Bulletin of the American Physical Society
Annual Meeting of the Four Corners Section of the APS
Volume 58, Number 12
Friday–Saturday, October 18–19, 2013; Denver, Colorado
Session K3: Condensed Matter IV: Theory/Computation |
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Chair: Charles Stafford, University of Arizona Room: 281 |
Saturday, October 19, 2013 11:15AM - 11:39AM |
K3.00001: Macroscopic quantum tunneling in Bose-Einstein condensates Invited Speaker: Lincoln Carr I present three studies on macroscopic quantum tunneling of Bose-Einstein condensates. First, I show how even at the simplest mean field level already the problem of escape through a barrier has new features compared to single particle physics: the tunneling time is not the inverse of the rate and interactions allow one to tune from bound to quasi-bound to unbound states freely. Second, I show how tunneling in a double-well system leads to Josephson junction and Schrodinger cat (NOON state) physics. I demonstrate that although a very small bias or tilt in the potential can destroy Cat-like states, by intentional use of bias the many body wavefunction can be used to protect such states from destruction (or internal decoherence). Third, I present a full many-body calculation of entangled quantum dynamics of the escape problem, exploring entanglement, number correlations, and other features not accessible by instanton or other methods. I show that the tunneling process is non-smooth, and actually occurs in bursts. When approximately half the particles have tunneled out of the well, the particles remaining are maximally entangled with the escaped portion. [Preview Abstract] |
Saturday, October 19, 2013 11:39AM - 11:51AM |
K3.00002: Computational Analysis of Energy Pooling to Harvest Low-Energy Solar Energy in Organic Photovoltaic Devices Michael LaCount, Sean Shaheen, Garry Rumbles, Jao van de Lagemaat, Nan Hu, Dave Ostrowski, Mark Lusk Current photovoltaic energy conversions do not typically utilize low energy sunlight absorption, leaving large sections of the solar spectrum untapped. It is possible, though, to absorb such radiation, generating low-energy excitons, and then pool them to create higher energy excitons, which can result in an increase in efficiency. Calculation of the rates at which such upconversion processes occur requires an accounting of all possible molecular quantum electrodynamics (QED) pathways. There are two paths associated with the upconversion. The cooperative mechanism involves a three-body interaction in which low energy excitons are transferred sequentially onto an acceptor molecule. The accretive pathway, requires that an exciton transfer its energy to a second exciton that subsequently transfers its energy to the acceptor molecule. We have computationally modeled both types of molecular QED obtaining rates using a combination of DFT and many-body Green function theory. The simulation platform is exercised by considering upconversion events associated with material composed of a high energy absorbing core of hexabenzocoronene (HBC) and low energy absorbing arms of oligothiophene. In addition, we make estimates for all competing processes in order to judge the relative efficiencies of these two processes. [Preview Abstract] |
Saturday, October 19, 2013 11:51AM - 12:03PM |
K3.00003: The Adsorption of Polyatomics on Carbon Surfaces Jared Burde, Mercedes Calbi We study the adsorption of hydrocarbon chains on several carbon surfaces. We focus on the kinetics of adsorption, working to elucidate the factors that have the greatest influence on the time needed for the system to reach equilibrium. Preliminary results suggest that a major factor is the \textit{effective energy}, which includes the binding energy, interaction energy with neighboring adsorbates, chemical potential and other system parameters. We use computational and analytical techniques to determine the relationship between the adsorption rate and effective energy of several hydrocarbon chains (including methane, ethane, and propane) as they condense on carbon substrates (like graphene and carbon nanotubes). [Preview Abstract] |
Saturday, October 19, 2013 12:03PM - 12:15PM |
K3.00004: Matching up the Kovalev and CDML space-group IRs Rebecca Winzer, Branton J. Campbell, Harold T. Stokes We are building a correspondence between two tables of the irreducible representations (IRs) of crystallographic space groups at incommensurate k-points. The first is the table of little-group IRs prepared by Kovalev. The second is the table of little-group IRs prepared by Cracknell, Davies, Miller, and Love (CDML), which were used to induce the complete space-group IRs recently tabulated by Stokes, Campbell and Cordes. To date, we have established a correspondence between the CDML and Kovalev k-vector tables and setting-dependent space-group operator tables, and have verified that the little-group irrep characters can be matched. A few ambiguities remain to be resolved. [Preview Abstract] |
Saturday, October 19, 2013 12:15PM - 12:27PM |
K3.00005: Porous Resonators for Chemical Sensing Steven Noyce, Robert Davis, Richard Vanfleet Porous resonators have the potential to overcome limitations in the micro-resonator field. For example, such structures are potentially capable of higher detection limits than solid resonators when used as sensors, due to their immensely larger surface area. We present a versatile micro-resonator fabrication process in which carbon nanotubes are grown from a patterned catalyst, after which the space between the tubes is filled to various degrees of porosity with carbon through Chemical Vapor Deposition. We present characterizations of this novel material, along with frequency shift data showing a surprising trend that opens the door to many future explorations. [Preview Abstract] |
Saturday, October 19, 2013 12:27PM - 12:39PM |
K3.00006: The role of shape anisotropy in the stabilization of magnetic antivortices in pound-key like structures Martin Asmat, Lin Li, Brian Shaw, Arabinda Haldar, Kristen Buchanan The magnetic vortex state has received increasing attention during recent years however its topological counterpart, the magnetic antivortex (AV), has not been explored with the same intensity. The antivortex spin configuration may have some advantages over vortices, especially for channeling spin waves emitted from the dynamic core reversal. In order to study the properties of antivortices it is necessary to have geometries and procedures that reliable stabilize antivortices, however this task is more challenging than forming a vortex. In this work, we use micromagnetic simulations to show that pound-key-like structures can be used to form stable AV's and to explore the role of shape anisotropy in the AV formation. Our results are compared to Magneto Optical Kerr Effect (MOKE) hysteresis measurements and magnetic force microscopy images. The simulations show that the AV nucleation field depends on the sample global and relative dimensions and these results are in good agreement with the experiments. Acknowledgement: This work was supported by the NSF. [Preview Abstract] |
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