Bulletin of the American Physical Society
Inaugural Fall 2009 Meeting of the Prairie Section of the APS
Volume 54, Number 17
Thursday–Saturday, November 12–14, 2009; Iowa City, Iowa
Session G2: Condensed Matter Physics II |
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Chair: Giovanni Vignale, University of Missouri Room: IMU 243 (Ballroom) |
Friday, November 13, 2009 2:00PM - 2:12PM |
G2.00001: Role of Competing Interactions on the Nature of Carriers at a Model Polar Oxide Interface Birabar Nanda, Sashi Satpathy Transport measurements of the polar oxide interfaces like LaAlO$_3$/SrTiO$_3$ have revealed a rich variety of physical properties such as a Kondo resistance minimum, metallicity, insulation, superconductivity, and possibly also magnetism under varying experimental conditions. We present a mean-field study of a model Hamiltonian, appropriate for the polar oxide interfaces, that includes the electron hopping, Jahn-Teller coupling, and Coulomb interaction terms. Our results predict the existence of a number of interesting phases, {\it viz.}, a 2D metallic or polaronic phase as well as 3D metallic or a polaronic phase depending on the relative strengths of these interactions. Under appropriate conditions, a mixed phase may also result consisting of 2D polarons and a 3D metal. In the polaronic phase, the combination of Jahn-Teller coupling and Coulomb terms could form magnetic centers. In addition, our results also reveal the possibility of a phase separation, where for some carrier concentrations, the system phase separates into a 2D like region and a 3D region. This will have important implication for the interpretation of the transport measurements. Work supported by the US Department of Energy. [Preview Abstract] |
Friday, November 13, 2009 2:12PM - 2:24PM |
G2.00002: In Situ XAFS studies of the oxygen reduction reaction on carbon supported Pt and PtNi(1:1) catalysts Qingying Jia, Emily Lewis, Corey Grice, Eugene Smotkin, Carlo Segre We have conducted Pt $L_3$ and Ni K edge in situ XAFS measurements on carbon supported Pt and PtNi(1:1) nanoparticle catalysts under a wide range of operating potentials. We observe that in PtNi alloys the Pt-Pt bond distance is shorter and the distribution of Pt and Ni is non-uniform: Pt has a tendency to be found on the surface while Ni is mostly in the interior of the catalyst nanoparticles. In addition, while a change in oxidation of the pure Pt nanoparticles is clearly observed at different potentials, the Pt in the PtNi alloy remains nearly oxygen-free at all potentials but an accompanying oxidation change of Ni has been observed instead. This phenomenon suggests that the presence of Ni inhibits the coverage of oxygen adsorbate on Pt surface, resulting in more active sites on the Pt surface. [Preview Abstract] |
Friday, November 13, 2009 2:24PM - 2:36PM |
G2.00003: High Speed Single Dopant Spin Manipulation with a Single Electrical Gate Victoria Povilus, J.-M. Tang, M.E. Flatt\'e Ultra-low-power computation with spin based electronics can be achieved through coherent spin manipulation. Naturally occurring Mn ions with a bound hole in GaAs provide a uniform system with the potential for fast, all electrical spin manipulation applicable to high-density scalable spin-based electronics [1] and can be probed optically [2]. In an effort to increase device scalability by utilizing a single gate we consider a configuration in which three fields, DC magnetic, DC electric and AC electric, are parallel. With a DC magnetic field of 2.5 T and total electric field strength of $200 kV/cm$, we predict Rabi periods on the order of picoseconds with high visibilities. Assuming each Mn experiences a random electric field, which modifies its spin precession, we performed an ensemble calculation using this Hamiltonian to predict polarization curves from a PL measurement on low concentration Mn in GaAs. In addition we calculate how these curves are affected by a bias DC electric field.\\[4pt][1] J.-M Tang, Jeremy Levy, and M. E. Flatt/'e, Phys. Rev. Lett. 97, 106803 (2006).\\[0pt] [2] R. C. Myers, et al. Nature Mat. 7, 203 (2008). [Preview Abstract] |
Friday, November 13, 2009 2:36PM - 2:48PM |
G2.00004: Jamming in Frictionless and Frictional Systems Leo Silbert The study of jamming of static packings of frictionless spheres has revealed many interesting features that signal the approach of the jamming transition as the packing fraction is varied. This has motivated recent efforts to investigate how jamming occurs under the influence of other parameters such as shearing and thermal agitation. Using granular dynamics simulations we compare and contrast how signatures of jamming in frictionless and frictional systems depend on how the jamming transition is approached. In static systems the packing fraction is varied as previously studied in purely frictionless systems. This method provides a suitable method to identify the random loose packed state. In the other case, structural and dynamical features are studied in granular flows down an inclined plane as the inclination angle is reduced towards the angle of repose. [Preview Abstract] |
Friday, November 13, 2009 2:48PM - 3:00PM |
G2.00005: Drude weight and optical conductivity in doped graphene: effects of electron-electron interaction Saeed H. Abedinpour, Marco Polini, Allan H. MacDonald, Giovanni Vignale The long wavelength behavior of collective modes in ordinary electron liquids with parabolic dispersion is protected from many-body renormalizations by Galilean invariance. In this talk, we will show how the absence of Galilean invariance in the massless Dirac Hamiltonian for a single layer doped graphene implies strong renormalization of Drude weight, optical conductivity and collective modes, even in the long wavelength limit. The physical origin of these renormalizations lies in the coupling between orbital and pseudospin degrees of freedom. Our predictions can be tested by using inelastic light scattering and infrared spectroscopy. [Preview Abstract] |
Friday, November 13, 2009 3:00PM - 3:12PM |
G2.00006: Coexistence between superconductivity and magnetism in the iron pnictides Rafael Fernandes, J\"org Schmalian In this work, we present a theory to analyze the coexistence between the antiferromagnetic and the superconducting phases in the iron arsenides. Particularly, we focus on how distinct regimes of competition between these two phases are related to different symmetries of the Cooper pair wave-function. Using a mean-field microscopic model where superconductivity and itinerant antiferromagnetism are caused by electrons sharing the same bands, we show that while the so-called $s^{++}$ state is generally incompatible to the antiferromagnetic phase, the unconventional $s^{+-}$ state can coexist with magnetism depending on the Fermi surface topology. Neutron diffraction data on $\mathrm{Ba(Fe_{1-x}Co_{x})_{2}As_{2}}$ supporting these conclusions are also presented. [Preview Abstract] |
Friday, November 13, 2009 3:12PM - 3:24PM |
G2.00007: New Insights into High-Performance Thermoelectric Tellurides from $^{125}$Te NMR Spectroscopy E.M. Levin, Y.-Y. Hu, B.A. Cook, J.L. Harringa, K. Schmidt-Rohr, M.G. Kanatzidis Thermoelectric materials are widely used for direct transformation of heat to electricity (Seebeck effect) and for solid state refrigeration (Peltier effect). Efforts to increase the efficiency of high-performance thermoelectrics, which include narrow-gap, doped tellurium-based semiconductors, require detailed knowledge of their local structure and bonding. We have used $^{125}$Te nuclear magnetic resonance (NMR) as a local probe for obtaining better understanding of these high-performance thermoelectric tellurides, specifically PbTe doped with Ag and Sb (LAST materials) and GeTe doped with Ag and Sb (TAGS materials). The resonance frequencies and line shapes of the NMR spectra, as well as spin-lattice relaxation times and chemical shift anisotropies are highly sensitive to the composition and synthesis conditions of LAST and TAGS materials, enabling studies of the \textit{local} composition, distortion, bonding, and carrier concentration. Several intriguing phenomena including electronic inhomogeneity and local distortions of the crystal lattice have been observed by NMR. [Preview Abstract] |
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