transport in DIII-D [1] is reproduced for TFTR and JET. The effect of using different neural network topologies has been investigated across all of the devices. It is found that the neural networks can consistently predict the total species' heat fluxes for all of the devices, however they have difficulty in predicting the individual components of the heat fluxes in presence of significant transient variations in stored energy (i.e. non-steady-state conditions). Such limitation has been addressed by providing the time-derivative information of the plasma parameters that are input to the neural network. Finally, an attempt is made to draw a connection between the most consistently successful neural network topologies and their relevance to the physics of heat transport in tokamak plasmas. [1] O. Meneghini, et al., Phys. Plasmas 21 (2014) 060702 [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z23.00004: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z23.00005: Incompleteness of General Relativity, Einstein's Errors, and Related Experiments C.Y. Lo General relativity is incomplete since it does not include the gravitational radiation reaction force and the interaction of gravitation with charged particles. Einstein's covariance principle is invalid. There is no bounded dynamic solution for the Einstein equation. Thus, Gullstrand is right and the 1993 Nobel Prize for Physics press release is incorrect. Awards to Christodoulou reflect the blind faith toward Einstein and accumulated errors. The Einstein equation with an electromagnetic wave source has no valid solution unless a photonic energy-stress tensor with an anti-gravitational coupling is added. Thus, the photonic energy includes gravitational energy and Einstein's theory needs improvement. The existence of anti-gravity coupling implies that the energy conditions in space-time singularity theorems of Hawking and Penrose cannot be satisfied in physics. The positive mass theorem of Yau and Schoen is misleading in physics, though considered an achievement by the Fields Medal. E $=$ mc$^{\mathrm{2}}$ is not valid for the electromagnetic energy alone. The discovery of the charge-mass interaction establishes the need for unification of electromagnetism and gravitation and would explain many puzzles. Experimental investigations for further results are important. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z23.00006: Mechanical modeling of structures exhibiting negative thermal expansion Joseph Schick, Andrew Rappe Materials that exhibit negative thermal expansion (NTE) over a wide range of temperatures are of great technological importance in creating materials that are structuraly stable despite being exposed to wide variations in operational temperature. One well-known NTE material, ZrW$_2$O$_8$, has been the subject numerous investigations and yet the details of the underlying causes of NTE in this material are not resolved. We present results from our theoretical investigation of a mechanical model in which the atoms and their bonds are replaced by point masses and anharmonic potentials. We demonstrate that negative thermal expansion can result solely from the differential expansion of the bonds. Implications for materials design will be discussed. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z23.00007: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z23.00008: Derrivitive Structure Enumeration: Trimming a Combinatoric Tree Wiley S. Morgan, Rodney W. Forcade, Conrad W. Rosenbrock, Gus L.W. Hart In computational material science, one frequently needs to have a list of the ``derrivative superstructures'' of a given lattice. For example many phases in metal alloys are merely ``superstructures'' of fcc, bcc, or hcp lattices (L1$_{2}$, B2, D0$_{19}$, etc.). When modeling potential alloys one needs to explore all possible arrangements of atoms on the lattice sites. The simple solution to this combinatorics problem is to generate the list of all possible configurations and then eliminate those that are symmetrically equivalent. This approach, however, suffers from the combinatoric explosion that happens when the supercell size is large or when there are more than two atom types. This problem persists even when there are only a relatively small number of unique configurations that survive the elimination process. Our new algorithm avoids this problem by generating ``partial configurations,'' then using group theory, it eliminates large classes of configurations in a single step. With this approach one can consider larger systems, such as multinary ground state searches, high entropy alloys, etc. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z23.00009: Robust Computational Physics and Automated Sanity Checks Conrad W. Rosenbrock, Gus L.W. Hart A good computational physics course teaches students to say ``well that's completely wrong'' anytime the computer gives them a result. Once cast in doubt, it is the scientist's responsibility to convince themselves that the result is in fact correct. As programs become more complicated, it usually becomes more difficult to guarantee that the final output is right. I will present a new framework that automates the production of robust, high quality Fortran code. The talk will include a brief overview of good coding principles and a demonstration of the most useful features of the framework that help automate implementation of these principles. By providing an XML-based documentation standard and automated unit testing, fortpy$^a$ helps researchers ensure that their code produces accurate physics and is easier to use by others. \vskip 1em \noindent \mbox{[a]} Conrad W. Rosenbrock \textit{Fortpy Auto-completion and Automated Unit Testing for Fortran: https://github.com/rosenbrockc/fortpy} [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z23.00010: Equilibrium shape of colloidal crystals Dimitrios Maroudas, Ray Sehgal Clusters of colloidal crystals exhibit a wide range of size dependent properties. Leveraging such properties requires a strong fundamental understanding of the thermodynamics of colloidal clusters. A first step in developing this understanding is to accurately describe the equilibrium structure and morphology of these assemblies. In this presentation, we report the results of a generalized Wulff construction that is able to accurately describe the equilibrium, i.e., of minimum free energy, shape of an assembly of colloidal particles. The colloidal system that we focus on is modeled with an interparticle interaction consisting of two terms, an electrostatic repulsion and an Asakura-Oosawa (AO) depletion attraction. The generalized Wulff construction can account for both surface facet and surface edge effects on the stable colloidal crystalline morphology. This construction results in a configuration of minimum free energy for given crystal volume. We carry out these equilibrium shape calculations over a range of crystal sizes to examine size dependent effects on the stability of colloidal clusters. These calculations enable the determination of cluster sizes which exhibit improved stability (lower free energy) compared to that of similar-size clusters. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z23.00011: Coarsening Simulation with an Energy-Stable, Semi-Implicit Time Step Benjamin Vollmayr-Lee Coarsening dynamics is effectively described by phase field models, which provide nonlinear field equations of motion that can be integrated numerically. However, these simulations are hampered by a numerical instability that imposes time marching with a fixed-size time step determined by the lattice and not by accuracy considerations. I will present an energy-stable semi-implicit method that removes this time-step restriction, and that has the additional feature of being linear in the implicit fields, allowing for efficient calculation with fast Fourier transforms. Using this method enables simulations to extend decades farther into the coarsening scaling regime. I will report measurements of a variety of coarsening exponents obtained with this method. [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z23.00012: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z23.00013: Developing Accurate and Transferable Artificial Neural Network Potentials for Li-Si Alloys Berk Onat, Ekin Dogus Cubuk, Brad Malone, Efthimios Kaxiras Investigation of the lithiation and delithiation of Si anode in Li-ion batteries using realistic simulations is important and requires large numbers of atoms and long time scales which is generally inaccessible with first-principle approaches. These simulations can be carried out using interatomic potentials that can capture the dependence of structure on chemical composition. Compared with the fixed functional form of empirical potentials, a promising approach to construct the potential energy surface is using artificial neural networks (ANN) that extends the time scales of simulations without sacrificing the accuracy and transferability. Using ab-initio density functional theory data for training, we developed an environment-dependent high-dimensional ANN potential for Li-Si alloys. Our calculations based on the geometry optimizations and molecular dynamic simulations show that the developed potential can accurately predict total energies and equilibrium structures of Li, Si and Li-Si alloys. Using several training databases that include different concentrations of Li in the alloy, we investigated the range of the validity of the ANN potential. Our results show that ANN potentials are widely transferable to Li-Si alloys with various concentrations of Li. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z23.00014: Mass, Energy, Space, and Time Systemic Unified Theory-MEST Dayong Cao Massenergy and spacetime build up a balance system of flat universe; massenergy equals negative spacetime. Like mass attract, opposite mass repel; like energy repel, opposite energy attract; like space attract, opposite space repel; like time repel, opposite time attract. In the balance macrosystem: $http://meetings.aps.org/link/BAPS.2014.MAR.Y33.9$ In the balance microsystem: $E+E'{\psi}=mc^2+m'{\psi}c'^2=0.$ $http://meetings.aps.org/link/BAPS.2011.MAR.K1.68$ There is a unified balance between macro-micro system. In cold area of CMB, the dark massenergy, the spacetime particle which has spacetime center, causes an expanding of period-wavelength of light and redshift which equals negative gravitational redshift of massenergy particle; in hot area, the dark massenergy causes more redshift of light. The sun and dark hole are a balance system-SDS which triggered periodic mass extinctions and created new life on our earth. The quantum orbits both of planets and dark comets of dark hole decided the period. Consciousness remotely change output voltages of solar cell and a balance between Electrons and electron holes which is a negative balance of SDS. By the nuclear energy of spacetime, consciousness should change the orbit of the balance of SDS for avoiding impaction. [Preview Abstract] |
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