Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B46: SPS Undergraduate II |
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Sponsoring Units: SPS Chair: Crystal Bailey, American Physical Society Room: Hilton Baltimore Holiday Ballroom 5 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B46.00001: Redesign of an AC Magnetic Susceptometer for Measurements in Smaller Samples Andres Vargas, Ryan Fukuda, Smitha Sunny, Pei-Chun Ho A new AC magnetic susceptometer was created for the purpose of measuring the magnetic properties of smaller samples, such as nanoparticles that are currently being synthesized in our lab. The susceptometer consists of a primary coil, a secondary coil, and a sample holder. The primary coil is the outer component of the susceptometer, which provides a magnetic field when current is applied due to Ampere's Law. Inside of the primary coil lies the secondary coil, which has two oppositely wound solenoids; they are oppositely wound to reduce background signal. The sample holder lies inside of the secondary coil with the sample. All of these go inside of a beryllium copper casing for protection. We tested the susceptometer by looking for the ferromagnetic phase transition of an 11 mg Gd sample. A $\sim 100 \mu$A AC current was applied to the primary coil, which created a magnetic field that polarized the magnetic moments in the sample. This induced a voltage on the secondary coil, which is proportional to the magnetic susceptibility. We measured the temperature dependency of the induced voltage from 10 K to 300 K. The results showed a sharp increase in the induced voltage around 293K, which agrees with the known ferromagnetic transition of Gd. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B46.00002: Electrospun fibers of PLA/P3HT blends for device and sensor applications William Serrano, Nicholas Pinto The thermoplastic aliphatic polyester, poly (lactic acid) (PLA) is a biodegradable polymer that is sometimes used in implant screws for bone repair. Our focus was to fabricate fibers of this polymer and its blends with p-doped poly (3-hexylthiophene)-(P3HT) in order to extend its use to devices and/or sensors. PLA/P3HT fibers were prepared in air at room temperature using the electrospinning technique that is cheap, fast and reliable. Scanning Electron Microscope images of the fibers reveal that the presence of P3HT does not affect the fabrication of PLA fibers at low or high polymer concentrations in chloroform, retaining the same morphological structure of pure PLA fibers. The fiber diameters were in the range 1-10 microns. A slight increase in fiber formation results with the addition of P3HT, most likely due to a reduction of the solution surface tension. Results of the electrical characterization of this material will be presented. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B46.00003: AC Circuit Measurements with a Differential Hall Element Magnetometer Matthew W. Calkins, B. Scott Nicks, Pedro A. Quintero, Mark W. Meisel As the biomedical field grows, there is an increasing need to quickly and efficiently characterize more samples at room temperature. An automated magnetometer was commissioned to do these room temperature magnetic characterizations. This magnetometer, which is inspired by a Differential Hall Element Magnetometer,\footnote{Yongquing Li\textit{ et al}., Appl. Phys. Lett. \textbf{80 }(2002) 4644.} uses two commercially available Hall elements wired in series. One Hall element measures the external magnetic field of a 9 T superconducting magnet and the other measures the same external field plus the field due to the magnetization of the sample that sits on top of the Hall element. The difference between these two Hall elements is taken while a linear stepper motor sweeps through the external magnetic field. The linear motor and data acquisition are controlled by a LabVIEW program. Recently, the system was outfitted for AC circuit measurements and these data will be compared to DC circuit data. In addition, the lowest signal to noise ratio will be found in order to deduce the smallest amount of sample needed to register an accurate coercive field. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B46.00004: Low Temperature Probe for Measuring Anisotropic Magnetotransport Galin Dragiev, Daniel Grant, Amlan Biswas Certain materials display a change in resistance when a magnetic field is applied on them. This resistance change is called magnetoresistance (MR). The value of MR may also depend on the direction of the magnetic field relative to the crystal structure of the material, which is called anisotropic MR (AMR). We built a probe which allows us to measure the AMR of a sample in a temperature range of 1.2 K to 300K in magnetic fields of up to 9 tesla. The probe design allows the angle between the magnetic field and a particular direction of the sample to be changed over almost the entire solid angle of 4$\pi $. In particular, this probe lets us measure the AMR of a sample with magnetic anisotropy when the magnetic field is applied either along the hard or easy axes, or somewhere in between. The probe allows us to change the orientation of the sample while it is inside the low temperature cryostat. We will present our data on hole-doped manganese oxide (manganite) thin films and discuss the possible origins of AMR in these materials. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B46.00005: Probing Quantum Turbulence in He II with a MEMS Oscillator Aleksander Levental, Josh Bauer, Miguel Gonzalez, Pan Zheng, Yoonseok Lee, Ho Bun Chan Micrometer scale mechanical oscillators based on MEMS technology have been developed for the study of quantum fluids and have been tested successfully at ultra low temperatures. Our recent low temperature test [1] in which the device was immersed in the superfluid phase of $^{4}$He revealed striking behavior below 400 mK: nonlinear and hysteretic resonance at high excitations. The observed phenomenon is thought to be related to vortices and quantum turbulence and warrants a systematic investigation for better understanding. We constructed an experimental set-up that allows us to cool a MEMS device in liquid $^{4}$He down to 50 mK at pressures up to 25 bar. We will discuss our new set-up and present our preliminary results performed at saturated vapor pressure. \\[4pt] [1] M. Gonzalez, B. Moon, P. Zheng, E. Garcell, H. B. Chan, and Y. Lee. \textit{Journal of Low Temperature Physics, Online First}$^{TM}$\textit{, 22 August 2012}, DOI: 10.1007/s10909-012-0682-8. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B46.00006: Exploration of Quartz Tuning Forks as Potential Magnetometers for Nanomagnets B. Scott Nicks, Matthew W. Calkins, Pedro A. Quintero, Mark W. Meisel A change in the resonance frequency, $f_0 \approx 32$ kHz, of quartz tuning forks is expected when nano-sized magnetic particles or films are applied to a fork that is then exposed to a variable magnetic field. This work explores the feasibility of using these forks, once removed from their protective canisters, as potentially inexpensive magnetometers operating at room temperature in fields up to 2~T, and eventually up to 9~T, by analyzing the responses of loaded forks in such a field. However, the forks are also dependent on subtle variations of the ambient temperature, and the magnetic leads may present a background signal that must be subtracted. Preliminary results are encouraging, but better understanding of the noise sources must be made for these forks to be used as envisioned. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B46.00007: Toward CN-VFET logic circuits Stephen Gilbert, Bo Liu, Mitchell McCarthy, Evan Donoghue, Andrew Rinzler Gate field modulation of the Fermi level in the low density of electronic states carbon nanotubes provides a new control mechanism for modulating the Schottky barrier between the nanotubes and a semiconductor to control charge injection across their interface. This has been exploited in the recently developed carbon nanotube-enabled vertical field effect transistor (CN-VFET) comprised of a bottom gate, dielectric layer, dilute nanotube source electrode, semiconducting channel layer, and drain electrode situated in a collinear, vertical stack. Since the channel length in this architecture is simply the thickness of a thin film, the naturally short channel lengths can overcome the relatively low mobility of organic semiconductors to source higher on-state currents or potentially improve operating speeds. Prototype logic gates using such organic transistors have yet to be demonstrated. As a step in this direction we have fabricated organic CMOS inverters utilizing a p-type and an n-type CN-VFET. The device fabrication, materials used, performance and progress toward a CN-VFET ring oscillator will be discussed. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B46.00008: Pressure Dependence of MEMS Oscillator Quality Factor Joshua Bauer, Sarah Geiger, Miguel Gonzalez, Pan Zheng, Yoonseok Lee This paper details a study in which the pressure dependence of the quality factor and resonance frequency of a micro-electro-mechanical device is examined. The results obtained will aid in the understanding of the effects of slide film damping in various gasses on oscillators operating at micrometer length scales. The device utilized was a capacitively driven plate oscillator positioned 1.25$\mu$m above a silicon substrate. The dominant damping mechanism for this geometry is slide film damping from the gaseous film between the oscillating plate and substrate. The mechanical resonance of the device was characterized as a function of pressure from 6 mTorr to 1 atm in air. We observed three distinct damping regimes in the quality factor. In addition to the characterization performed in air, pressure dependences in helium and argon were also examined at pressure ranges of 6.5 mTorr to 5 Torr and 750mTorr to 760 Torr, respectively. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B46.00009: Intrinsic Localized Modes in nonlinear two-dimensional electrical lattices J.F. Stormes, L.Q. English, F. Palmero, P.G. Kevrekidis We report on the generation of stationary and traveling intrinsic localized modes (ILMs), also called discrete breathers or discrete solitons, in two dimensions in damped-driven electrical lattices. ILMs are spatially localized eigenmodes that arise due to the nonlinearity of the system, not due to spatial impurities. Since solitons are generally unstable in two dimensions, the existence of these ILMs relies on the discreteness of the lattice. We show experimentally that depending on the frequency and amplitude of the spatially uniform driving, different numbers of ILMs can be induced in both square and hexagonal lattices. In lattices that allow ILM motion, we furthermore study the interaction of such modes. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B46.00010: Thermal effects of laser illumination on coated quartz crystal microbalance surfaces Benjamin Keller, Keeley Stevens, Liming Pan, Jacqueline Krim Prior work on the thermal sensitivity of quartz crystal microbalances (QCM) has shown them to be powerful tools, capable of measuring milli-Kelvin temperature impulses while also presenting a well-understood response to steady state heating [1]. This has been demonstrated for physical contact to the QCM surface via a STM tip with a temperature differential [2]; here we present a novel application wherein a laser is focused onto the coated QCM, thus applying a non-contact thermal pulse. By applying variable length (second to minute) exposures from a laser source we can isolate the thermal shock, time decay and gross heating effects. The system is sensitive to the coating used, showing significant differences in heating for absorbative and reflective coatings. This method is unique in that the QCM measures energy lost into the substrate, unlike standard techniques which focus primarily on material efficiency. This has potential to characterize various coatings used in solar cells and thermal collectors, as well as in photovoltaic materials.\\[4pt] [1] Wolsky, S.\ P.\ and Zdanuk, E.\ J., editors. \textit{Ultra Micro Weight Determination in Controlled Environments} {\bf 1969}.\\[0pt] [2] Pan, L.\ and Krim, J.\ {\it Rev.\ Sci.\ Instr.\ }{\bf 2012}, in press. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B46.00011: Spectrum, symmetries, and dynamics of Heisenberg spin-1/2 chains Kira Joel, Davida Kollmar, Lea Santos Quantum spin chains are prototype quantum many-body systems. They are employed in the description of various complex physical phenomena. Here we provide an introduction to the subject by focusing on the time evolution of Heisenberg spin-1/2 chains with couplings between nearest-neighbor sites only. We study how the anisotropy parameter and the symmetries of the model affect its time evolution. Our predictions are based on the analysis of the eigenvalues and eigenstates of the system and then confirmed with actual numerical results. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B46.00012: High temperature series expansion and the exact solution study of the 1/5 depleted square lattice Ising model Simeon Hanks, Trinanjan Datta, Jaan Oitmaa The critical behavior of the 1/5 depleted square-lattice Ising model with nearest neighbor ferromagnetic interaction has been investigated by means of both a high-temperature series expansion and an exact solution. The critical point in the coupling constant has been accurately determined with a series expansion up to order eighteen in the high temperature expansion parameter. For the exact solution we use a set of decoration transformations to recast the original model in terms of a set of nearest neighbor, next-nearest neighbor, and four spin interaction Ising model. This is followed by a transformation to a staggered 8-vertex model. As the vertex weights satisfy the free-fermion condition the free energy and critical point are obtainable by standard methods. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B46.00013: Developing an Embedded Atom Method Potential for Copper Ben Stortenbecker, Brian Demaske, Vasily Zhakhovsky, Ivan Oleynik A new embedded-atom method (EAM) interatomic potential for copper has been developed in order to improve upon the predictive power of atomistic simulations under extremes of pressures and temperatures induced by shock compression and ultrashort laser irradiation. Several candidate potentials were fit to a database consisting of \textit{ab initio} cold pressure tensor components calculated for a wide range of hydrostatic and uniaxial deformations as well as experimental properties near equilibrium conditions. The close relationship between the stress tensor and interatomic forces under naturally-occurring material states ensures the accuracy of the potential without the need for a large number of fitting points. After fitting, the candidates were then screened against the experimental melting point in order to select a single best potential. This final potential will be verified against the experimental melting line, liquid-vapor coexistence curve, and the shock Hugoniot. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B46.00014: Surface-induced reduction of the spin coherence times of nitrogen-vacancy centers in diamond Jeffrey M. Moore, Michael E. Flatt\'e The exceptionally long room-temperature spin coherence times of nitrogen-vacancy (NV) centers in diamond indicate their potential utility for quantum information processing. The remarkable sensitivity of the spin dynamics of NV centers to electric and magnetic fields, and to strain, also suggests these centers can be used in novel sensors. The sensitivity and spatial resolution of such a sensor will depend on the depth of the NV center below the diamond surface. Local relaxation of the atomic positions near the diamond surface, however, will strain the NV center and consequently reduce its spin coherence time. We evaluate this effect by calculating the strain near a (001) diamond surface using density functional theory. The strain for a specific NV-center depth was evaluated using the linearized augmented plane wave (LAPW) method and the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional within the WIEN2k density functional code. The effect of the resulting strain values on the spin coherence times were determined using a low-energy effective Hamiltonian for the NV-center energies and wave functions, and their strain dependence. This work was supported by an AFOSR MURI. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B46.00015: Density of States of Type-II Superconductors in High Magnetic Field and Low Temperatures Renzo Villazon, Owen Lehmer, Julian Irwin, Sasha Dukan In high magnetic fields and at low temperatures, electronic energies are quantized in the form of Landau levels. The inclusion of Landau level quantization in the superconducting pairing (both diagonal and off-diagonal) leads to gapless points on the Fermi surface. Within this theory, the density of states of a type-II superconductor in the range of magnetic fields 0.2B$_{c2}$ \textless\ B \textless\ B$_{c2}$ is calculated. The influence of disorder on the density of states is investigated for a range of impurity concentrations and scattering potential strengths. We compare our theoretical predictions to experimental results for superconductor YNi$_{2}$B$_{2}$C and find that our model is reliable at high magnetic fields but has limited applicability at lower fields. [Preview Abstract] |
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