Bulletin of the American Physical Society
79th Annual Meeting of the APS Southeastern Section
Volume 57, Number 16
Wednesday–Saturday, November 14–17, 2012; Tallahassee, Florida
Session LB: Condensed Matter Physics II |
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Chair: Laurie McNeil, University of North Carolina - Chapel Hill Room: DoubleTree Salon AB |
Saturday, November 17, 2012 8:30AM - 8:42AM |
LB.00001: Heatless Motive Force for Producing Grid Turbulence in Liquid Helium near Absolute Zero Gary Ihas, Kyle Thompson, Roman Chapurin, Lydia Munday, Greg Labbe, Peter McClintock Flow through a grid is a standard method to produce isotropic, homogeneous turbulence for laboratory study. This technique has been used to generate quantum turbulence (QT) above 1 K in superfluid helium,\footnote{S. R. Stalp, L. Skrbek, and R. J. Donnelly, \textit{Phys. Rev. Lett}. \textbf{\textit{82}}, 4831 (1999).} where QT seems to mimic classical turbulence. Efforts have been made recently\footnote{G.~G.~Ihas, G.~Labbe, S-c.~Liu, and K.~J.~Thompson,\textit{J. Low Temp. Phys}. \textbf{150}, 384 (2008).} to make similar measurements near absolute zero, where there is an almost total absence of normal fluid and hence classical viscosity. This presents the difficulty that most motive force devices produce heat which overwhelms the phenomena being investigated. The process of designing and implementing a ``dissipation-free'' motor for pulling a grid through superfluid helium at millikelvin temperatures has resulted in the development of new techniques which have broad application in low temperature research. Some of these, such as Meissner-effect magnetic drives, capacitive and inductive position sensors, and magnetic centering devices (bearings) will be described. [Preview Abstract] |
Saturday, November 17, 2012 8:42AM - 8:54AM |
LB.00002: The Design and Testing of a Device to Limit Superfluid Helium Film Flow David G. Haase, Dillon K. Frame, James R. Rowland A proposed new measurement of the electric dipole moment of the neutron (the nEDM project) uses a volume of superfluid liquid $^4$He to trap neutrons from the Spallation Neutron Source as ultracold neutrons. Polarized $^3$He atoms dissolved in the liquid serve as a co-magnetometer. The measurement process will require the distillation of 10$^{-10}$ concentrations of depolarized $^3$He from liquid $^4$He. Unfortunately, a superfluid $^4$He film flows up the walls of its container where it eventually evaporates and would overwhelm the $^3$He vapor removal. Following a design employed by NASA on the cryogenic helium-cooled XRS satellite [1] we have had manufactured at NCSU 300 micron thick, 0.75 inch square silicon wafers which include gas flow orifices and rows of etched ridges. The reduced radius of curvature of the superfluid film at the atomically sharp ridges reduces the film thickness to stop film flow. We have designed and constructed a closed-cycle cryocooler-based cryostat to test and to characterize these silicon ``film pinners.'' We will discuss the design and operation of the cryostat and initial tests of the etched pinners. \\[4pt] [1] P. J. Shirron and M. J. DiPirro, Adv. in Cryo. Engineering, vol. 43, p. 949, 1998. [Preview Abstract] |
Saturday, November 17, 2012 8:54AM - 9:06AM |
LB.00003: Diverging Thermodynamic Derivatives in Critical Phenomena in a Binary Liquid Mixture James Baird The opposite sides of the coexistence curve of a binary liquid mixture with a miscibility gap converge at the critical solution temperature where certain of the thermodynamic derivatives go to infinity [1]. We examine three cases of solids in contact with a mixture of isobutyric acid $+$ water, which has a critical solution temperature near 26 C: (a) The temperature derivative of the solubility, s, of a metal oxide goes to infinity as the temperature, T, approaches the critical temperature [2]. (b) When charcoal comes into contact with the liquid mixture, the derivative of isobutyric acid mole fraction with respect to chemical potential goes to infinity as T approaches the critical temperature [3]. (c) When the hydroxide form of an anion exchange resin comes into contact with isobutyric acid $+$ water, the isobutyrate anion exchanges with the hydroxide ion. If the fraction of resin sites occupied by isobutyrate anions is assumed to be governed by the Langmuir adsorption isotherm, then the derivative of isobutyric acid mole fraction with respect to resin site mole fraction diverges as T approaches the critical temperature. We will show that all of these diverging derivatives follow as a consequence of the principle of critical point universality [1].\\[4pt] [1] R. B. Griffiths and J. C. Wheeler, Phys. Rev. A 2, 1047 (1970).\\[0pt] [2] B. Hu, J. K. Baird, R. D. Richey, and R. G. Reddy, J. Chem. Phys. 134, 154505 (2011).\\[0pt] [3] T. J. Giesy, A. S. Chou, R. L. McFeeters, J. K. Baird, and D. A. Barlow, Phys. Rev. E 83, 061201 (2011). [Preview Abstract] |
Saturday, November 17, 2012 9:06AM - 9:18AM |
LB.00004: Superconducting Properties of Nb/Ni Bilayers Timothy Ahrenholz, Emily Davis, Phillip Broussard We grew Nb/Ni bilayers in order to observe their behavior as it compares to Usadel theory. The bilayers themselves were grown in a magnetron sputtering system, each consisting of a 33 nm film of Nb and a 0-7 nm film of Ni. These films were deposited on Si wafers. Measurements of the thicknesses of the films were performed with a contact profilometer. We measured the transition temperatures ($T_c$) of the films by cooling them as low as 5 K in a Janis Cryocooler. We measured the $T_c$ with four different methods to insure accuracy: resistively, inductively, through third harmonic analysis, and resistively from upper critical field measurements. The $T_c$ with respect to Ni layer thickness for each of these measurements seemed to confirm Usadel theory's predictions to a great extent. In the case of the resistive measurements, the $T_c$ dropped from 8.31 K to 6.66 K as the thickness of Ni went from 0 nm to 0.48 nm, but then it rose to 7.28 K as Ni thickness reached 3.97 nm, and thereafter remained constant. This pattern of behavior was evident across each of the measurements of $T_c$. [Preview Abstract] |
Saturday, November 17, 2012 9:18AM - 9:30AM |
LB.00005: Predicting the temperature for the solid-solid phase transition in ZnS and CdS Douglas Barlow At atmospheric pressure, many of the II-VI semiconducting alloys are known to undergo a zinc-blende to wurtzite solid-solid transition at elevated temperatures below the melting point. Few experimental values for these transitions temperatures have been reported. We show here that chemical potentials for one of the components in a solid solution with the other can be used to estimate the transition temperature. The non-ideal behavior of the solvent component is addressed via an activity coefficient which is computed using the quasi-chemical model. The chemical potentials for each case, zinc-blende and wurtzite are then taken to be equal at the transition temperature. Predicted transition temperatures are reported here, and compared with the experimental values for ZnS and CdS. [Preview Abstract] |
Saturday, November 17, 2012 9:30AM - 9:42AM |
LB.00006: Highly tunable electron transport in epitaxial topological insulator (Bi$_{\mathrm{1-x}}$Sb$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ thin films Tong Guan, Xiaoyue He, Kehui Wu, Yongqing Li Three dimensional topological insulators have emerged as a novel type of quantum materials that may lead to ground-breaking applications such as quantum computation and spintronic devices. These applications, however, often require an insulating bulk. A lot of progress has been made in suppressing the bulk conductivity. Here we report the growth of single crystalline (Bi$_{\mathrm{1-x}}$Sb$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ films on SrTiO$_{\mathrm{3}}$(111) substrates by molecular beam epitaxy (MBE). A full range of Sb-Bi compositions have been studied in order to obtain the lowest possible bulk conductivity. For the samples with optimized Sb compositions (x $=$ 0.5$\pm$0.1), the carrier type can be tuned from n-type to p-type with the help of a back-gate. Linear magnetoresistance has been observed at gate voltages close to the maximum in the longitudinal resistance of a (Bi$_{\mathrm{1-x}}$Sb$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ sample. These highly tunable (Bi$_{\mathrm{1-x}}$Sb$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ thin films provide an excellent platform to explore the intrinsic transport properties of the three dimensional topological insulators. [Preview Abstract] |
Saturday, November 17, 2012 9:42AM - 9:54AM |
LB.00007: Magnetic and Structural Investigations on As-grown and Annealed Pulsed Laser Deposited SnO$_{2}$:Co Thin Films Gratiela M. Stoian, P.A. Stampe, R.J. Kennedy, Y. Xin, S. von Molnar Dilute Magnetic Semiconductor SnO$_{2}$:Co films were deposited on r-cut sapphire substrates via PLD from a doped target with a nominal Co concentration of 5 at.{\%}. To study the role of oxygen vacancies and other defects in tuning the ferromagnetic (FM) and electrical properties of these materials, films were deposited at different growth rates, temperatures and oxygen pressures. In addition, some films were annealed at various conditions. For samples grown at optimal conditions, magnetometer measurements show that films are FM at room temperature for thicknesses between 30 nm and 100 nm, with a saturation magnetization of approximately 20 emu/cm$^{3}$. Crystallinity improves as the thickness decreases and the moment per surface area varies linearly with the film thickness increase, suggesting the magnetism in our materials is a volume property of the films rather than a surface effect. Moreover, we have also noticed a decrease of the saturation magnetization with increasing growth rate. Annealing films grown at a higher deposition rate than optimal under the same conditions used for their growth, led to an initial rapid increase in the saturation magnetization for short annealing times followed by constant saturation magnetization after further annealing. [Preview Abstract] |
Saturday, November 17, 2012 9:54AM - 10:06AM |
LB.00008: The Density of States: Counting that Counts Gregory Brown Wang-Landau method and associated Multicanonical Monte Carlo methods make it possible to calculate the density of states g(E) for thermodynamic systems. This is particularly useful for first-principles calculations, where temperature-dependent quantities like the specific heat and susceptibility can be calculated from, say, 50000 configurations. We have successfully employed an approach combining the classical Heisenberg model of magnetism with first-principles LSMS computations to calculate Tc in transition metal alloys. The thermodynamic properties of the system can be calculated directly from the ln[g(E)] and its derivatives. [Preview Abstract] |
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