Bulletin of the American Physical Society
76th Annual Meeting of the Southeastern Section of APS
Volume 54, Number 16
Wednesday–Saturday, November 11–14, 2009; Atlanta, Georgia
Session GC: Condensed Matter II |
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Chair: Yiping Zhao, University of Georgia Room: Paris |
Friday, November 13, 2009 8:30AM - 8:42AM |
GC.00001: Motional averaging of quadrupole interactions in Al$_{4}$Sr and Ga$_{4}$Sr studied using PAC Samantha Cawthorne, Gary S. Collins Perturbed angular correlation of gamma rays (PAC) was used to study diffusion of $^{111}$In/Cd probe atoms in Al$_{4}$Sr and Ga$_{4}$Sr phases. These phases have the tetragonal Al$_{4}$Ba structure, with two inequivalent Al sublattices that have axially symmetric and collinear electric field gradients. Measurements of nuclear quadrupole interactions in each phase revealed three dynamical regimes. Static quadrupole interactions were observed below 500$^{\circ}$C. Between 500 and 700$^{\circ}$C, the two quadrupole interaction signals were observed to approach each other and merge at 700$^{\circ}$C (slow fluctuation regime). Only a single quadrupole interaction signal was observed above 700$^{\circ}$C (fast fluctuation regime). These changes are attributed to jumping of probe atoms between the two sublattices, leading to motional averaging of the two electric field gradients as temperature and jump frequency increase. The two signals merge at a temperature at which the jump frequency between the sublattices is equal to the difference in the static quadrupole interaction frequencies. Temperature dependences of the quadrupole interaction frequencies will be contrasted with those measured earlier on phases including Al$_{4}$Ba and In$_{4}$Ba. [Preview Abstract] |
Friday, November 13, 2009 8:42AM - 8:54AM |
GC.00002: Possible deviation of magnetoresistivity of V$_{3}$Si from Kohler Rule S. Moraes, A.A. Gapud, P. Favreau, J.R. Thompson, D.K. Christen The normal-state transport magnetoresistivity \textit{$\rho $(H) }in high-quality samples of A15 compound V$_{3}$Si is studied. Contrary to the results of a previous study by Zotos \textit{et al.} (\textit{Sol. State Comm. }\textbf{50 }(5), 1984, p. 453) which found that \textit{$\rho \quad \sim $} $H^{2}$ over a wide range of temperatures, the result of the current study is closer to \textit{$\rho \quad \sim $} $H^{1.5}$ at higher temperatures. However, there seems to be also a deviation from this form at lower temperatures, thus showing a slight temperature dependence for \textit{$\rho $(H)} -- in other words, a temperature-dependent deviation from the traditional Kohler's Rule, \textit{$\Delta \rho $/$\rho $}$_{0 }$\textit{= f(H/$\rho $}$_{0}). $Possible reasons for this are discussed. [Preview Abstract] |
Friday, November 13, 2009 8:54AM - 9:06AM |
GC.00003: Effects of charge density wave phase on free flux flow in weak-pinning single crystals of NbSe$_{2}$ P. Favreau, A.A. Gapud, S. Moraes, L. Delong, T. Besara, E. Prettner, A.P. Reyes, J.R. Thompson, D.K. Christen The co-existence of two different ordering schemes in the conduction electrons of superconductors -- charge density waves (CDWs) and superconductivity, particularly in the motion of quanta of magnetic flux -- is studied in high-quality samples of the compound, NbSe$_{2}$. Of specific interest is the magnetic-field dependence of transport critical current density $J_{c}$ and that of the Lorentz-driven free flux flow (FFF) resistivity associated with the ordered motion of vortices when the CDWs are switched on and off -- as effected by doping with Ta. The CDW phase is manifested as a broadening of NMR peaks and as a ``knee'' in the temperature dependence of normal-state resistivity. While both doped and non-doped samples show similar field dependence in FFF resistivity, $J_{c}(H)$ reveals a surprising double peak, probably due to sample inhomogeneity combined with the effects of crystal anisotropy. [Preview Abstract] |
Friday, November 13, 2009 9:06AM - 9:18AM |
GC.00004: Comparative study of radiation-induced magnetoresistance oscillations in MBE material from different sources Ramesh Mani, Werner Wegscheider, Vladimir Umansky Transport studies of GaAs/AlGaAs electron systems have shown microwave- and terahertz- radiation-induced, large amplitude, periodic-in-the-inverse-magnetic-field, magnetoresistance oscillations that saturate into novel radiation-induced zero- resistance states (RIZRS) at the lowest temperatures.[1] The origin of these RIZRS remains an open topic for further experimental investigation, as does the dependence of these phenomena on the impurity configuration and the material quality. It remains to be understood if similar material prepared in different laboratories yield a similar response. In addressing this issue, we examine here the radiation-induced magnetoresistance oscillations in GaAs/AlGaAs material prepared by W. Wegscheider and co-workers, and compare the results to our own previous results obtained on specimens prepared by V. Umansky and co-workers. \\[4pt] [1] R. G. Mani, W. B. Johnson, V. Umansky, V. Narayanamurti, and K. Ploog, Phys. Rev. B 79, 205320 (2009). [Preview Abstract] |
Friday, November 13, 2009 9:18AM - 9:30AM |
GC.00005: Transport measurements of GaAs/AlGaAs devices in the ``anti-Hall-bar within a Hall bar" geometry Annika Kriisa, Ramesh Mani Hall effect measurements are often carried out in the Hall geometry, which is a thin rectangular plate with current and Hall voltage contacts at the external boundary. The motivation of this study is to further understand the impact on Hall effect when a hole is inserted inside the Hall geometry. One way on conducting this investigation is to superimpose an ``anti-Hall bar'' inside the standard Hall bar, where the anti Hall bar is actually the hole inside the Hall device with contacts on the inside boundary of this hole. This configuration is thought to generate an ordinary Hall effect within the interior boundary. One believes that it might also be possible to simultaneously realize multiple independent Hall effects by injecting multiple currents into the multiply connected device [1]. We have experimentally studied the Hall effect in the doubly connected ``anti-Hall bar within a Hall bar'' geometry fabricated out of the GaAs/AlGaAs semiconductor system, and convey the results in this presentation. \\[4pt] [1] R. G. Mani and K. von Klitzing, Z. Phys. B 92, 335 (1993). [Preview Abstract] |
Friday, November 13, 2009 9:30AM - 9:42AM |
GC.00006: Transport studies of Graphite Aruna Ramanayaka, Ramesh Mani Transport studies of single layers of carbon, known as Graphene, have shown striking new features in two dimensional transport, arising from the linear dispersion relation and analogies to quantum electrodynamics. Highly Oriented Pyrolytic Graphite (HOPG) consists of stacked sheets of Graphene with weak interlayer interactions, which gives rise to anisotropic transport with striking differences between in-plane and perpendicular transport. In-plane transport measurements indicate plateaus in the Hall resistance (Rxy) along with Shubnikov-de Haas (SdH) oscillations in the diagonal resistance (Rxx) in three-dimensional HOPG. The transport data for graphite indicate a strong deviation from the resistivity rule proposed for canonical quantum Hall systems. The origin of this anomaly in graphite is identified here by examining the relative phases of the oscillatory diagonal and off-diagonal resistances. [Preview Abstract] |
Friday, November 13, 2009 9:42AM - 9:54AM |
GC.00007: Polarized Raman Spectroscopic Study of ZnGeP$_{2}$ Ramazan Atalay, Goksel Durkaya, Nikolaus Dietz The ternary semiconductor ZnGeP$_{2}$ alloy is one of the most technologically important material systems among the II-IV-V$_{2 }$compound family since it is promising in device applications based on non-linear optics. In order to employ ZnGeP2 material system in device applications such as; frequency doubling, parametric optical amplification, second order differential frequency coupling, the ZnGeP$_{2}$ crystal structure must have an oriented distortion that results in birefringent effect. The orientation of the distortion in the crystal structure can be promoted into a preferred direction so that only one plane of the crystal becomes birefringent. In this study, we present detailed study of the polarization dependent behavior of the phonon modes in ZnGeP2 crystal using Polarized Raman Spectroscopy (PRS). The results of our studies demonstrate non-linear mixing of radiation only occurs in the crystalline plane where conservation of momentum or so-called phase matching is present. The effect of birefringence in phonon modes of ZnGeP$_{2}$ crystal in Raman spectra is studied. [Preview Abstract] |
Friday, November 13, 2009 9:54AM - 10:06AM |
GC.00008: Stick-slip transition at the granular critical state Nick Gravish, Paul Umbanhowar, Daniel I. Goldman We study the force on a flat plate (3.8~cm width, 7.0~cm depth) dragged at constant velocity $v$ through the surface of a granular medium (250~$\mu$m glass beads) as a function of volume fraction $0.57<\phi<0.63$. The dynamics of the drag force $F_d$ are sensitive to $\phi$: we find a sharp transition in the form of $F_d$ at a critical volume fraction $\phi_c=0.605$. For $\phi<\phi_c$, $F_d$ increases with time and saturates, while for $\phi>\phi_c$ $F_d$ exhibits an initial peak followed by periodic oscillations at frequency $f$ about a constant mean. The standard deviation in force (a measure of the fluctuations) shows a sharp transition at $\phi_c$. The force oscillations suggest that the granular media periodically jams and flows as the plate is horizontally translated. Examining the bed surface we observe a spatially periodic scalloped feature of length $\lambda$ which is equal to $v/f$, independent of $v$, and increases linearly with $\phi$ for $\phi>\phi_c$. By measuring the displaced volume after the drag $\Delta V$, we observe a transition from media compaction ($\Delta V<0$) for $\phi<\phi_c$ to dilation ($\Delta V>0$) for $\phi>\phi_c$. [Preview Abstract] |
Friday, November 13, 2009 10:06AM - 10:18AM |
GC.00009: Gas Diffusion in Porous Silicon Sensors Serdar Ozdemir, James Gole Nanopore covered microporous silicon conductometric gas sensors have been produced via electrochemical etching and standard microfabrication techniques. Reversible and sensitive gas sensors working at room temperature have been fabricated. Sensing of NH$_{3}$, NO$_{x}$ and PH$_{3}$ at or below the ppm level have been observed. The porous surface has been modified using selective coatings including electroless tin, gold, nickel and copper solutions to increase the response to NO$_{x}$, NH$_{3}$, PH$_{3}$ respectively. The diffusion of the analyte species has been investigated in'the nanopore and micropore regimes by numerical analysis. Comparing the response time of the hybrid porous sensor surface with numerical diffusion calculations on the pores, it has been observed that Knudsen diffusion time scales dominate the sensor response. A transduction model is proposed based on nanopore limited gas diffusion and the experimental response and recovery data. [Preview Abstract] |
Friday, November 13, 2009 10:18AM - 10:30AM |
GC.00010: General Approach to the Formation of Sensitive, Selective, Rapidly Responding Conductometric Sensors James Gole, Serdar Ozdemir Rapidly responding, reversible, sensitive, and selective porous silicon-based gas sensors are formed with a highly efficient electrical contact to a nanopore covered microporous array. A general approach to facilitate significant changes in sensor surface sensitivity for a variety of gases, based on a complementary concept to that of hard and soft acid and base (HSAB) interactions and commensurate with a basis in physisorption has now been formulated to create highly selective surface coatings at the nanoscale. The technology, implemented on ``phase matched'' nanoporous silicon layers positioned on porous silicon micropores facilitates the application of nanostructured metals, metal oxides, and nanoparticle catalytic coatings, and provides for notably higher sensitivities and selectivity verses, for example, metal oxide systems. Nanomaterials are applied to the PS surface to provide for the detection of gases including NO, NO$_{2}$, CO, NH$_{3}$, PH$_{3}$, SO$_{2}$, H$_{2}$S, and HCl in an array-based format at the sub-ppm level. The value of this sensor technology results from (1) sensitivity and short recovery time, (2) operation at room temperature with an insensitivity to temperature drift, (3) ease of coating with diversity of clearly mapped gas-selective materials to form sensor arrays, (5) its low cost of fabrication. [Preview Abstract] |
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