Bulletin of the American Physical Society
78th Annual Meeting of the Southeastern Section of the APS
Volume 56, Number 9
Wednesday–Saturday, October 19–22, 2011; Roanoke, Virginia
Session BB: Nano Materials |
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Chair: Michel Pleimling, Virginia Polytechnic Institute and State University Room: Crystal Ballroom B |
Thursday, October 20, 2011 8:30AM - 9:00AM |
BB.00001: Graphene: it's all about the surface Invited Speaker: Every atom of graphene, a monolayer of graphite, belongs to the surface. Therefore, the environment of graphene -- the substrate onto which graphene is deposited and the coating on top of graphene -- intimately affects the properties of graphene. In this talk, we demonstrate that both mechanical and electrical properties of graphene can be greatly tuned by varying its environment. First, we discuss ultraclean graphene devices suspended in vacuum. We achieve a carrier mobility in excess of 200,000 cm$^{2}$/Vs in these devices and demonstrate previously inaccessible transport regimes, including ballistic transport and the fractional quantum Hall effect. Second, we explore the electrical properties of graphene surrounded by liquid dielectrics. We find that the ions in liquids can cause strong scattering in graphene and demonstrate very large values for room temperature mobility ($>$60,000 cm$^{2}$/Vs) in ion-free liquids with high dielectric permittivity. Finally, we demonstrate that the environment of graphene affects its mechanical properties. We develop a novel technique to study the mechanical properties of graphene films attached to substrates by measuring the temperature-dependent deflection of a ``bimetallic'' cantilever composed of graphene and silicon nitride or gold layers. We demonstrate that the built-in strain, the substrate adhesion force and even the thermal expansion coefficient of graphene depend on the substrate under it. [Preview Abstract] |
Thursday, October 20, 2011 9:00AM - 9:30AM |
BB.00002: Flat-band Nanostructures Invited Speaker: The electronic band structure of many systems, e.g., carbon-based nanostructures, can exhibit essentially no dispersion. Models of electrons in such flat-band lattices define non-perturbative strongly correlated problems by default. Here strong interactions can give rise to novel quantum phases of matter with intriguing collective excitations. Flat bands therefore allow the possibility of discovering emergent physics determined solely by interactions. I will review work that theoretically explores strongly correlated lattice models with flat bands. Zero-field flat-band lattice systems offer arenas to study quantum crystals, quantum liquids, and magnetism. I will also discuss recent results from microscopic modeling of a specific flat-band system, electrons in graphene nanoribbons with zig zag edges. Here I will show that interactions can lead to quantum crystals with ferromagnetic order. [Preview Abstract] |
Thursday, October 20, 2011 9:30AM - 10:00AM |
BB.00003: Spin-dependent quantum transport in nanoscaled geometries Invited Speaker: We discuss experiments where the spin degree of freedom leads to quantum interference phenomena in the solid-state. Under spin-orbit interactions (SOI), spin rotation modifies weak-localization to weak anti-localization (WAL). WAL's sensitivity to spin- and phase coherence leads to its use in determining the spin coherence lengths Ls in materials, of importance moreover in spintronics. Using WAL we measure the dependence of Ls on the wire width w in narrow nanolithographic ballistic InSb wires, ballistic InAs wires, and diffusive Bi wires with surface states with Rashba-like SOI. In all three systems we find that Ls increases with decreasing w. While theory predicts the increase for diffusive wires with linear (Rashba) SOI, we experimentally conclude that the increase in Ls under dimensional confinement may be more universal, with consequences for various applications. Further, in mesoscopic ring geometries on an InAs/AlGaSb 2D electron system (2DES) we observe both Aharonov-Bohm oscillations due to spatial quantum interference, and Altshuler-Aronov-Spivak oscillations due to time-reversed paths. A transport formalism describing quantum coherent networks including ballistic transport and SOI allows a comparison of spin- and phase coherence lengths extracted for such spatial- and temporal-loop quantum interference phenomena. We further applied WAL to study the magnetic interactions between a 2DES at the surface of InAs and local magnetic moments on the surface from rare earth (RE) ions (Gd3+, Ho3+, and Sm3+). The magnetic spin-flip rate carries information about magnetic interactions. Results indicate that the heavy RE ions increase the SOI scattering rate and the spin-flip rate, the latter indicating magnetic interactions. Moreover Ho3+ on InAs yields a spin-flip rate with an unusual power 1/2 temperature dependence, possibly characteristic of a Kondo system. We acknowledge funding from DOE (DE-FG02-08ER46532). [Preview Abstract] |
Thursday, October 20, 2011 10:00AM - 10:30AM |
BB.00004: Synthesis of nanostructures by combination of electrospinning and sputtering techniques Invited Speaker: Electrospinning and sputtering are well known techniques for the formation of different materials in the shape of fibers and films, respectively. Both techniques offer the advantage of being able to prepare a broad range of materials, from metals to insulators, in a different range of compositions and structures. Their combined used offers then a unique opportunity to explore the fabrication of different materials with tailored compositions and nanostructures. An interesting application results when the electrospun fibers are used as templates for sputtering of palladium metal. Palladium (Pd) is one of the most prominent materials studied for the detection of hydrogen gas. Hydrogen rapidly dissociates on its surface and diffuses into subsurface layers forming palladium hydride with consequent changes in optical, mechanical and electrical properties that are easily detected. Materials with nanoscale morphologies are promising to improve sensor performance as they provide large surface areas for adsorption, and smaller crystallite size reducing the time needed for ``bulk'' diffusion. In this presentation it will be shown how Pd nanoribbons and nanoshells are prepared by magnetron sputtering deposition on top of the mat of polymer fibers. Sputtering is a line-of-sight deposition process and the fibers become a variable angle-substrate for the incoming Pd flux. A larger amount of palladium is deposited on top of the fiber where the incoming flux is perpendicular to the surface compared to the sides where the flux is incident at a glancing angle. The top and sides of the fibers shadow their bottom parts closer to the substrate preventing any substantial deposition there. The end result of the deposition is the formation of Pd nanostructures, thicker in the middle region than at the edges, with a large void network. The high sensitivity and response time shown to 1\% or less of hydrogen in nitrogen is understood to result from the reduced dimensions combined with this unique nanostructure. A description will be given of the conductance changes with hydrogen concentration as result of the competing mechanisms of percolation and scattering. [Preview Abstract] |
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