Bulletin of the American Physical Society
79th Annual Meeting of the APS Southeastern Section
Volume 57, Number 16
Wednesday–Saturday, November 14–17, 2012; Tallahassee, Florida
Session MC: National High-Field Magnet Laboratory II |
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Chair: Timothy Murphy, National High-Field Magnet Laboratory Room: DoubleTree Adams-Park |
Saturday, November 17, 2012 10:45AM - 10:57AM |
MC.00001: Electronic Multicriticality in Bllayer Graphene Robert Throckmorton, Vladimir Cvetkovic, Oskar Vafek We use renormalization group (RG) methods to investigate the symmetry-breaking phases of bilayer graphene. We derive the flow equations for different coupling constants that appear in a low-energy effective theory for the system, and show how they may be used to determine the different symmetry-breaking phases in the system. We are able to map out all of the possible phases that the system is unstable to. We also apply our methods to the special case of finite-range, monotonically-decreasing, density-density interactions. We map out which phase(s) that the system is unstable to as a function of the overall interaction strength and of the range. We find that the system is unstable to an antiferromagnetic state for short-range interactions and to a nematic state, in which the parabolic degeneracy of the low-energy modes splits into two Dirac-like cones, for long ranges. Finally, we investigate, within the framework of variational mean field theory, the behavior of the antiferromagnetic state in the presence of a magnetic field applied perpendicular to the sample. We show how to determine the energy gap in the system, and find a slight non-monotonic behavior at low fields and a quasi-linear behavior at high fields. We then compare this result to experimental findings. [Preview Abstract] |
Saturday, November 17, 2012 10:57AM - 11:09AM |
MC.00002: Effects of Lateral Diffusion on the Dynamics of Desorption Tjipto Juwono, Ibrahim Abou Hamad, Per Arne Rikvold The adsorbate dynamics during simultaneous action of desorption and lateral adsorbate diffusion is studied in a simple lattice-gas model by kinetic Monte Carlo simulations. It is found that the action of the coverage-conserving diffusion process during the course of the desorption has two distinct, competing effects: a general acceleration of the desorption process, and a coarsening of the adsorbate configuration through Ostwald ripening. The balance between these two effects is governed by the structure of the adsorbate layer at the beginning of the desorption process. It is found that when starting with larger cluster sizes, the acceleration effect of diffusion dominates, while starting with smaller cluster sizes results in the dominance of coarsening. It is also found that the initial size distribution of the adsorbate determines both the quantitative and the qualitative features of the distribution as it develops during the desorption process. [Preview Abstract] |
Saturday, November 17, 2012 11:09AM - 11:21AM |
MC.00003: Gauge Fields and Interlayer Coherence in Bilayer Composite Fermion Metals Robert Cipri, N.E. Bonesteel The $v$= 1 (=1/2+1/2) bilayer quantum Hall system exhibits at least two distinct phases as a function of layer spacing, $d$. In the limit of large layer spacing ($d>> l$, where $l$ is magnetic length) the system decouples into two distinct compressible $v$= 1/2 ``composite Fermi liquid'' states. In the opposite limit of small layer spacing ($d<< l)$ the system enters an incompressible bilayer quantum Hall state (the ``111'' state). After decades of study, the transition between these two states is still poorly understood. Recently, Alicea et al. [1] have proposed an interesting new state which might exist in this system for intermediate layer spacing ($d\sim $ l). In this so-called ``interlayer phase coherent'' state, composite fermions tunnel coherently between layers and form well-defined bonding and antibonding Fermi seas, despite the fact that there is no actual tunneling of physical electrons. We study the effect of the Chern-Simons gauge fields associated with the composite fermions on the formation of such an interlayer phase coherent state. We show that scattering from these gauge fields leads to layer-dependent fluctuations in the Aharonov-Bohm phase of the composite fermions which strongly suppress interlayer phase coherence. This suppression manifests itself through the appearance of a contribution to the ground state energy which is logarithmically singular in the order parameter characterizing this interlayer coherence. \\[4pt] [1] J. Alicea, O. I. Motrunich, G. Refael, M. P. A. Fisher, Phys. Rev. Lett. 103, 256403 (2009) [Preview Abstract] |
Saturday, November 17, 2012 11:21AM - 11:33AM |
MC.00004: Field-induced spin-flop transitions in single-crystalline La$_{0.25}$Pr$_{0.75}$Co$_{2}$P$_{2}$ Tongshuai Xu, Kirill Kovnir, Ping Chai, Michael Shatruk, Peng Xiong We report measurements of anisotropic magnetization and magnetoresistance (MR) on single crystals of La$_{0.25}$Pr$_{0.75}$Co$_{2}$P$_{2}$, a novel magnetic alloy with the ThCr$_{2}$Si$_{2}$-type structure, which is a parent structure of the ferro-pnictide high-temperature superconductors. The material exhibits two magnetic transitions in the $c$-axis direction at 240 K and 10.5 K. Below 10K, the magnetization and MR measurements provide corroborating evidence that applying a magnetic field along the $c$-axis induces two successive spin-flop transitions at 0.5 T and 5.5 T. For a magnetic field applied in the \textit{ab }plane, spins cannot be fully polarized at up to 7 T and the MR changes sign from negative to positive at $\sim $20 K with increasing temperature. The observations are consistent with a magnetic configuration, in which the Pr and Co magnetic moments are oriented along the $c$ axis and ordered ferromagnetically within the \textit{ab} plane but antiferomagnetically along the $c$ axis. Details of the microscopic origin of the spin-flop transitions will be discussed. [Preview Abstract] |
Saturday, November 17, 2012 11:33AM - 11:45AM |
MC.00005: Carrier density dependence of spin lifetime in Fe/AlGaAs heterostructures Joon-Il Kim, Jennifer Misuraca, Konstantinos Kountouriotis, Stephan von Molnar, Peng Xiong, Kangkang Meng, Jun Lu, Lin Chen, Xuezhe Yu, Jianhua Zhao Electrical Hanle-type measurements have been performed to determine spin lifetimes at various carrier densities in Si:Al$_{0.3}$Ga$_{0.7}$As, a persistent photoconductor (PPC). The carrier density of this material can be tuned, changing it from insulating to metallic \textit{in situ }via photo excitation. Utilizing this PPC effect, we conduct electrical measurements of spin accumulation and transport under the same experimental conditions without the necessity of making replicas to realize different doping levels. We report the carrier density dependence of the spin lifetime derived from Hanle measurements with spin devices formed on wafers which have different graded junctions and Si doping levels. Carrier densities ranged from 3.5x10$^{16}$ to 2.4x10$^{17}$ cm$^{-3}$ and from 7.2x10$^{16}$ to 6.5x10$^{17}$ cm$^{-3}$ in two different samples. The spin lifetimes (determined using Lorentzian fits to the Hanle curves) ranged from 0.5 to 2.8 ns. From optical studies, the spin lifetime at zero bias and at low temperature in n-GaAs was reported to be larger than 100 ns on the insulating side and $\sim$80 ns on the metallic side. Based on our measurements in Si:Al$_{0.3}$Ga$_{0.7}$As, the extrapolated spin lifetime at zero bias and at 5 K is found to be only $\sim$2.3 ns on the insulating side and decreases with increasing bias current. [Preview Abstract] |
Saturday, November 17, 2012 11:45AM - 11:57AM |
MC.00006: Measurements of Molecule-Based Magnets using micro-SQUID on a Waveguide Guang Yue, Wolfgang Wernsdorfer, Irinel Chiorescu We describe a setup design which plans to use a dc micro-SQUID to measure the magnetization signal of a magnetic sample while the spins are excited by microwave pulses. Such on-chip techniques are gathering more and more interest. In a first approach, the SQUID chip containing the studied sample, will be placed on top of another chip containing the waveguide. Computer simulations are done to verify that the field strength is sufficiently strong and uniform in this case. In another approach, we integrate a microwave waveguide on the same chip as the SQUID. The presented setup will be able to sense the magnetic flux generated by rotating spins, without the need of analyzing the output microwave signal. The microwave will only serve as an input pulse, to excite the spin dynamics. [Preview Abstract] |
Saturday, November 17, 2012 11:57AM - 12:09PM |
MC.00007: Magnetic Dipolar Interactions in Nanoparticle Assemblies Manabendra Chandra, Kenneth Knappenberger I will present evidence of a magnetic dipolar contribution to the nonlinear optical (NLO) response of colloidal metal nanostructures. Second-order NLO responses from several small assemblies of solid gold nanospheres (SGN) were examined using polarization-resolved second harmonic generation (SHG) spectroscopy at the single-particle level. Unambiguous circular dichroism in the SH signal was observed for many of the nanostructures, indicating that the plasmon field located within the interparticle gap was chiral. Detailed analysis of the polarization line shapes of the SH intensities obtained by continuous polarization variation suggested that the effect resulted from strong magnetic-dipole contributions to the nanostructure's optical properties. [Preview Abstract] |
Saturday, November 17, 2012 12:09PM - 12:21PM |
MC.00008: Relaxation Dyanamics of Electronically Excited Metal Nanoclusters Thomas Green, Kenneth Knappenberger Recent advances describing optical properties and relaxation dynamics of electronically excited nanoclusters are presented. Femtosecond time-resolved transient absorption and magneto-photoluminescence data were used to study properties of excited states located near the HOMO-LUMO energy gap for anionic and neutral Au$_{\mathrm{25}}$(SCH$_{\mathrm{2}}$CH$_{\mathrm{2}}$Ph)$_{\mathrm{18}}$. With these data, we show the optical properties of these nanoclusters are charge-state dependent. Transient absorption measurements employing NIR probe pulses reveal a previously unobserved relaxation process for neutral nanoclusters with a lifetime of several hundred picoseconds. Information regarding the electronic g-factor and photoluminescence rates of these clusters were obtained from temperature- and magnetic field-dependent measurements. Together these data indicate neutral nanocluster emission proceeds through multiple channels, including both high- and low-spin states. Quantum-confined, ligand-protected gold nanoclusters represent a class of nanomaterials with potential application across a wide range of fields. These results will impact several technologies, including: optical imaging, energy conversion, and catalysis, which all feature the nanocluster platform. [Preview Abstract] |
Saturday, November 17, 2012 12:21PM - 12:33PM |
MC.00009: Scalable Lipid Multilayer Microarraying Troy Lowry, Steven Lenhert A need exists for scalable, high thoughput screening of both lipophilic drug candidates and biosensors. Surface-supported lipid multilayer nanostructures are lipid molecules thicker than a bilayer with controlled thickness physisorbed to a substrate and have promising applications in scalable drug screening...$^{1}$ and biosensing....$^{2}$ Uniform lipid nanostructures with multilayers of controlled thickness are essential for drug screening in order to control the dosage of drug candidates and for adhesion of cells. Methods for controlling lipid multilayer thickness have been shown using DPN nanolithography$^{2}$ and multilayer stamping.$^{3}$ In order for scalability to be achieved, multiple different lipid multilayers need to be arrayed onto the same surface. In this work, liposomal lipid concentrations are microarrayed onto an `ink-palette' made of PDMS and in combination with multilayer stamping, lipid nanostructures are patterned onto a substrate. Characterization of the patterns is shown using fluorescence and atomic force microscopy. The method is used to test how different lipid-drug combinations alter the efficacy of drugs. .1 Kusi-Appiah, A. E., Vafai, N., Cranfill, P. J., Davidson, M. W. {\&} Lenhert, S. Lipid multilayer microarrays for in vitro liposomal drug delivery and screening. \textit{Biomaterials} \textbf{33}, 4187-4194, doi:10.1016/j.biomaterials.2012.02.023 (2012). 2 Lenhert, S.\textit{ et al.} Lipid multilayer gratings. \textit{Nat. Nanotechnol.} \textbf{5}, 275-279, doi:10.1038/nnano.2010.17 (2010). 3 Nafday, O. A., Lowry, T. W. {\&} Lenhert, S. Multifunctional Lipid Multilayer Stamping. \textit{Small} \textbf{8}, 1021-1028, doi:10.1002/smll.201102096 (2012). [Preview Abstract] |
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