Bulletin of the American Physical Society
Fall 2011 Meeting of the APS Prairie Section
Volume 56, Number 13
Thursday–Saturday, November 10–12, 2011; Cedar Falls, Iowa
Session F1: Condensed Matter Physics: Experiement and Theory |
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Chair: Carlos Wexler, University of Missouri Room: UNI Center for Enery and Environmental Education Auditorium |
Friday, November 11, 2011 2:00PM - 2:50PM |
F1.00001: Correlating microstructure, charge transport and device performance in small molecule organic photovoltaic cells Invited Speaker: Russell Holmes Organic semiconductors continue to receive attention for use in photovoltaic cells (OPVs) due to their unique optoelectronic properties and compatibility with high-throughput processing. In these materials, the molecular excited state is a tightly-bound, electron-hole pair known as an exciton. Consequently, the generation of photocurrent requires the efficient dissociation of the exciton into its component charge carriers. Often, this is accomplished using a heterojunction between electron donating and accepting materials. In this work we explore the electrical and structural behavior of uniformly mixed films of boron subphthalocyanine chloride (SubPc) and C$_{60}$ and their performance in OPVs. Device performance shows a strong dependence on active layer donor-acceptor composition with peak efficiency realized at 80 wt.{\%} C$_{60}$. The origin of this unusual C$_{60}$-rich optimum composition is elucidated in terms of morphological changes in the active layer upon diluting SubPc with C$_{60}$. While neat SubPc is found to be amorphous, mixed films containing 80 wt.{\%} C$_{60}$ show clear nanocrystalline domains of SubPc. Supporting electrical characterization indicates that this change in morphology coincides with an increase in the hole mobility of the mixture, with peak mobility observed at a composition of 80 wt.{\%} C$_{60}$. Cells constructed using this optimum ratio realize a power conversion efficiency of (3.7$\pm $0.1){\%} under 100 mW cm$^{-2}$ simulated AM1.5G solar illumination. [Preview Abstract] |
Friday, November 11, 2011 2:50PM - 3:02PM |
F1.00002: Physisorption strain in CVD graphene on copper substrates Rui He, Liuyan Zhao, Nicholas Petrone, Michael Roth, James Hone, Philip Kim, Abhay Pasupathy, Aron Pinczuk Strain and morphology of CVD (chemical vapor deposition) graphene layers grown on Cu substrates are studied by Raman spectroscopy and scanning tunneling microscopy (STM). We find that CVD graphene on Cu surfaces are subject to strain which depends on the orientation of the underlying Cu surfaces. The strain is compressive on Cu (111) surface. For graphene grown on Cu (100) surface, the strain is highly nonuniform and includes both compressive and tensile components. Molecular dynamics (MD) simulations show that the compressive strain in graphene on Cu (111) is on the order of 0.5 percent expressed through the presence of hexagonal superstructures and highly compressed domain walls. MD simulations of graphene on Cu (100) show highly nonuniform strain patterns including linear superstructures, consistent with the patterns seen in STM. For graphene grown on Cu foil the strain is partially released after graphene is removed from Cu surfaces and transferred onto oxidized Si substrate. [Preview Abstract] |
Friday, November 11, 2011 3:02PM - 3:14PM |
F1.00003: Measuring Ultrasonic Backscatter in the Presence of Nonlinear Propagation Timothy Stiles, Quinton Guerrero A goal of medical ultrasound is the formation of quantitative ultrasound images in which contrast is determined by acoustic or physical properties of tissue rather than relative echo amplitude. Such images could greatly enhance early detection of many diseases, including breast cancer and liver cirrhosis. Accurate determination of the ultrasonic backscatter coefficient from patients remains a difficult task. One reason for this difficulty is the inherent nonlinear propagation of ultrasound at high intensities used for medical imaging. The backscatter coefficient from several tissue-mimicking samples were measured using the planar reflector method. In this method, the power spectrum from a sample is compared to the power spectrum of an optically flat sample of quartz. The results should be independent of incident pressure amplitude. Results demonstrate that backscatter coefficients can vary by more than an order of magnitude when ultrasound pressure varies from 0.1 MPa to 1.5 MPa at 5.0 MHz. A new method that incorporates nonlinear propagation is proposed to explain these discrepancies. [Preview Abstract] |
Friday, November 11, 2011 3:14PM - 3:26PM |
F1.00004: ABSTRACT WITHDRAWN |
Friday, November 11, 2011 3:26PM - 3:38PM |
F1.00005: One Dimensional Magnetic Dichalcogenide Nanostructures Timothy Kidd, Aaron O'Shea, Paul Shand, Laura Strauss The transition metal dichalcogenides are a class of layered 2D materials with a rich variety of electron and, with doping, magnetic characteristics. They are structurally akin to graphite and, like their carbon based analogs, can be induced to form various low dimensional nanostructures. One dimensional forms are of particular interest, with the dichalcogenides forming nanometer scale tapes, rods, or hollow tubes. Given the novel properties of the bulk analogs of these materials, these 1D nanostructures could provide novel pathways for studying the effects of dimensional confinement. However, one aspect of the macroscopic systems that has not been repeated for the 1D nanostructures is the ability to dope the nanostructures with magnetic ions. By modifying the synthesis process, we have been able to overcome this obstacle to create highly 1D dichalcogenide structures with spin-glass like or ferromagnetic ground states. We hope to extend this technique to be able to create a variety of such nanostructures to explore how quantum size effects can alter the base magnetic properties of these materials. [Preview Abstract] |
Friday, November 11, 2011 3:38PM - 3:50PM |
F1.00006: Magnetic clustering at a quantum critical point: A modified percolation theory John Gaddy, Tom Heitmann, Jagat Lamsal, Wouter Montfrooij The formation and dynamics of magnetic clusters have proven to be important for materials that have been driven to a quantum critical point via substantial chemical substitution. Tiny variations in the local exchange interaction lead to the formation of a distribution of Kondo temperatures, which in turn gives rise to a fragmentation of the magnetic lattice. Importantly, the temperature scale at which the clusters form is typically low enough that finite-size effects force the clusters to order internally as soon as they form. We argue that this process defies description by a standard percolation model but that a slight restriction-- whereby ordered clusters are not allowed to be broken up-- succeeds, but with the consequence that a new universality class emerges. We demonstrate this model with analytics as well as Monte-Carlo numerical results. [Preview Abstract] |
Friday, November 11, 2011 3:50PM - 4:02PM |
F1.00007: Skyrmions in Solids K.V. Shanavas, S. Satpathy The recent observation of skyrmions in magnetic solids has raised considerable interest in this new magnetic state. The skyrmion state is a novel, vortex-like spin structure in a magnetic solid, anticipated to produce unconventional spin-electronic functions such as the topological Hall effect. Experiments have confirmed their existence in crystals such as MnSi and FeGe when subjected to small magnetic fields. Competition between ferromagnetic exchange, the Dzyaloshinskii-Moriya interaction, and the Zeeman coupling to the external magnetic field are expected to stabilize this unique magnetic arrangement. Of these, the anisotropic Dzyaloshinskii-Moriya $\vec{D}\cdot\vec{S_i}\times\vec{S_j}$ interaction, can exist in crystals with spin-orbit interaction but no inversion symmetry. We discuss the origin of the Dzyaloshinskii-Moriya interaction in these solids based on density-functional study of the electronic structure of MnSi, the prototypical solid for skyrmions. The present state of research in this area as well as open questions and future outlook will be discussed. [Preview Abstract] |
Friday, November 11, 2011 4:02PM - 4:14PM |
F1.00008: The reversibility of the adsorption of methane-methyl mercaptan mixtures in nanoporous carbon Carlos Wexler, Monika Golebiowska, Lucyna Firlej, Bogdan Kuchta, Michael Roth The results of extensive molecular dynamics simulations and theoretical considerations of the adsorption of methane-methyl mercaptan mixtures in slit-shaped carbon nanopores are presented. We observe significant mobility of both methane and mercaptan molecules within the pore volume, between pores, and between adsorbed and gas phases for a wide range of temperatures and pressures. Although mercaptans adsorb preferentially relative to methane, the process remains reversible, provided non-oxidizing conditions are maintained. A mercaptan/methane ratio of the order of 200 ppm in the adsorbed phase is sufficient for the gas phase to have a mercaptan concentration above the human threshold for detection. The reversibility of the adsorption process and low concentration of mercaptans makes it unlikely that these would be harmful for adsorbed natural gas storage systems. See also: doi:10.1016/j.carbon.2011.08.039. [Preview Abstract] |
Friday, November 11, 2011 4:14PM - 4:26PM |
F1.00009: Analysis of dimer dynamics with an enhanced bond-fluctuation model Frank Bentrem A recently introduced enhancement to the bond-fluctuation model has been shown to both increase efficiency and extend the applicability of the bond-fluctuation model for polymer simulations. In order to better understand the increase in performance, a theoretical analysis of the dynamics for the simplest case--an isolated dimer--is presented along with a comparison between the original and enhanced bond-fluctuation models. In particular, we find the equilibrium bond-length probability distribution for the dimer using each of the models. [Preview Abstract] |
Friday, November 11, 2011 4:26PM - 4:38PM |
F1.00010: Many-body calculation of spin-orbit induced mixed-parity pairing in Sr$_2$RuO$_4$ John Deisz, Tim Kidd The unusual superconducting state in Sr$_2$RuO$_4$ has long been viewed as being analogous to a superfluid state in liquid $^3$He. Nevertheless, calculations based on this odd-parity state are presently unable to completely reconcile the properties of Sr$_2$RuO$_4$. Using a self-consistent quantum many-body scheme that employs realistic parameters, we are able to model several signature properties of the normal and superconducting states of Sr$_2$RuO$_4$. We find that the dominant component of the model superconducting state is of even parity and closely related to superconducting state for the high-$T_c$ cuprates although a smaller odd-parity component is induced by spin-orbit coupling. This mixed pairing state gives a more complete representation of the complex phenomena measured in Sr$_2$RuO$_4$. [Preview Abstract] |
Friday, November 11, 2011 4:38PM - 4:50PM |
F1.00011: Observation of quantum criticality with ultracold atoms Cheng Chin As the temperature approaches absolute zero, thermal fluctuations of observables cease and quantum fluctuations dominate. Competition between different energies, such as kinetic energy, interactions or thermodynamic potentials, can induce a quantum phase transition between distinct ground states. Near a continuous quantum phase transition, the many-body system is quantum critical, exhibiting scale invariant and universal collective behavior. We report the observation of quantum critical behavior in a two-dimensional Bose gas in optical lattices near the vacuum-to-superfluid quantum phase transition. We observe universal scaling of the equation of state at sufficiently low temperatures, locate the quantum critical point, and determine the critical exponents. The universal scaling laws also allow determination of thermodynamic observables. Our experiment provides a prototypical method to study quantum criticality with ultracold atoms, and prepares the essential tools for further study on quantum critical dynamics. [Preview Abstract] |
Friday, November 11, 2011 4:50PM - 5:02PM |
F1.00012: Fisher's zeros, complex RG flows and confinement in LGT models Yannick Meurice, Alexei Bazavov, Alan Denbleyker, Daping Du, Yuzhi Liu, Bugrahan Oktay, Don Sinclair The zeros of the partition function in the complex beta plane (Fisher's zeros) play an important role in our understanding of phase transitions and RG flows. Recently, it has been argued that they act as gates or separatrices for complex RG flows. Using histogram reweighting to construct the density of states, we calculate the Fisher's zeros for pure gauge SU(2) and U(1) on $L^4$ lattices. For SU(2), these zeros appear to move almost horizontally when the volume increases. They stay away from the real axis which indicates a confining theory at zero temperature. We discuss the effect of an adjoint term on these results. In contrast, using recent multicanonical simulations for the U(1) model for L up to 8 we find that the imaginary part of the zeros scales as $L^{-3.07}$ and pinches the real axis at beta near 1.0113. Preliminary results concerning higher volumes will be presented. We will also discuss recent results for SU(3) with various numbers of flavors. [Preview Abstract] |
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