Bulletin of the American Physical Society
2012 Annual Fall Meeting of the APS Prairie Section
Volume 57, Number 14
Thursday–Saturday, November 8–10, 2012; Lawrence, Kansas
Session D2: Condensed Matter Physics III |
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Chair: Siyuan Han, University of Kansas Room: Oread Hotel Griffith Room |
Friday, November 9, 2012 4:15PM - 4:27PM |
D2.00001: Effect of Oxygen on the Stability of Ag islands on Si(111)-(7x7) Dahai Shao, Xiaojie Liu, Ning Lu, C.Z. Wang, Kai-Ming Ho, Michael Tringides, Patricia Thiel We are working to determine whether an electronic effect known as the quantum size effect can influence chemisorption on Ag islands of different height. We have used scanning tunneling microscopy to probe the effect of oxygen exposure on an ensemble of Ag islands separated by a Ag wetting layer on Si(111)-(7x7). Starting from a distribution dominated by islands that are 1 layer high (measured with respect to the wetting layer), coarsening in ultrahigh vacuum at room temperature leads to growth of 2-layer islands at the expense of 1-layer islands, which is expected. If, however, the sample is exposed to oxygen, coarsening leads to growth of 3-layer islands. There is no evidence for oxygen adsorption on top of Ag islands, but there is clear evidence for adsorption in the wetting layer. Density functional theory supports a model in which traces of oxygen on top of Ag islands can change the height-dependent relative stabilities of the islands. Dahai Shao, et al. Surf. Sci. 606, 1871 (2012). [Preview Abstract] |
Friday, November 9, 2012 4:27PM - 4:39PM |
D2.00002: Characterizing the Superconducting Properties of NbSe2 Using Point Contact Spectroscopy James Hansen, Laura Greene Superconductivity has applications from MRI machines in hospitals to high energy particle accelerators like those at CERN. However to find more applications in medicine, research, and industry we must better understand superconductivity and discover higher temperature superconductors. Point Contact Spectroscopy (PCS) is an important tool for studying the electron interactions inside a material. Using PCS I studied the electronic properties of niobium diselenide (NbSe2) which exhibits a charge density wave (CDW) below 33.5 K and superconducts below 7.2 K. The superconducting energy gap was determined by the spectroscopic data obtained being fitted to the Blonder-Tinkham-Klapwijk theory of conductance. Several temperature evolutions of the PCS conductance revealed no signatures of the CDW. The data obtained was also an important diagnostic of PCS junction quality. [Preview Abstract] |
Friday, November 9, 2012 4:39PM - 4:51PM |
D2.00003: Optical Properties of Lead Borate Glasses Containing Ag Nanoparticles P.K. Babu, Akinloluwa Olumoroti, Saisudha Mallur We prepared a series of lead borate glasses containing Ag nanoparticles. Ag nanoparticles were derived from silver nitrate that was added as a precursor during glass preparation. Thermochemical reduction of silver nitrate to silver atom was achieved by controlled annealing near the glass transition temperature. Transmission electron microscope (TEM) images confirm the formation of Ag nanoparticles and the variation of their sizes with the duration of annealing. Optical absorption experiments show that a well-defined surface plasmon resonance (SPR) peak can be observed only for samples that were annealed for 36 hrs. We also investigated the effect of Ag nanoparticles on the fluorescence of Pb$^{2+}$ ions. The excitation spectra obtained at two different emission wavelengths clearly show that Ag nanoparticles create new Pb$^{2+}$ emission centers by altering the chemical environment of lead ions. Comparing our results with earlier investigations on Pb$^{2+}$ fluorescence reveals that the new emission centers represent lead dimers and lead aggregates. Detailed analysis of the emission spectra show that in lead borate glasses containing Ag nanoparticles, the fraction of lead aggregates increases systematically with heat treatment. [Preview Abstract] |
Friday, November 9, 2012 4:51PM - 5:03PM |
D2.00004: Optical techniques to study electronic transport in solids Hui Zhao In most transport studies, currents are generated and detected by electrical techniques. Although these techniques can have superior sensitivities and high spatial resolution, most of them require contacts and device fabrications, can only study steady-state transport, and are insensitive to spin. I will present three optical techniques based on ultrafast lasers that are complementary to these traditional techniques, and can overcome some of the limitations. First, a coherent control technique by utilizing quantum interference between multiple interband transitions can be used to generate ballistic charge and spin currents. Second, by incorporating a differential detection scheme in transient absorption microscopy, we can monitor transport of electrons with sub-nanometer spatial resolution and femtosecond time resolution, even with micrometer-sized laser spots. Third, we use nonlinear optical effects induced by currents to achieve non-invasive and non-destructive detection of charge and spin currents in real time. By combining these techniques, we have recently studied intrinsic spin Hall effect, plasma oscillation, optical effects of spin currents in GaAs, and charge carrier diffusion in several nanoscale materials. [Preview Abstract] |
Friday, November 9, 2012 5:03PM - 5:15PM |
D2.00005: Development of doped and plasmonic graphene for transparent conductive electrodes and photodetector Jianwei Liu, Guowei Xu, Rongtao Lu, Rongqing Hui, Judy Wu Graphene nanohole arrays (GNAs) were fabricated using nanoimprint lithography. The improved optical transmittance of GNAs is primarily due to the reduced surface coverage of graphene from the nanohole fabrication. The exposed edges of the nanoholes provided effective sites for chemical doping using thionyl chloride was shown to enhance the conductance by a factor of 15-18 in contrast to only 2-4 for unpatterned graphene. We fabricated plasmonic graphene using thermally assisted self-assembly of silver nanoparticles on graphene. The localized-surface-plasmonic effect is demonstrated with the resonance frequency shifting from 446 nm to 495 nm when the lateral dimension of the Ag nanoparticles increases from about 50 nm to 150 nm. The plasmonic graphene shows much improved electrical conductance by a factor of 2-4 as compared to the original graphene, making the plasmonic graphene a promising advanced transparent conductor with enhanced light scattering for thin-film optoelectronic devices. Along this direction, we developed a scheme of photodetection based on ionic liquid gated graphene with plasmonic metal nanostructures. [Preview Abstract] |
Friday, November 9, 2012 5:15PM - 5:27PM |
D2.00006: Micromechanical Model for Structural Transition of Secondary Phase Oxide Nanorods in Epitaxial YBa2Cu3O7$-$\textunderscore Films Jack Shi, Judy Wu A micromechanical model based on the theory of elasticity has been developed to study the configuration of self-assembled secondary phase oxide nanostructures in high temperature superconducting YBa2Cu3O7$-$\textunderscore (YBCO) films. With the calculated equilibrium strain and elastic energy of the impurity doped film, a phase diagram of lattice mismatches vs. elastic constants of the dopant was obtained that predicts the energetically-preferred orientation of secondary phase nanorods. The structural transition of the nanorods orientation was studied with impurity doped YBCO films on vicinal SrTiO3 substrates. It was found that the increase of the vicinal angle of the substrate leads to a substantial change of the strain field in the film, resulting in a transition of the nanorod orientation from the normal to in-plane direction of the film. The calculated threshold vicinal angle for the onset of the transition and lattice deformation of the YBCO film due to the inclusion of the nanorods are in very good agreement with experimental observations. This result sheds lights on understanding of the role of the film/substrate lattice mismatch in controlling self-assembly of dopant nanostructures in matrix films. [Preview Abstract] |
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