Bulletin of the American Physical Society
14th Annual Meeting of the Northwest Section of the APS
Volume 57, Number 7
Thursday–Saturday, October 18–20, 2012; Vancouver, British Columbia, Canada
Session C4: Condensed Matter I |
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Chair: Simon Watkins, Simon Fraser University Room: SFU Harbour Centre 1520 Barrick Gold Lecture Room |
Friday, October 19, 2012 1:30PM - 2:06PM |
C4.00001: Free Electron Beams with Helical Wavefronts and Quantized Angular Momentum Invited Speaker: Benjamin McMorran Electron vortex beams, composed of helical electron wavefunctions that carry quantized orbital angular momentum (OAM), are analogous to optical vortices in beams of light. Electrons in the beam possess quantized amounts of orbital angular momentum (OAM) and have an associated magnetic moment. To produce such states, we use nanofabricated diffraction holograms to coherently imprint a phase vortex onto free electron matter waves in a transmission electron microscope (TEM). We also use this approach to place free electrons in coherent superpositions of orbital states, and we apply this to observe the Gouy phase for matter waves and to measure the orbital magnetic moment of the vortex state. Electron vortex beams can interact with surfaces and materials in unique ways. For example, electron vortex beams can transfer quantized OAM to an atom through inelastic scattering, inducing the same atomic and molecular transitions induced by circularly polarized light. Such OAM-dependent scattering provides a ``dichroic'' signal that can be measured in electron energy loss spectra from samples in a TEM. We will discuss our efforts toward using this to measure optical, electronic, and magnetic properties of samples with sub-nanometer resolution. [Preview Abstract] |
Friday, October 19, 2012 2:06PM - 2:18PM |
C4.00002: Precision microwave spectroscopy of the heavy fermion superconductor CeCoIn$_{5}$ reveals nodal quasiparticle dynamics Colin Truncik, Wendell Huttema, Patrick Turner, Sibel Ozcan, Natalie Murphy, Paul Carriere, Eric Thewalt, Kevin Morse, John Sarrao, David Broun CeCoIn$_{5}$ is a heavy fermion superconductor with strong similarities to the high-T$_{c}$ cuprates, including quasi-two-dimensionality, proximity to antiferromagnetism, and probable d-wave pairing arising from a non-Fermi-liquid normal state. Each system therefore forms an important testing ground for ideas relevant to the other system. Experiments that allow detailed comparisons of electronic properties are of particular interest. Here we use low temperature microwave spectroscopy to carry out a high-resolution study of the charge dynamics of the CeCoIn$_{5}$ superconducting state. The similarities to cuprates, in particular to ultra-clean YBa$_{2}$Cu$_{3}$O$_{y}$, are striking: the frequency and temperature dependence of the quasiparticle conductivity are instantly recognizable, a consequence of rapid suppression of quasiparticle scattering below T$_{c}$; and penetration depth data, when properly treated, reveal a clean, linear temperature dependence of the quasiparticle contribution to superfluid density. The measurements also expose key differences, including prominent multiband effects and a temperature-dependent mass renormalization. [Preview Abstract] |
Friday, October 19, 2012 2:18PM - 2:30PM |
C4.00003: The Local Properties of Superconducting LiFeAs: From the Pure Crystal to the Influence of Defects Stephanie Grothe, Shun Chi, Pinder Dosanjh, Ruixing Liang, Walter N. Hardy, Sarah A. Burke, Doug A. Bonn, Yan Pennec The iron pnictide superconductor LiFeAs is of particular interest as it is superconducting without chemical substitution and therefore presents a clean system to study the mechanisms behind unconventional superconductivity. We study the unreconstructed surface of LiFeAs by scanning tunneling microscopy and spectroscopy [1]. In regions free of defects, spectra at 2 K show two nodeless superconducting gaps, homogeneous over tens of nanometers, as well as a dip-hump structure with an energy scale consistent with a magnetic resonance recently reported by inelastic neutron scattering [2]. The gaps close at the bulk T$_c$, indicating that the surface accurately represents the bulk properties. We study how the superconducting phase of LiFeAs is modified in the vicinity of defects. The most commonly observed Fe site defect exhibits a bound state near the edge of the smaller gap. Three other common defects, including another one on an Fe site, are pair-breaking indicated by clear in-gap bound states, in addition to states near the smaller gap edge. Spectroscopic mapping reveals the high complexity of the real space bound state patterns.\\[4pt] [1] Chi et al., Phys. Rev. Lett. 109, 087002 (2012)\\[0pt] [2] Qureshi et al., Phys. Rev. Lett. 108, 117001 (2012) [Preview Abstract] |
Friday, October 19, 2012 2:30PM - 2:42PM |
C4.00004: Doping is Good: Enhancing Hall-Effect Sensor Performance with Doped Bismuth Ricky Chu, Nigel David, Taras Chouinard, Adam Schneider, David Broun Hall-effect sensors are quantitative magnetic flux detectors with sensitivity comparable to that of superconducting quantum interference devices (SQUIDs), but with superior spatial resolution [S.J.\ Bending, Adv.\ Phys.\ \textbf{48}, 449 (1999)]. Applications of Hall sensors include the imaging of microscopic magnetic structures such as vortices in superconductors, nanoscale domains in magnetic thin films, and nanoparticles in bioassay samples. Bismuth is being tested as a Hall probe material in order to avoid problems associated with excess noise, which arise in semiconductor Hall sensors as they are miniaturized [A. Sandhu \textit{et al}, Jpn.\ J.\ Appl.\ Phys.\ \textbf{40}, L524 (2001)]. However, bismuth is a compensated metal, and the presence of both electrons and holes reduces its native sensitivity due to cancellations in the Hall coefficient. We present experimental results for thin films and sensors that show hole doping by Pb can be used to empty the electron band, thereby breaking the compensation and increasing flux sensitivity. [Preview Abstract] |
Friday, October 19, 2012 2:42PM - 2:54PM |
C4.00005: Microwave measurements of vortex dynamics in the heavy fermion superconductor CeCoIn$_5$ Natalie Murphy, Eric Thewalt, Wendell Huttema, Colin Truncik, Kevin Morse, John Sarrao, David Broun Magnetic fields penetrate superconductors as a lattice of quantized tubes of magnetic flux, or ``vortices.'' A transport current, passed through such a superconductor, exerts a transverse component of force on the vortex lattice. Subsequent motion of the vortices results in dissipation. The frictional force experienced by a moving flux line is parameterized by a \emph{vortex viscosity}, and arises from induced electric fields coupling to charge excitations in the vicinity of the vortex core. We present vortex viscosity data on the heavy fermion superconductor CeCoIn$_5$, obtained using sensitive new microwave apparatus that operates at temperatures down to 0.07~mK and magnetic fields up to 9~T. The data we obtain is surprising, and indicates a breakdown of Bardeen--Stephen theory in this material; instead of arising from normal currents in the vortex cores, the frictional forces on the vortices appear to be caused by interactions with $d$-wave quasiparticles \emph{outside} the cores. This is evident in two ways: from the temperature dependence of the viscosity, which mirrors that of the $d$-wave quasiparticle conductivity; and from the observation of a new type of Volovik effect, in which the vortex viscosity has a $\sqrt{B}$ dependence on magnetic field. [Preview Abstract] |
Friday, October 19, 2012 2:54PM - 3:06PM |
C4.00006: SC2IT: a cloud computing interface that makes computational science available to non-specialists Kevin Jorissen, Fernando Vila, John Rehr Computational work is a vital part of much scientific research. In materials science research in particular, theoretical models are usually needed to understand measurements. There is currently a double barrier that keeps a broad class of researchers from using state-of-the-art materials science (MS) codes: the software typically lacks user-friendliness, and the hardware requirements can demand a significant investment, e.g. the purchase of a Beowulf cluster. Scientific Cloud Computing (SCC) has the potential to breach this barrier and make computational science accessible to a wide class of non-specialists scientists. We present a platform, SC2IT, that enables seamless control of virtual compute clusters in the Amazon EC2 cloud and is designed to be embedded in user-friendly Java GUIs. Thus users can create powerful High-Performance Computing systems with preconfigured MS codes in the cloud with a single mouse click. We present applications of our SCC platform to the materials science codes FEFF9, WIEN2k, and MEEP-mpi. SC2IT and the paradigm described here are applicable to other fields of research beyond materials science, although the computational performance of Cloud Computing may vary with the characteristics of the calculations. [Preview Abstract] |
Friday, October 19, 2012 3:06PM - 3:26PM |
C4.00007: BREAK |
Friday, October 19, 2012 3:26PM - 3:38PM |
C4.00008: Influence of Solvent Polarization on Electric Double Layer Interactions in Nanochannels Sushanta Mitra, Siddhartha Das We discuss the influence of solvent polarization effect on the Electric Double Layer (EDL) electrostatic potential distribution and the resulting EDL interaction between similar and oppositely charged surfaces in thin nanochannels with overlapping EDLs. We invoke a Langevin-Bikerman type free energy model that explicitly accounts for the solvent polarization and the finite size (of the ions and the water dipoles) effect in delineating the EDL interactions. We witness that the solvent polarization effects leads to a weaker EDL potential gradient and a larger interaction force between the surfaces. The solvent polarization effect can successfully explain the lowering of the relative permittivity of the solvent from bulk towards the charged surface. Most importantly, the EDL interaction force with finite solvent polarization can explain the large mismatch between the corresponding experimental and existing theoretical (computed using simple Poisson-Boltzmann model) results. [Preview Abstract] |
Friday, October 19, 2012 3:38PM - 3:50PM |
C4.00009: Investigating the morphology of ionic graft copolymers using SAXS and SANS techniques Rasoul Narimani, Emily M.W. Tsang, Ami Yang, Laurent Rubatat, Steven Holdcroft, Barbara Frisken We have studied the morphology of an ion-containing graft copolymer system using small angle x-ray and neutron scattering (SAXS and SANS), in addition to transmission electron microscopy (TEM). Our SAXS measurements on dry samples reveal that the poly(vinylidene difluoride) backbone of this copolymer forms quasi-spherical domains embedded in a continuous matrix of the poly(styrene) side chains. By analyzing the data we are able to calculate the size and spacing between these domains. According to SANS measurements, the ionic groups aggregate to form water-rich domains when the samples are hydrated. By comparing the SANS and SAXS results we find that the swelling properties at the nano-scale are consistent with bulk membrane properties. These results provide insight into the proton conductivity of these materials. [Preview Abstract] |
Friday, October 19, 2012 3:50PM - 4:02PM |
C4.00010: Growth and strain relaxation of GaAs/GaSb core/shell nanowires Omid Salehzadeh Einabad, Karen Kavanagh, Simon Watkins The nanowire geometry allows the fabrication of highly mismatched heterostructures beyond the critical thicknesses known for thin films. The GaAs/GaSb structure is of particular interest due to its staggered type-II band alignment and large valence band offset. This staggered band line-up spatially confines the carriers at opposite sides of the GaAs/GaSb interface resulting in desirable excitonic properties. Also, the large band offset makes the fabrication of infrared optoelectronic devices feasible. Previous work has focussed on the growth of axial heterostructures of GaAs/GaAs using the vapor-liquid-solid (VLS) growth mechanism. In this work we demonstrate the growth of GaAs-core/GaSb-shell heterostructures using a combination of VLS and vapour solid (VS) growth. The nanowire growth was carried out by metalorganic vapor phase epitaxy (MOVPE) at 410 \r{ }C. The large lattice mismatch between GaSb and GaAs (7.8{\%}) results in GaSb island formation on the GaAs NW facets. For shell thicknesses less than 1.8 nm, the GaSb shell is coherently strained to the GaAs core. For thicker shells, equal axial and radial strain relaxation between the GaAs NWs and the GaSb islands is observed, associated with the formation of periodic misfit dislocations. The degree of strain relaxation for the same shell thickness decreases from 100{\%} to 74 $\pm $ 3{\%} with decreasing core diameter from 50 to 15 nm. Strain relaxation was calculated from the spot spacing of selected area diffraction patterns, Moir\'{e} fringe spacing, dislocation spacing and high resolution TEM images. [Preview Abstract] |
Friday, October 19, 2012 4:02PM - 4:14PM |
C4.00011: Relaxation after a quench in the Bose Hubbard Model Malcolm Kennett, Denis Dalidovich Cold bosonic atoms confined in an optical lattice potential give a realization of the Bose Hubbard model, which has allowed the study of the phase transition between a superfluid and a Mott insulator as the depth of the optical lattice is varied. We study the real time dynamics of the Bose Hubbard model in the presence of time-dependent hopping using the Schwinger-Keldysh technique. Using a strong-coupling approach, we determine the effective action in the vicinity of the zero-temperature transition between superfluid and Mott insulating phases. We then study the solutions of the resulting saddle-point dynamical equations as the hopping is varied to sweep across the phase transition from the superfluid to insulating phase. We find that the dynamics can be understood within a picture where there are two timescales for relaxation. First, there is local equilibration, and on longer timescales there is mass transport. We discuss the implications of our results to realizations of the Bose Hubbard model in a harmonic trap. [Preview Abstract] |
Friday, October 19, 2012 4:14PM - 4:26PM |
C4.00012: Effects of gas adsorption on the conductance of suspended carbon nanotubes Boris Dzyubenko, Hao-Chun Lee, Oscar Vilches, David Cobden We have studied the effects of adsorbing a variety of gases on the electrical properties of individual suspended single-walled nanotubes, as a function of pressure and temperature. The quantity of gas adsorbed can be determined from the shift in the mechanical resonance frequency of the nanotube. We find that the conductance is sensitive to extremely small changes in density and can be measured on a timescale of milliseconds, permitting studies of the dynamics of the adsorbed atoms/molecules. The conductance varies nonmonotonically with coverage as a monolayer builds up and contains a contribution corresponding to charge transfer from the adsorbates of the order of one or two electrons in total. For noble gases, measurements below the 2D critical point on some devices show sharp features and fluctuations; in others these are absent. The reason for this is unclear and under investigation. In the nonlinear regime we observe features in the I-V characteristics as phase transitions are induced by the current and nonequilibrium stationary states occur. [Preview Abstract] |
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