Bulletin of the American Physical Society
Fall 2015 Joint Meeting of the Texas Section of the AAPT, Texas Section of the APS and Zone 13 of the Society of Physics Students
Volume 60, Number 15
Thursday–Saturday, October 29–31, 2015; Waco, Texas
Session N2: Condensed Matter Physics II |
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Chair: Greg Benesh, Baylor University Room: A.108 |
Saturday, October 31, 2015 10:30AM - 10:42AM |
N2.00001: DFT and DFT$+$U Calculations of Lead Halide Perovskites Modeling Polaron Behavior and Doping Eric Welch, Paul Erhart, Luisa Scolfaro, Alex Zakhidov Due to the ever present drive towards improved efficiencies in solar cell technology, new and improved materials are emerging rapidly. One class of materials, organic halide perovskites, are a promising prospect with efficiencies surpassing 20{\%}. This requires a fundamental understanding of the perovskite structures as well as an explanation for physical phenomena. However, explanations of certain physical phenomena, specifically a high recombination rate and low mobility of charge carriers are still controversial. One possible explanation is the formation of self trapped holes, something seen in perovskites like SrTiO3. Methylammonium lead iodide (MAPbI3) and methylammonium lead iodine doped with chlorine (MAPbI$_{-x}$Cl$_{x})$ are the perovskites studied in this paper as they are the most promising of the organic halide perovskites. It is shown that a partially localized state is realized using DFT$+$U calculations. Then, the addition of chlorine is shown to have an effect on the band structure of the MAPbI such that certain crystal directions have greater dispersion (concavity) and thus a larger mobility. It is predicted that a study of the polaron behavior in the chlorine doped molecule will show a similar localized state as the pure MAPbI molecule. [Preview Abstract] |
Saturday, October 31, 2015 10:42AM - 10:54AM |
N2.00002: Stability study of tin alloyed acanthite Cu$_{\mathrm{2}}$S by cluster expansion method Sajib Barman, Muhammad Huda As a promising solar absorber material, Cu$_{\mathrm{2}}$S is widely known in scientific community. Understanding its complex phase structures and phase stabilities are very important to apply Cu$_{\mathrm{2}}$S as solar absorber materials. Even though recent theoretical study show that acanthite like structure of Cu$_{\mathrm{2}}$S shows good electronic properties with suitable band gap, no experimental finding of this structure has been reported yet. Hence stabilizing this structure remains as a big challenge. In this work alloying Cu$_{\mathrm{2}}$S with tin with a goal to stabilizing this structure will be presented. We have used density functional theory systematically to study this alloyed structures with different tin concentrations in copper layer. Alloy configurations of tin in acanthite Cu$_{\mathrm{2}}$S are determined by cluster expansion method utilizing DFT energies. We will present our results based on these alloy configurations. [Preview Abstract] |
Saturday, October 31, 2015 10:54AM - 11:06AM |
N2.00003: Coupling Reactions of Formaldehyde on Rutile TiO$_{\mathrm{2}}$(110) Zhenrong Zhang, Ke Zhu, Yaobiao Xia, Miru Tang, Zhi-Tao Wang, Igor Lyubinetsky, Gingfeng Ge, Zdenek Dohnalek, Kenneth Park We have studied reactions of formaldehyde on reduced rutile TiO$_{\mathrm{2}}$(110) surfaces using scanning tunneling microscope (STM). Images from the same area at viable temperatures (75 $-$ 170 K) show two coupling reactions channels. Formaldehyde adsorbed on bridging-bonded oxygen vacancy (V$_{\mathrm{O}})$ couples with Ti-bound CH$_{\mathrm{2}}$O form a diolate species, which stays stable at room temperature. Consequently, exposure of formaldehyde at room temperature leads to diolate as the majority species on the surface and no V$_{\mathrm{O}}$-bound formaldehyde is observed. STM images directly visualize a second coupling reaction channel, a low-temperature channel. Two V$_{\mathrm{O}}$-bound formaldehyde molecules can couple and form Ti-bound species, which desorbs above 215 K. This coupling reaction heals both the V$_{\mathrm{O}}$ sites indicating formation and desorption of ethylene. Statistical analysis shows that the total visible reaction products observed on the surfaces can only account for a half of the consumption of the initial V$_{\mathrm{O}}$ coverage, which further supports the desorption of the coupling reaction product, ethylene, after formaldehyde exposures between 215 and 300 K. [Preview Abstract] |
Saturday, October 31, 2015 11:06AM - 11:18AM |
N2.00004: First-Principles Study of the Structural, Electronic, and Vibrational Properties of the Type-II Clathrate Compounds A$_{\mathrm{x}}$M$_{\mathrm{136\thinspace \thinspace }}$(A~$=$~Na,K,Rb,Cs; M~$=$~Si,Ge,Sn; 0$\le $~x$\le $~24) Dong Xue, Charles Myles Powder X-Ray diffraction (XRD) experiments along with density functional theory (DFT) studies of the Type II clathrate compound Na$_{\mathrm{x}}$Si$_{\mathrm{136}}$ have found lattice contraction as the Na content x increases for 0\textless x\textless 8, and lattice expansion as x increases for 8\textless x\textless 24. This unusual phenomenon is explained by XRD data which shows that as x increases, the 28-atom Si cages are filled first by Na guests for x\textless 8 and that the 20-atom Si cages are then filled for x\textgreater 8 [1]. Motivated by this previous work, we have performed a systematic, first-principles computational study of the Type II clathrate compounds A$_{\mathrm{x}}$M$_{\mathrm{136}}$ for 0$\le $x$\le $24. A is an alkali metal atom. M is Si, Ge, or Sn. Our calculations utilize DFT as implemented in the VASP code. We present results for some structural, electronic and vibrational properties of some of these compounds. Our results include lattice parameters, elastic constants, covalent bond lengths, and electronic state densities. Where possible, our results are compared with experiment. We also show that the low frequency, localized guest vibrational modes can be explained by an avoided crossing effect. These low frequency modes should contribute to a minimization of lattice thermal conductivity. [1] M. Beekman, E.N. Nenghabi, K. Biswas, C.W. Myles,~ M. Baitinger, Y. Grin, and G.S. Nolas, \underline {Inorganic Chemistry} \underline {49}, 5338 (2010). [Preview Abstract] |
Saturday, October 31, 2015 11:18AM - 11:30AM |
N2.00005: Minimizing Reflections from Artificial Boundaries in Electronic Structure Calculations G. A. Benesh, Roger Haydock Boundary conditions imposed on a local system that is joined to a larger substrate system often introduce unphysical reflections that affect the calculation of electronic properties. These problems are common in atomic cluster, slab, and supercell calculations alike. However, solutions of the Schr\"{o}dinger equation for a physical system carry current smoothly across the (artificial) boundary between the local system and the underlying medium. Previously, Haydock and Nex derived a non-reflecting boundary condition for discrete systems [Phys. Rev. B 75, 205121 (2006)]. Solutions satisfying this maximal breaking of time-reversal symmetry (MBTS) boundary condition carry current away from the boundary at a maximal rate---in much the same way as the exact wave functions for the physical system. The MBTS boundary condition has proved useful in discrete systems for constructing densities of states and other distributions from moments or continued fractions. The MBTS approach has now been extended to studies employing continuous spatial wave functions. Results are presented for a model system and an Al(001) surface. Comparisons are made with slab calculations, embedding calculations, and experiment. [Preview Abstract] |
Saturday, October 31, 2015 11:30AM - 11:42AM |
N2.00006: PuO$_{\mathrm{2}}$ (111) surface study of electronic and magnetic properties using hybrid density functional theory Shafaq Moten, Raymond Atta-Fynn, Asok Ray, Muhammad Huda PuO$_{\mathrm{2}}$ (111) surface have been investigated using an all-electron hybrid density functional theory. The periodic slab models of the surface, from one to six molecular layers, were examined to find surface properties and slab size effects. Both ferromagnetic (FM) and anti-ferromagnetic (AFM) configurations were considered with and without spin-orbit coupling for the 1x1 slab. The effect of periodicity was explored between AFM configured 1\texttimes 1 and 2\texttimes 2 super cell. Results based on surface energies, work function, band gaps and density of states will be presented. It will be shown that except for the top layer, the surface in general retains the Mott-insulator property. [Preview Abstract] |
Saturday, October 31, 2015 11:42AM - 11:54AM |
N2.00007: Magnets satisfy the Landau-Lifshitz equation AND the Bloch equation. Wayne Saslow Magnets possess distinct contributions to their magnetization. On the one hand, the magneization direction is associated with a quantization axis, and has an associated the equilibrium magnitude M. On the other hand, the excitations -- when the system is out of equilibrium -- are specified by a distribution function with moments that can yield an additional, non-equilibrium magnetization contribution, called the spin accumulation m. This is true for both conductors and insulators. The direction of the quantization axis satisfies the Landau-Lifshitz equation, and with irreversible thermodynamics one can show that the spin accumulation m satisfies a Bloch equation with diffusion. The boundary conditions on the magnetization direction follow from the equations of motion evaluated at the boundaries. The boundary conditions on the spin accumulation m involve the spin flux, and are a generalization of the equation for the bulk spin flux. [Preview Abstract] |
Saturday, October 31, 2015 11:54AM - 12:06PM |
N2.00008: Sheet resistance measurements on a Fe-doped NiO ReRAM Test Chip. James Shook, Yubo Cui, Md Abdul Ahad Taludker, Tang Xi, Greg McClendon, Alex Zakhidov, Luisa Scolfaro, Wilhelmus Geerts Transition metal oxide, specifically NiO, shows promise in its potential application in Resistive Random Access Memory (ReRAM) devices. In ReRam the resistance of a thin oxide layer is reversible switched via soft breakdown between a low and high resistance state. We investigated the sheet resistance of Fe doped NiO test structures. The ReRAM device wafer consisted of a50 nm thick Ni0.9Fe0.1-oxide film sandwiched between a bottom (20 nm Ti/150 nm Co) and top electrode (2 nm Ti/78nm Co). All devices were made by RF sputtering using an AJA system. 2pp and 4pp resistance measurements were made using an HP4145A semiconductor parameter analyzer (SPA) connected to a wafer prober. The measured sheet resistances were: 1.02E9$+$/-0.20E9 Ohm/square for the oxide, 11.3$+$/-1.51 Ohm/square for the bottom electrode, and 51 Ohm/square for the top electrode. Of particular interest is the behavior of the PyO test structures under probing by the SPA in 4pp mode using current sourcing. Initial attempts to obtain resistance resulted in non-linear measurements of Voltage against current. Upon introducing a hold and delay time to the measurements, linearity of voltage versus current emerged suggesting that the oxide test structures displays characteristics of a capacitance device. We acknowledge financial support from Texas State University (Research Enhancement grant) and from DOD (HBCU/MI grant W911NF-15-1-0394). [Preview Abstract] |
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