Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D4: Undergraduate Research/Society of Physics Students II |
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Sponsoring Units: APS Chair: Cortney Bougher, American Physics Society Room: Mayor Cockrell Room 004 |
Monday, March 2, 2015 2:30PM - 2:42PM |
D4.00001: Photoresponsive memory device based on Graphene/Boron Nitride heterostructure Salman Kahn, Jairo Velasco Jr, Long Ju, Dillon Wong, Juwon Lee, Hsin Zon Tsai, Takashi Taniguchi, Kenji Watanabe, Alex Zettl, Feng Wang, Michael Crommie Recent technological advancements have allowed the stacking of two dimensional layered material in order to create van der Waals heterostructures (VDH), enabling the design of novel properties by exploiting the proximal interaction between layers with different electronic properties. We report the creation of an optoelectronic memory device using a Graphene/Boron Nitride (hBN) heterostructure. Using the photo-induced doping phenomenon, we are able to spatially ``write'' a doping profile on graphene and ``read'' the profile through electrical transport and local probe techniques. We then utilize defect engineering to enhance the optoelectronic response of graphene and explore the effect of defects in hBN. Our work introduces a simple device architecture to create an optoelectronic memory device and contributes towards understanding the proximal effects of hBN on Graphene. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D4.00002: High quality factor titanium nitride and aluminum resonators for increased superconducting qubit coherence N.E. Frattini, A. Dove, D.M. Toyli, S. Hacohen-Gourgy, A. Eddins, I. Siddiqi Superconducting qubits have successfully realized effective two--level quantum systems whose state can be read out by dispersive coupling to a linear resonator. Superconducting films which exhibit low loss in the microwave frequency regime at millikelvin temperatures and single photon excitation powers are an essential ingredient in realizing high-coherence qubits and high-fidelity readout. To explore the magnitude of these losses and their correlation with fabrication recipes, we prepare and characterize both lumped element and distributed element resonators derived from titanium nitride on silicon and aluminum on sapphire. We study the role of substrate annealing, film growth conditions, and lithographic technique on resonator quality factor. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D4.00003: the Characteristic Phase Transitions of Co-doped BaFe2As2 Synthesized via Flux Growth C.H. Shea, C. Roncaioli, C. Eckberg, T. Drye, M.C. Sulliavan, J. Paglione Since the discovery of a new family of type II superconductors in 2008, the iron pnictides, researches have had suspicions that they might bear similar electronic properties to the well-known (but not easily understood) oxide superconductors. For this reason studies on this family of compounds has been of great interest to the materials science community. Our efforts have been aimed at single crystal growth and measurement of a particular member of this family, BaFe2As2. While this material is not superconducting at standard pressure, the partial substitution of cobalt on the iron site has been shown to suppresses an anti-ferromagnetic phase transition occurring at lower temperatures allowing for the appearance of a superconducting phase. Transport and low field magnetization measurements taken on our samples show clean transitions, indicating Tc's of up to 24 K in optimally doped samples. We will discuss the growth methods and temperature dependent phase transitions of this material at different cobalt concentrations. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D4.00004: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 3:18PM - 3:30PM |
D4.00005: Measurements of Viscosity and Dynamics of Thin Films of Organic Glass TPD via Hole Growth Dewetting Studies Kareem Wahid, Yue Zhang, Mu Li, Zahra Fakhraai In this study, we aim to measure the viscosity of thin glassy films of the small organic molecule N,N$\prime $-Bis(3-methylphenyl)-N,N$\prime $-diphenylbenzidine (TPD). Organic glasses such as TPD have various applications in organic light emitting diodes (OLED), and organic photovoltaics. An understanding of the origin for nano-scale properties (e.g. viscosity) would allow for better design of such devices in future applications. Viscosity is simple to measure in bulk systems but challenging at the nanometer scale. Dewetting experiments provide a simple and non-invasive method to measure viscosity in thin film systems. By following dewetting kinetics over time, we are able to identify material related viscous dissipation and substrate related frictional dissipation involved during dewetting. Both homogeneously and heterogeneously nucleated holes have been observed on TPD films of various thicknesses or various substrates. There is reasonable agreement between these observations and with full-slip dewetting models among heterogeneously nucleated holes. However, this is not observed in homogeneously nucleated holes. Careful substrate treatment and control of substrate properties influence the slip length and the dewetting dynamics. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D4.00006: Monitoring Residual Solvent Additives and Their Effects in Solution Processed Solar Cells Derek M. Fogel, James I. Basham, Sebastian Engmann, Sujitra J. Pookpanratana, Emily G. Bittle, Oana D. Jurchescu, David J. Gundlach High boiling point solvent additives are a widely adopted approach for increasing bulk heterojunction (BHJ) solar cell efficiency. However, experiments show residual solvent can persist for hours after film deposition, and certain common additives are unstable or reactive. We report here on the effects of residual 1,8-diiodooctane on the electrical performance of poly(3-hexylthiophene-2,5-diyl) (P3HT): phenyl-C71-butyric acid methyl ester (PC[71]BM) BHJ photovoltaic cells. We optimized our fabrication process for efficiency at an active layer thickness of 220 nm, and all devices were processed in parallel to minimize unintentional variations between test structures. The one variable in this study is the active layer post spin drying time. Immediately following the cathode deposition, we measured the current-voltage characteristics at one sun equivalent illumination intensity, and performed impedance spectroscopy to quantify charge density, lifetime, and recombination process. Spectroscopic ellipsometry, FTIR, and XPS are also used to monitor residual solvent and correlated with electrical performance. We find that residual additive degrades performance by increasing the series resistance and lowering efficiency, fill factor, and free carrier lifetime. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D4.00007: Solution Processed Polymeric Semiconductors for Carbon Nanotube-Enabled, Vertical, Organic Field Effect Transistors Alexander Schachtner, Nicholas S. Cunningham, Christopher C. Samouce, Maxime G. Lemaitre, Andrew G. Rinzler Carbon nanotube-enabled, vertical, organic field effect transistors (CN-VFETs) based on the small molecule dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) have demonstrated high current, low-power operation suitable for driving active matix organic light emitting diode (AMOLED) displays [1]. This performance is achieved without the need for costly high-resolution patterning, despite the low mobility of the organic semiconductor, by employing sub-micron channel widths, defined in the vertical devices by the thickness of the semiconducting layer. Replacing the thermally evaporated small molecule semiconductor with a solution-processed polymer would possibly further simplify the fabrication process and reduce manufacturing cost. Here we investigate several polymer systems as wide bandgap semiconducting channel layers for potentially air stable and transparent CN-VFETs. The field effect mobility and optical transparency of the polymer layers are determined, and the performance and air stability of CN-VFET devices are measured. 1. M. A. McCarthy et al. Science 2011 332, 570 [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D4.00008: Synthesis and Characterization of Rare Earth Nanoparticles in a non-aqueous environment E.I. Paredes Aulestia, R.H. Fukuda, M.M. Castro De La Torre, P.-C. Ho, S. Attar, M. Golden, D. Margosan Magnetic nanoparticles have several potential applications, such as in biomedicine and for magnetic information storage due to their reduced size and magnetization properties. We synthesize gadolinium and neodymium nanoparticles by applying the reverse micelle method. This method consists of using a surfactant with a large nonpolar-solvent-to-polar-solvent ratio to form spherical cages around a reactant. Most studies related to the reverse micelle method use water as the polar solvent, but the use of water is not suitable for our project since both Gd and Nd are highly reactive in water. Instead, we employ methanol as our polar solvent. Hexane and heptane are tested as nonpolar solvents. DDAB and AOT are used as surfactant molecules. A solution containing a reducing agent is then added to produce the desired Nd and Gd nanoparticles. Our samples are analyzed using light microscopy, SEM (Scanning Electron Microscopy) and EDX (Energy Dispersive X-ray). We found heptane, methanol and AOT produce Neodymium particles with diameters less than 5 micrometers. Heptane, methanol and DDAB yield Gadolinium particles with diameters less than 1 micrometer. The synthesis procedure is currently being revised, in order to produce cleaner samples and particles of smaller size. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D4.00009: Atomic Structure of Grain Boundaries in Graphene Otho Ulrich, Joseph Gonzalez, Kien Nguyen Cong, Ivan Oleynik In its pristine form, graphene is one of the strongest materials measured, and possesses a wide range of technologically appealing characteristics. Several recent experiments have explored the mechanical properties of graphene which contains grains, some with contradictory results. To explore the atomic structure of grain boundaries in graphene, we employ a complex of computational approaches. A set of unit cells of graphene bicrystals with variable grain misorientation is generated by applying a conjugate gradient method with periodic boundary conditions using the SEDREBO potential for carbon-carbon interaction. Structures are classified by formation energy and atomic coordination, and identification of physically viable samples is achieved using these statistics. The defective regions constituting the grain boundaries are defined using the atomic energy distribution. Formation energies of any viable structures are normalized according to cell height and compared by indexing misorientation angles. Lack of a functional relationship between misorientation angle and formation energy indicates a greater complexity in the mechanisms of the grain boundaries. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D4.00010: Phase Boundaries of the Pseudogap Anderson Impurity Model Aaron Mohammed, Tathagata Chowdhury, Kevin Ingersent As the temperature of metals containing dilute concentrations of magnetic impurities reach very low temperatures, a phenomenon known as the Kondo effect takes place in which the resistance increases. This is due to the domination of spin-exchange processes that occur between the electrons of the metal and the electrons of the magnetic impurity near absolute zero. The Anderson model is a quantum impurity model that was developed in the 1960s to explain this phenomenon. It involves a single magnetic impurity tunnel-coupled to the conduction band of a metal. If the conduction band of this system contains a pseudogap, or a power-law decrease in the density of states around the Fermi energy, then quantum phase transitions will occur. The phase boundaries of the pseudogap Anderson impurity model have been previously approximated using poor man's scaling analysis. Here, we focus on using the more accurate numerical renormalization group method to calculate the location of these boundaries. We then compare these numerical results with the predictions derived from the scaling approximations. The development of nanotechnology like quantum dots and STM have rekindled interest in the Kondo effect since it can now be studied within controlled settings. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D4.00011: Intrinsic localized modes in antiferromagnetic sheets: the role of shape-dependent demagnetization fields Benjamin Kimock, Lars English We investigate numerically the role of global demagnetization fields on the formation of energy-localized patterns in two-dimensional sheets of antiferromagnetically-coupled, easy-axis spins. These global fields depend on the macroscopic shape of the lattice, and three scenarios can be delineated depending on whether the uniform mode is above, below or coincident with the long-wavelength spin waves in frequency. Each scenario leads to a different localization pathway and pattern. In the context of spin sheets, we can now also consider lattice anisotropies and their effects on the properties and interactions of intrinsic localized modes. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D4.00012: Computational Electromagnetic Modeling of Optical Responses in Plasmonically Enhanced Nanoscale Devices Fabricated with Nanomasking Technique Eric Novak, Desalegn Debu, Cameron Saylor, Joseph Herzog This work computationally explores plasmonic nanoscale devices fabricated with a recently developed nanomasking technique that is based on the self-aligned process. Computational electromagnetic modeling has determined enhancement factors and the plasmonic and optical properties of these structures. The nanomasking technique is a new process that is employed to overcome the resolution limits of traditional electron beam lithography and can also be used to increase resolution in photolithography fabrication as well. This technique can consistently produce accurate features with nanostructures and gaps smaller than 10 nm. These smaller dimensions can allow for increased and more localized plasmonically enhanced electric fields. These unique metal devices encompass tunable, enhanced plasmonic and optical properties that can be useful in a wide range of applications. Finite element methods are used to approximate the electromagnetic responses, giving the ability to alter the designs and dimensions in order to optimize the enhancement. Ultimately, we will fabricate devices and characterize the plasmonic properties with optical techniques, including dark-field spectroscopy, to confirm the properties with the goal of generating more efficient devices. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D4.00013: Trap State Introduction versus Band Gap Narrowing in Nitrogen-Doped La$_{2}$Ti$_{2}$O$_{7}$ Brandon Yost, Scott Cushing, Nianqiang Wu, Alan Bristow Nitrogen doping was reported to extend lanthanum dititanate's (LTO), La$_{2}$Ti$_{2}$O$_{7}$, absorption from 380 nm to 500 nm by narrowing the band gap without introducing trap states [1]. N-LTO holds promise for solar water splitting if, unlike in N-doped TiO$_{2}$, spectral coverage can be increased without decreasing carrier lifetimes and decrementing the overall performance. Therefore, in this presentation, the effect of N-doping on LTO is confirmed using transient absorption spectroscopy with a supercontinuum and THz probe. The supercontinuum probe reveals carrier evolution in both band edge and mid-gap defect states. By exciting above and below the band edge, the influence of N-doping on the density of trap states is directly compared to the band edge position. Further, comparison of dynamics measured with the supercontinuum and THz probes reveals which changes in lifetime correspond to increased mobility or increased trapping, showing how the shifted band edge modifies carrier dynamics, and that N-doping in LTO is an efficient strategy for solar energy harvesting.\\[4pt] [1] F. Meng, et. al. Nano Res., 5, 213 (2012). [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D4.00014: Measurement of Specific Heat of Pr$_{\mathrm{1-x}}$Nd$_{\mathrm{x}}$Os$_{4}$Sb$_{12}$ from 11K-300K Taylor McCullough-Hunter, Shoji Hishida, Pei-Chun Ho, Brian Maple, Tatsuya Yanagisawa The filled skutterudite compound PrOs$_{4}$Sb$_{12}$ exhibits heavy fermion behavior and unconventional superconductivity at low temperatures (T$_{\mathrm{c}}=$1.85K). The exact causes of these behaviors are unknown. The compound NdOs$_{4}$Sb$_{12}$ exhibits ferromagnetism at a Curie temperature near 1 K. Originally, Nd doped compounds of the form Pr$_{\mathrm{1-x}}$Nd$_{\mathrm{x}}$Os$_{4}$Sb$_{12}$ were developed to investigate the effect of ferromagnetism on the unconventional superconductivity and heavy fermion behavior of PrOs$_{4}$Sb$_{12}$. The specific heat of Pr$_{\mathrm{1-x}}$Nd$_{\mathrm{x}}$Os$_{4}$Sb$_{12}$ (where x$=$0.25, 0.5, 0.75, 0.8, and 1) is measured at 11K-300K to investigate the compounds' normal state properties. The specific heat is measured using relaxation calorimetry of finite heat pulse in a cryocooler system. Values of the electronic specific heat coefficient, $\gamma $, for these compounds are estimated to be 10-60 mJ/K$^{2}$-mol. This contrasts with previous low temperature measurements (\textless 10K) of NdOs$_{4}$Sb$_{12}$ with $\gamma $ approximately 520 mJ/K$^{2}$-mol. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D4.00015: Colossal Piezoresistance in strained La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ thin films Maria Viitaniemi, In Hae Kwak, Amlan Biswas Piezoresistance is the change in electrical resistance as a function of strain. A known mechanism leading to piezoresistance is thermodynamic phase separation. It has been shown that the compound (La$_{\mathrm{1-y}}$Pr$_{\mathrm{y}}$)$_{\mathrm{1-x}}$Ca$_{\mathrm{x}}$MnO$_{3}$ (LPCMO) exhibits colossal piezoresistance (CPR) at low temperatures due to electronic phase separation. For use in many applications, such as sensors, materials must exhibit CPR near room temperature. A possible candidate compound is La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) which has a Curie temperature of approximately 350 K. However, bulk LSMO single crystals do not show CPR since such samples are uniformly ferromagnetic and metallic with no phase separation. In this study, we examine the piezoresistance of ultrathin LSMO films grown on SrTiO$_{3}$ (STO) substrates using a three-point beam bending method to control the compressive and tensile strain. It has been suggested that the lattice mismatch strain due to the substrate induces phase separation in these thin films. We have observed CPR in such strained LSMO thin films even at room temperature. [Preview Abstract] |
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