Bulletin of the American Physical Society
Joint Fall 2011 Meeting of the Texas Sections of the APS, AAPT, and Zone 13 of the SPS
Volume 56, Number 7
Thursday–Saturday, October 6–8, 2011; Commerce, Texas
Session L5: Condensed Matter Physics and Materials Research IV |
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Chair: Heather Galloway, Texas State University--San Marcos Room: Science Building 127 |
Saturday, October 8, 2011 9:25AM - 9:37AM |
L5.00001: Effect of free-carrier concentration on the phase transition and vibrational properties of VO$_{2}$ Mohammad Nazari, Changhong Chen, Ayrton Bernuusi, Zhaoyang Fan, Mark Holtz Vanadium dioxide (VO$_{2})$ shows a reversible first-order phase transition from low monoclinic semiconductor phase (M1) into high temperature tetragonal metallic rutile phase (R) near temperature 340 K. The metal-insulator transition is accompanied by 4-5 orders of magnitude change in the electrical resistivity and an abrupt change in the optical properties. The effects of native defect doping concentration on the phase transition properties of vanadium dioxide thin films are investigated. The onset temperature of the metal-insulator transition is found to depend on the free-carrier concentration and to correlate with an abrupt change in the temperature dependence of the vibrational energies of the V-O related vibration. Raman scattering study at different temperatures (20-80 \r{ }C) shows the V-O related phonon band at $\sim $614 cm$^{-1}$ behaves differently from the other Raman bands. The dependence of this band on temperature is found to vary for VO2 with different hole concentrations. Based on Raman and electrical resistance measurements, a phase diagram is proposed identifying insulating, intermediate, and conducting regimes. The intermediate region is attributed to a mixed metallic/insulating phase. [Preview Abstract] |
Saturday, October 8, 2011 9:37AM - 9:49AM |
L5.00002: Epitaxial growth of BiFeO$_{3}$ thin films on SrTiO$_{3}$/Si substrates Ryan Laughlin, Daniel Currie, Gokul Radhakrishnan, Weerasinghe Priyantha, Rocio Contreras-Guererro, Ravindranath Droopad, Nikoleta Theodoropoulou We are using molecular beam epitaxy (MBE) to grow BiFeO$_{3}$ (BFO) thin films. SrTiO$_{3 }$ (STO) on Si is used as a virtual substrate to enable the growth of BFO. Commensurate growth of STO on Si using MBE has been achieved by using co-deposition with the fluxes adjusted for stoichiometric growth and the growth rate is determined using RHEED intensity oscillations. The native oxide of the Si substrates is removed in-situ by deoxidation at around 750\r{ }C using a flux of Sr. The substrate is cooled to 500\r{ }C and additional Sr is added to form template with a (2x1) surface structure. BFO is then deposited on well-characterized STO (2-20nm thick) on Si using Fe and oxygen plasma with an overpressure of Bi flux- the growth rate being controlled by the incoming Fe flux. The RHEED pattern taken during deposition of BFO shows 2-D growth front with a 6-fold surface reconstruction. The structural and magnetic properties of the BFO samples have also been measured. [Preview Abstract] |
Saturday, October 8, 2011 9:49AM - 10:01AM |
L5.00003: Magnetic and structural properties of BiFeO$_{3}$ thin films grown epitaxially on SrTiO$_{3}$/Si substrates Daniel Currie, Ryan Laughlin, Gokul Radhakrishnan, Weerasinghe Priyantha, Rocio Contreras-Guererro, Ravindranath Droopad, Nikoleta Theodoropoulou Commensurate growth of SrTiO$_{3}$ (STO) on Si using molecular beam epitaxy (MBE) has been achieved. STO on Si is used as a virtual substrate to enable the growth of BiFeO$_{3}$ (BFO). Having a crystalline oxide surface on Si is an enabler for deposition of other functional oxides that would not have been possible directly on silicon. The dependence of the magnetic and structural properties of BFO on the growth conditions is investigated. The crystalline nature of the BFO film has been confirmed by X-Ray diffraction showing the expected peak positions for (100) oriented oxide films. The BFO/STO/Si films exhibit antiferromagnetic behavior with high transition temperatures, thus leading to the possibility of room temperature magnetoelectric coupling-based devices integrated onto Si CMOS circuitry. Atomic Force Microscopy measurements of such films display rms values indicative of atomically smooth films. [Preview Abstract] |
Saturday, October 8, 2011 10:01AM - 10:13AM |
L5.00004: Electrochemical Performance of Lithium Iron Phosphate Doped with Tungsten Hanu Arava, Andrew Trenchard, Gan Liang, Hui Fang Due to its high thermal stability, low cost and high theoretical charge capacity, LiFePO4 has emerged as one of the most promising cathode materials for large-scale lithium ion batteries. In this work, we systematically investigated the effect on structure and electrochemical properties brought by W doping on Fe site of LiFePO$_{4}$. LiFe$_{1-x}$W$_{x}$PO$_{4}$ (x= 0. 0.01, 0.02, 0.03) samples with and without carbon coating were prepared by using solid-state reaction. The phase and structure of as prepared powders were characterized by X-ray diffraction. Cycling charge and discharge measurement at various C-rates and cyclic voltammetry were employed to reveal the electrochemical properties. Results showed that C coating dramatically improved the capacity at fast C-rate. 2 at.{\%} W doping further enhances the capacity and retention of capacity. [Preview Abstract] |
Saturday, October 8, 2011 10:13AM - 10:25AM |
L5.00005: Temperature dependent conductivity mechanisms in p-type amorphous silicon thin films* K. Shrestha, T.M. Beig, P. Gali, P. Nukala, C. Littler, V.C. Lopes, U. Philipose, A.J. Syllaios Temperature dependent conductivity measurements were performed on p-type $\alpha $-Si:H thin films grown by Plasma Enhanced Chemical Vapor Deposition from 80K to 450K. The purpose of this study was to better understand carrier transport mechanisms in ``as- grown'' and ``annealed'' thin films of $\alpha $-Si. In both samples, the results can be described in terms of two competing, parallel conduction mechanisms, where exponential carrier activation (Arrhenius) conduction dominates at high temperature and a variable range (Mott) hopping conduction dominates at low temperatures. From our analysis, we observed an increase in both the activation energy (from 0.22eV to 0.28eV) and the Arrhenius conductivity prefactor (from 22 $\Omega $-cm$^{-1 }$to 63 $\Omega $-cm$^{-1})$ as a result of annealing. Since annealing minimize voids and defects, the resulting conduction results from an improvement in the amorphous nature of the material. [Preview Abstract] |
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