Bulletin of the American Physical Society
Joint Fall 2012 Meeting of the Texas Sections of the APS, AAPT, and Zone 13 of the SPS
Volume 57, Number 10
Thursday–Saturday, October 25–27, 2012; Lubbock, Texas
Session E7: Invited & Contributed Papers: Condensed Matter & Materials Physics II |
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Chair: Wilhelmus Geerts, Texas State University Room: Holiday Inn Towers Petroleum B |
Friday, October 26, 2012 3:30PM - 4:10PM |
E7.00001: Integration of functional oxides and semiconductors Invited Speaker: Alex Demkov The astounding progress of recent years in the area of oxide deposition has made possible the creation of oxide heterostructures with atomically abrupt interfaces. The ability to control the length scale, strain, and orbital order in these materials structures offers a uniquely rich toolbox for condensed matter physicists. Because the oxide layers are very thin, the physics is often controlled by the interface. The electronic properties of oxide interfaces are governed by a subtle interplay of many competing interactions such as strain, polar catastrophe, electron correlation, and Jahn-Teller coupling, as well as by defects and phase stability. It is not clear which, if any, of these newly discovered systems will find applications in future high-tech devices. However, they undoubtedly hold tremendous promise, particularly when integrated with conventional semiconductors such as Si. In this talk I will review our recent results in theoretical modeling and experimental realization of several epitaxial oxide heterostructures. I will set the stage with a brief discussion of extrinsic magnetoelectric coupling at the interface of a perovskite ferroelectric and conventional ferromagnet. I will then describe our recent successful attempt to integrate anatase, a photo-catalytic polymorph of TiO$_{2}$, with Si (001) using molecular beam epitaxy. In conclusion, I will talk about strain stabilized ferromagnetism in correlated LaCoO$_{3}$ (LCO) and monolithic integration of LCO and silicon for possible applications in spintronics. The integration is achieved \textit{via} the single crystal SrTiO$_{3}$ (STO) buffer epitaxially grown on Si. Superconducting quantum interference device magnetization measurements show that, unlike the bulk material, the ground state of the strained LaCoO$_{3}$ on silicon is ferromagnetic with a $T_{C}$ of 85 K. [Preview Abstract] |
Friday, October 26, 2012 4:10PM - 4:22PM |
E7.00002: Electronic structure of self-passivated Cu-delafossite nanocrystals Muhammad N. Huda A unique class of highly stabile, self-saturated and self-charge-compensated delafossite nanocrystals has been identified. The density functional theory (DFT) study of structural and electronic properties of these nano-crystalline Cu-based delafossites will be presented. To have a better estimate of the electronic excitation energies, and consequently the optical gap, time dependent DFT has been employed as well. The goal is to show, first of all, that these unique set of nanocrystals exists, and to study whether the nano-phase can enhance the electronic properties for its application as photocatalysts. [Preview Abstract] |
Friday, October 26, 2012 4:22PM - 4:34PM |
E7.00003: Positive Muonium in Indium Oxide Brittany Baker, Y.G. Celebi, R.L. Lichti, P.W. Mengyan Using Muon Spin Relaxation (MuSR) measurements, we are investigating the diffusion of the positively charged ionic state of muonium (muonium is a muon plus a captured electron) defects in In$_{2}$O$_{3}$. The muonium is treated as a light hydrogen analog. Zero field (ZF) measurements were taken from 100 K up to 750 K. This range of temperatures allows for investigation of how the muonium defect center diffuses through the material. The global diffusion barrier energy is being pursued at higher temperatures. In$_{2}$O$_{3}$ is a semiconducting material in the class of transparent conducting oxides (TCO) that are commonly being used in semiconductor optical devices, such as solar cells and LEDs. [Preview Abstract] |
Friday, October 26, 2012 4:34PM - 4:46PM |
E7.00004: Rare earth doped upconverting nano and micron size particles for photonic applications Madhab Pokhrel, Ajith Kumar Gangadharan, Dhiraj Kumar Sardar Upconverting nano and micron size particles are known to exhibit extraordinary characteristics and have a wide range of applications which utilizes their unique properties. A thorough optical characterization of the rare earth doped materials is of critical importance in evaluating them as potential for various photonic applications. My studies include absorption, excitation, emission, and fluorescence lifetime measurements in upconverting powder. Along with the spectroscopic analysis, we have successfully measured the quantum yield for the different concentration of Er/Yb doped M$_{\mathrm{2}}$O$_{\mathrm{2}}$S (M$=$La,Y, Gd). We have found that quantum yield in Er/Yb doped M$_{\mathrm{2}}$O$_{\mathrm{2}}$S is power dependent. Measurements done under the identical condition with respect to reported most efficient upconverting phosphor NaYF$_{\mathrm{4}}$ doped with Er/Yb show that the new phosphor system is three times more efficient. The high quantum yield under near infrared excitation enables the application of upconverting phosphors as potential candidate for biomedical imaging application. Luminescence measurement shows that present upconverting phosphors system is also powerful candidates for display application. [Preview Abstract] |
Friday, October 26, 2012 4:46PM - 4:58PM |
E7.00005: Measurement of Junction Temperature in High Brightness Light-Emitting Diodes Logan Hancock, Mark Holtz LEDs are becoming increasingly important due to the large energy consumption of residential and industrial lighting. This motivates the need for LEDs with high brightness capabilities, with higher electrical compliances than standard LEDs (upwards of 100V/5A), and with long lifetimes. Monitoring the junction temperature in these devices is a significant concern as self heating has effects on efficiency, peak wavelength, output power and overall device lifetime. However, due to the small size and encapsulation of the LED, direct measurement of the junction temperature is difficult and invasive. Spectroscopic methods for assessing the junction temperature suffer from competition between band filling and temperature-induced shifts, making extraction of the junction temperature approximate at best. We present a method involving a straightforward electrical measurement of the forward voltage and drive current. By controlling the ambient temperature of the junction during small pulsed currents, a calibration curve for V$_{f}$ vs T$_{j}$ can be established and used to determine the junction temperature during normal operation levels. The details of the measurement will be discussed as well as the effects of the junction temperature on the electroluminescence emission peak for various sample LEDs. [Preview Abstract] |
Friday, October 26, 2012 4:58PM - 5:10PM |
E7.00006: Thermally Stimulated Impedance Spectroscopy: Setup And Analysis Hanu Arava, Wim Geerts Impedance Spectroscopy involves applying a sinusoidal AC voltage (usually) and observing the current response. It's been gaining momentum as a valuable electrical characterization technique for organic polymers lately due to its speed and reliability along with the extensive analysis that one can perform on the data to obtain vital information (e.g. electron mobility, double layer capacitance). Our setup has the HP4192A Impedance Analyzer as the base system which was connected to a Janis CCS-350S cryostat. The temperature sweeps are controlled by the Lakeshore 331 temperature controller. We wrote the code in Labview (National Instruments) to remotely control the instruments. Preliminary tests were conducted to attain optimal parameters for the instrumentation, e.g. PID loop values for temperature controller, OSC voltage level for the Impedance Analyzer etc. The non-invasive nature of Impedance Spectroscopy additionally helps make it a valuable tool when working with active materials. Our research primarily focuses on highly n-doped polymers and observing their defect behavior. We argue the need for a thermally stimulated set up and will present data from test samples including their equivalent circuit models. [Preview Abstract] |
Friday, October 26, 2012 5:10PM - 5:22PM |
E7.00007: Temperature dependence of the optical properties of VO$_{2}$ deposited on sapphire with different orientations Mohammad Nazari, Yong Zhao, Vladimir Kuryatkov, Zhaoyang Fan, Ayrton Bernussi, Mark Holtz Vanadium dioxide exhibits a reversible first-order metal-insulator phase transition (MIT) at temperature $T_{MIT} = 350$ K. The transformation brings structural phase transition and abrupt changes in electrical conductivity and optical properties. Despite intensive studies of this material, little is understood about the optical properties and their connection with the structural properties across the phase transition. We report spectroscopic ellipsometry and Raman investigations of the optical properties of vanadium dioxide on sapphire substrates with $c-$, $m-$ and $r-$ orientations. For the $m-$ and $r$-plane substrates, VO$_{2}$ is strained such that the material transforms from the monoclinic M$_{1}$ phase directly to the rutile R structure. In contrast, $c$-plane sapphire produces strains favoring transformation from M$_{1}$ into monoclinic M$_{2}$ material, prior to reaching the R phase. These structural differences result in distinct variations of the optical transitions observed in the ellipsometry results. While in $m$-plane sample the energy gap collapses over a narrow temperature range, for the $c$-plane case, a broad temperature range is obtained over which the energy gap is small but not fully collapsed. Raman studies show diverse phonon behavior across the phase transitions. [Preview Abstract] |
Friday, October 26, 2012 5:22PM - 5:34PM |
E7.00008: Calculation of energy barriers for magnetic vortices in sub-100 nm dots Pavel Lapa, Andrew King, Igor V. Roshchin In a magnetic vortex, the magnetization is curling in plane everywhere except the ``core,'' where it is out of plane. Interest in switching of magnetic vortices in nanodots is stimulated by their potential application for magnetic memories and nano-oscillators. By combining analytical and micromagnetic techniques, we calculated energy barriers for vortex switching in 20 nm-thick iron dots as a function of applied in-plane field and dot diameter. Using analytical formula for magnetization distribution in the vortex,\footnote{N. A. Usov and S. E. Peschany, J. Magn. Magn. Mater. \textbf{118}, 290 (1992).} we performed micromagnetic calculations of the dot energy for different vortex core positions. In contrast to the ``rigid body approximation,'' the core size and core shape in our calculations were varied to achieve the energy minimum for every core displacement. The energy barriers required for vortex nucleation and annihilation were calculated as a function of magnetic field. By comparing these barriers to the thermal energy k$_{\mathrm{B}}$T we obtained the temperature dependences of the vortex nucleation and annihilation fields in good agreement with the experiment.\footnote{R. K. Dumas \textit{et al}., Appl. Phys. Lett. \textbf{91}, 202501 (2007).} [Preview Abstract] |
Friday, October 26, 2012 5:34PM - 5:46PM |
E7.00009: Charging of a Fused Silica Optic by an Electrostatic Drive Colin Fitzgerald, Dennis Ugolini Gravitational-wave interferometers with suspended optics may use an electrostatic drive (ESD) as a position fine adjust. An ESD is an electrode pattern printed onto a reaction mass with alternating positive and negative voltages. According to measurements at Moscow State University (MSU), the ESD contributes displacement noise by redistributing charge on the optic. We sought to verify the MSU result by measuring the induced charge distribution from an ESD with a Kelvin capacitive probe. A fused silica optic was exposed to an ESD and then moved via a motorized translation stage to the probe for measurement at high vacuum (10$^{-5}$ torr). Calibration with a known voltage source determined a sensitivity 1V = 6$\times $10$^{-12}$ C/cm$^{2}$. A voltage of $\pm $ 600V was applied to the electrodes for a period of one week, then shut off for one week, and reversed for another week. The charging rate was found to be 7$\times $10$^{-15}$ C/cm$^{2}$/hr, fourteen times slower than the MSU result. The decay time constant found was 3700 $\pm $ 900 hours, about 3 times faster than the MSU result. [Preview Abstract] |
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