Bulletin of the American Physical Society
2014 Annual Meeting of the Mid-Atlantic Section of the APS
Volume 59, Number 9
Friday–Sunday, October 3–5, 2014; University Park, Pennsylvania
Session C6: Phase Transitions/Thin Films |
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Chair: Xuemei May Cheng, Bryn Mawr College Room: Life Sciences Building 011 |
Saturday, October 4, 2014 10:30AM - 11:06AM |
C6.00001: Pressure-driven magnetic and structural transitions in the 122-pnictides Invited Speaker: Michael Widom Pnictides of the family AFe2As2, where A is an alkali earth element, exhibit several phase transitions in their structure and magnetic order as functions of applied pressure. We employ density functional theory total energy calculations at T=0K to model these transitions for the entire set of alkali earths (A=Ca, Sr, Ba, Ra) which form the 122 family. Three distinct types of transition occur: an enthalpic transition [1] in which the striped antiferromagnetic orthorhombic (OR-AFM) phase swaps thermodynamic stability with a competing tetragonal phase; a magnetic transition in which the OR-AFM phase loses its magnetism and orthorhombicity; a lattice parameter anomaly in which the tetragonal c-axis collapses. We identify this last transition as a ``Lifshitz transition'' [2] caused by a change in Fermi surface topology. Depending on the element A, the tetragonal state exhibiting the Lifshitz transition might be metastable (A=Ca) or stable (A=Sr, Ba and Ra). \\[4pt] [1] M. Widom and K. Quader, Phys. Rev. B 88 (2013) 045117\\[0pt] [2] I. M. Lifshitz, Sov. Phys. JETP 11, 1130 (1960) [Preview Abstract] |
Saturday, October 4, 2014 11:06AM - 11:18AM |
C6.00002: Geometric Magnetic Frustration in the Double Perovskites Ba2YMoO6, Ba2YRuO6, and Ba2CaOsO6 studied with neutron scattering and muon spin relaxation J.P. Carlo, J.P. Clancy, C.M. Thompson, B.D. Gaulin, J.E. Greedan Geometrically frustrated materials, in which the arrangement of ions inhibits the development of magnetic order, has been of substantial interest owing to their rich phase diagrams featuring exotic ground states and emergent properties. Typically associated with triangular or tetrahedral coordination of antiferromagnetically (AF) coupled moments, frustration manifests in a variety of lattices, including spinel, garnet, pyrochlore and Kagome systems. The double perovskites (DPs) A$_2$BB'O$_6$ also exhibit frustration, with magnetic B' ions comprising a lattice of edge-sharing tetrahedra. DPs can be synthesized with most elements in the periodic table, enabling systematic studies as a function of moment size, lattice distortion, ionic radius and even relativistic spin-orbit coupling. Here we report on neutron scattering and muon spin relaxation measurements of three frustrated DP systems - Ba$_2$YMoO$_6$ (4$d^1$ Mo$^{5+}$), Ba$_2$CaOsO$_6$ (5$d^2$ Os$^{6+}$), and Ba$_2$YRuO$_6$ (4$d^3$ Ru$^{5+}$). Our findings include long-range order with evidence for anisotropy due to spin-orbit coupling in the Ru system, to an exotic gapped state consistent with a spin singlet in the molybdate. These results are compared to other systems and to theoretical expectations. [Preview Abstract] |
Saturday, October 4, 2014 11:18AM - 11:30AM |
C6.00003: Effects of Particle Size on the Magnetic Properties of Maghemite Nanoparticles Kelly L. Pisane, Mohindar S. Seehra The effects of particle size on the magnetic properties of oleic-acid-coated maghemite ($\gamma $-Fe$_{2}$O$_{3})$ nanoparticles (NPs) with average diameters of 3.2 nm and 7.0 nm are reported. These samples were prepared by identical procedures and characterized by x-ray diffraction, transmission electron microscopy, FTIR spectroscopy and temperature-dependent ac and dc magnetometry. The zero field-cooled and field-cooled magnetization M vs. T data under H $=$ 100 Oe yield the blocking temperature T$_{\mathrm{B}} \approx$ 21 K (35 K) for the 3.2 nm (7.0 nm) NPs. Changes in T$_{\mathrm{B}}$ with changes in the measuring frequency f$_{\mathrm{m}}$ (10 Hz to 10 kHz) are used to determine the Neel-Brown relaxation time and the strength of inter-particle interaction. Above T$_{\mathrm{B}}$, the data of M vs. H up to H $=$90 kOe are used to determine magnetic moment per particle and to understand the effects of size distribution on the measured properties. Below T$_{\mathrm{B}}$, the plots of M vs. H show surprisingly negligible hysteresis with coercivity H$_{\mathrm{C}} \approx$ 20 Oe for both NPs. Interpretation of these results will be presented along with comparison with results obtained from bulk maghemite. [Preview Abstract] |
Saturday, October 4, 2014 11:30AM - 11:42AM |
C6.00004: Thermoelectric properties of amorphous ZnO-based materials using {\em Ab initio} methods Anindya Roy, Yu-Ting Cheng, Michael Falk We use a combination of computational methods - viz., molecular dynamics and density functional theory, to predict thermoelectric properties of amorphous ZnO-based materials. We use BoltzTraP [1] to calculate properties such as Seebeck coefficient and electrical conductivity within semiclassical Boltzman transport theory, and compare with available experimental results. Additionally, we investigate the change in the thermoelectric parameters caused by alloying amorphous ZnO with tin and other elements. Our preliminary calculations suggest that the thermoelectric performance of amorphous ZnO is on par with the crystalline counterpart. This is encouraging - since amorphous materials are yet to be studied in depth for their potential as thermoelectric materials, and they could see much improvement with sustained effort. Also, while {\it ab initio} methods are routinely used to predict properties of crystalline systems, their application in amorphous systems is a less-explored area. The present work reports exciting advance in the latter direction. \\[4pt] [1] BoltzTraP: Madsen, G. K. and Singh, D. J., Comput. Phys. Commun. {\bf 175}, 67-71 (2006). [Preview Abstract] |
Saturday, October 4, 2014 11:42AM - 11:54AM |
C6.00005: Control of the interfacial chemical coupling between organic adsorbates and semiconductor surfaces revealed with Raman spectroscopy Floyd Hilty, Andrew Kuhlman, Alexey Zayak In the search for methods of studying chemical properties of surfaces and heterogeneous interfaces we focus on Raman scattering, aiming to reveal physical and chemical processes that vary on the scale of a few chemical bonds, with information not only about a particular chemical species, but also about its immediate chemical environment. While the so-called ``chemical enhancement'' on metallic surfaces has been previously investigated in the context of SERS, in this work we use first-principles calculations to reveal general trends of the chemical modification of Raman spectra of organic species after being chemically absorbed on semiconductor surfaces. We examine the binding of a trans-1,2-two(4-pyridyl) ethylene molecule to various crystallographic facets of a PbSe, showing that Raman spectra of adsorbed species vary significantly on different crystallographic facets of PbSe, which is correlated with the electronic structure of each type of semiconductor surface. Based on that picture, we demonstrate the possibility of tuning the interfacial coupling by applying an external electric potential, achieving not only chemical tunability of the interface, but also a direct method of studying surface chemistry with Raman. [Preview Abstract] |
Saturday, October 4, 2014 11:54AM - 12:06PM |
C6.00006: Detecting the Presence of Quartz-Dissolution Precursors with DFT-based Molecular Dynamics Mark DelloStritto, Jorge Sofo, James Kubicki The study of water/oxide interfaces presents challenges as a result of the low symmetry of the system, the highly structured nature of water, and the difficulty of accurately modelling H-bond interactions. An example is the quartz-water interface, where adding ions to solution increases the dissolution rate by an order of magnitude, but without any change in the activation energy. This suggests that the ions alter the interfacial structure such that the dissolution reaction configuration becomes more accessible to the reactants, thereby decreasing the entropic barrier and increasing the rate of reaction. As the interfacial structure and dissolution reaction are dominated by H-bonding and proton transfer, DFT calculations are useful for simulating the interfacial configuration. We use DFT molecular dynamics simulations of the quartz-water interface with statistical measures of the interfacial structure and vibrational analysis to test whether or not ion-induced stable configurations increase the population of dissolution precursor states. We find that intrasurface proton transfer is likely to be a common precursor when an ion is near the surface, and that both direct proton transfer to the surface and nucleophilic attack of Si by H$_{2}$O are unlikely to be reaction precursors. [Preview Abstract] |
Saturday, October 4, 2014 12:06PM - 12:18PM |
C6.00007: LEED Study of the Structure of $Si(111)-\left( {2\sqrt 3 \times 2\sqrt 3 } \right)R30^{\circ }-4B+14Sn$ Phase Ying-Tzu Huang, Renee Diehl, Weisong Tu, Daniel Mulugeta, Paul Snijders, Paul Kent, Hanno Weitering While doping of bulk materials to control their properties has been hugely successful, doping of inherently low-dimensional surface-based electronic structures remains a challenge: dopant atoms not only affect the electronic carrier density but also significantly disrupt the surface atomic structure, and often lead to localized changes of atomic and electronic properties. We developed a modulation doping approach to dope surface structures with the dopants that are located below the surface of a two-layer thick Sn film on Si(111). The doped Sn film exhibits a temperature dependent structural and electronic phase transition that is absent in the undoped structure. In this study, we have used low-energy electron diffraction (LEED) to determine the surface structure. The LEED experiment was performed at 90 K. The energy range was 40-500 eV, for a total of 7126eV from 20 independent beams. The structure models considered include those based on the scanning tunneling microscopy (STM) images and density functional theory (DFT) calculations. The best agreement obtained was proposed in a recent DFT study produces a Pendry R-factor of 0.36. However, the level of agreement is not yet convincing and many more structure models are being tested. [Preview Abstract] |
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