Bulletin of the American Physical Society
Fall 2014 Meeting of the APS New England Section
Volume 59, Number 17
Friday–Saturday, November 7–8, 2014; Boston, Massachusetts
Session E2: Extremely Small Scale Physics |
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Chair: Franz Rueckert, WIT Room: 002 |
Saturday, November 8, 2014 8:30AM - 8:42AM |
E2.00001: Parameterized tight-binding models of ferromagnetic bilayer manganites M. Baublitz, Christopher Lane, Hsin Lin, Hasnain Hafiz, R.S. Markiewicz, B. Barbiellini, Z. Sun, D.S. Dessau, A. Bansil Half-metallic behavior in materials has been a subject of extensive research due to its potential for applications in spintronics. Ferromagnetic manganites have been seen as a good candidate, and aside from a small minority-spin pocket observed in La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ ($x =$ 0.38), transport measurements show that ferromagnetic manganites essentially behave like half metals. Here we develop robust tight-binding models to describe the electronic band structure of the majority as well as minority spin states of ferromagnetic, spin-canted antiferromagnetic, and fully antiferromagnetic bilayer manganites. Both the bilayer coupling between the MnO$_{\mathrm{2}}$ planes and the mixing of the \textbar $x^{2} - y^{2}$ \textgreater\ and \textbar 3$z^{2}$ $- r^{}$ \textgreater\ Mn 3d orbitals play an important role in the subtle behavior of the bilayer splitting. Effects of $k_{z}$ dispersion are included. [Preview Abstract] |
Saturday, November 8, 2014 8:42AM - 8:54AM |
E2.00002: Braids as a representation space of SU(5) Daniel Cartin The Standard Model of particle physics provides very accurate predictions of phenomena occurring at the sub-atomic level, but the reason for the choice of symmetry group and the large number of particles considered elementary, is still unknown. Along the lines of previous preon models positing a substructure to explain these aspects, Bilson-Thompson showed how the first family of elementary particles is realized as the crossings of braids made of three strands, with charges resulting from twists of those strands with certain conditions; in this topological model, there are only two distinct neutrino states. Modeling the particles as braids implies these braids must be the representation space of a Lie algebra, giving the symmetries of the Standard Model. This presentation makes this representation explicit, obtaining the raising operators associated with the Lie algebra of $SU(5)$, one of the earliest grand unified theories. Because the braids form a group, the action of these operators are braids themselves, leading to their identification as gauge bosons. Although this realization of particles as braids is lacking a dynamical framework, it is very suggestive, especially when considered as a natural method of adding matter to loop quantum gravity. [Preview Abstract] |
Saturday, November 8, 2014 8:54AM - 9:06AM |
E2.00003: Modeling magnetic nanoparticle biosensors Daniel Reeves, John Weaver Magnetic nanoparticles have attracted attention as sensors for specific molecules. By choosing an appropriate coating for the particles, they specifically interact with another substance. If the substance of choice is found, the particles can bind to it and form clusters. In clumping, the ensemble rotational Brownian dynamics of the particles are changed drastically, and through magnetic spectroscopy, the change can be quantified. In this work, we show the high sensitivity that is possible using this scheme. Additionally, with stochastic Langevin equation simulations of the nanoparticle dynamics, we show how the increases in the characteristic timescales of the particles due to binding can be modeled using log-normal distributions for particle sizes. [Preview Abstract] |
Saturday, November 8, 2014 9:06AM - 9:18AM |
E2.00004: Van der Waals forces between large molecules and nanoscale structures Allan Pierce Theories of forces between multi-atom structures go back to Fritz London; each structure is an assembly of electrons and nuclei, with net separation distance $R$. Coulomb-potential terms in the overall Hamiltonian, expanded in inverse powers of $R$, yield a leading relevant interaction Hamiltonian that varies asymptotically as $R^{-3}$. A second-order perturbation procedure yields the the correction to the lowest energy level, varying as $R^{-6}$. The principal difficulty is evaluating the matrix elements, products of which appear in each term of the sum giving the coefficient of $R^{-6}$. Each such matrix element involves tensorial products of dipole moment vector operators. An approximation, derived from experimental results used by Denbigh (1940) in regard to the polarizability of molecules, is that the dominant contribution comes from separate covalent bonds and that the contributions are additive. Matrix elements for a given covalent bond can be satisfactorily and simply calculated using an LCAO approximation. An additional approximation is that only two states are relevant for any given covalent bond. Results yield approximate insight that forces should be viewed as forces between bonds and that the forces depend tensorially on the bond directional orientations. [Preview Abstract] |
Saturday, November 8, 2014 9:18AM - 9:30AM |
E2.00005: A Possible Origin of the Fine Structure Constant Ernst Wall We relate the Bohr hydrogen atom to a finite sized, revolving charge, vortex model of the electron that has a light speed Compton wavelength orbit. (Defs: Comptons (or Cmptns)$=$unit length, electron rotation time$=$unit t, Cmptn/rot$=$unit v. ) We utilize their radial dimensions to determine an origin of the Bohr atom's inverse fine structure constant, and that value is 137.019, which is within 120 ppm of observed. Its value is determined primarily by a train of 43 Cmptn wavelets including both those emitted by the electron toward the nucleus and those reflected back to it, this combined distance being approximately twice the value of the Bohr radius. In addition, there is a 0.58969 Cmptn wavelet, whose dimension is determined by a three quarter rotation of the electron charge from its 90 degree emission point, and corrected for the finite sized electron. That value is added to the 43 Cmptns so that 1/$\alpha = $ (43 $+$ 0.58969) $\pi = $ 136.94103 Cmptns. We multiply this by 1.000572, the combined potential well and reduced mass corrections to the wavelength, and we obtain the above value. -- References: \textbf{www.tachyonmodel.com}, Ernst Wall, \textbf{The Physics of Tachyons}, Hadronic Press (1995) [Preview Abstract] |
Saturday, November 8, 2014 9:30AM - 9:42AM |
E2.00006: Super-Adiabatic Particle Number in Schwinger and de Sitter Particle Production Robert Dabrowski, Gerald Dunne We consider the time evolution of the adiabatic particle number in both time-dependent electric fields and in de Sitter spaces, and define a superadiabatic particle number in which the (divergent) adiabatic expansion is truncated at optimal order. In this superadiabatic basis, the particle number evolves smoothly in time, according to Berry's universal adiabatic smoothing of the Stokes phenomenon. This superadiabatic basis also illustrates clearly the quantum interference effects associated with particle production, in particular, for sequences of time-dependent electric field pulses, and in eternal de Sitter space where there is constructive interference in even dimensions and destructive interference in odd dimensions. [Preview Abstract] |
Saturday, November 8, 2014 9:42AM - 9:54AM |
E2.00007: Unexpected Broadband Visible Emission from Sol-Gel Derived Yttrium Silicate Nanopowders under 803.5 nm Laser Diode Excitation Murat Erdem, Joseph Liguori, Bryan Sitt, Gonul Ozen, Baldassare Di Bartolo We report the generation of efficient, wideband white light (WL) with a spec-trum ranging from 400 to 800 nm obtained from sol-gel derived $\gamma $-Y$_{2}$Si$_{2}$O$_{7}$ nano powders with average particle size $\sim$ 40nm when excited with the 803.5 nm emission of a laser diode. The WL intensity was found to increase with decreasing environment pressure and increasing laser pumping power. The decay time was 0.41 ms. The intensity reached its max value18 sec. After switching on the laser. The International Commission on Illumination (CIE) coordinates were found to be $x =$ 0.3408 and $y =$ 0.3100 under 803.5~nm excitation. These values lie in the yellowish region and correspond to color temperature values of 5030~K with a color rendering index of 85. The illuminance value was found to be 1456~lux. [Preview Abstract] |
Saturday, November 8, 2014 9:54AM - 10:06AM |
E2.00008: Kinetics of crystalline to smectic A (K--SmA) phase transition of 4-decyl-4-biphenylcarbonitrile liquid crystal Dipti Sharma In this study crystalline to smectic A (K--SmA) phase transition of 4-decyl-4-biphenylcarbonitrile (10CB) liquid crystal was studied using calorimetric technique. Two types of scans -- heating and cooling were performed from 250 to 350 K, and from 350 to 250 K. A clear difference in K--SmA phase transition was observed between heating and cooling scans. An inclination effect in K--SmA transition was observed on cooling which is completely absent on heating. The inclination of the K--SmA transition peak increased and showed the presence of a heating rate kinetics in the K-SmA transition which can be explained in terms of the presence of time lag and increased activation based on the density and nature of the material. [Preview Abstract] |
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