Bulletin of the American Physical Society
Annual Meeting of the Four Corners Section of the APS
Volume 57, Number 11
Friday–Saturday, October 26–27, 2012; Socorro, New Mexico
Session E4: Materials Physics III |
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Chair: Nicolai Kalugin, New Mexico Institute of Mining and Technology Room: Macey Center Silver |
Friday, October 26, 2012 4:30PM - 4:42PM |
E4.00001: First principles study of transition metal (TM=Pb, Cu) oxides/sulfides Sean Caudle, Meng Tao, Xihong Peng Earth-abundant transition meal oxides/sulfides have inspired special research attention recently due to their potential applications in solar cells. A clear understanding of the fundamental properties of these materials, especially the electronic properties and their tunability via chemical doping, are critically important towards the applications. In this presentation, we report first principles density-functional theory (DFT) study on the electronic structures of Pb and Cu oxides/sulfides and their oxysulfides compositions. The band structure and bandgap can be systematically tuned by increasing S component in the metal oxides. For example, the DFT predicted bandgap for PbO is 1.72 eV. While the bandgaps for PbO$_{0.937}$ S$_{0.063}$, PbO$_{0.875}$S$_{0.125}$, and PbO$_{0.75}$S$_{0.25}$ are 1.64 eV, 1.43 eV, and 0.79 eV, respectively. For Cu2O, the standard DFT seriously underestimates the bandgap to be 0.49 eV, compared to the experimental value of 2.17 eV. Two methods, DFT+U and hybrid functional (HSE06), were implemented to overcome this problem. Our results showed that DFT+U method fails and the bandgap doesn't further open up by providing a U potential. The hybrid functional predicts the bandgap to be 2.00 eV, which is in a good agreement with the experimental value. [Preview Abstract] |
Friday, October 26, 2012 4:42PM - 4:54PM |
E4.00002: Failure of RVB theory to give superconductivity in layered quasi-two dimensional organic charge transfer solids Niladri Gomes, R. Torsten Clay, Sumit Mazumdar The discovery of high Tc superconductivity in doped cuprates and the consequent proposal of Anderson's RVB theory have brought much focus on the Mott insulators as a novel phase. Following this, RVB mean field and other related theories developed in 1/2-filled band systems have also been applied to study the pressure-induced Mott insulator-to-superconductor transition (without external doping) in the layered quasi-two-dimensional organic charge transfer solids (CTS) like $\kappa$-(BEDT-TTF)$_2$X. We show that although the antiferromagnetic phase of these highly dimerized CTS can be described within effective half-filled band Hubbard or Hubbard-Heisenberg models, the superconducting phase is absent within these models. The so-called valence bond solid that has been found in EtMe$_{3}$P[Pd(dmit)$_{2}$]$_{2}$ is also absent within the effective 1/2-filled band model. We conclude that the effective 1/2-filled band models give an incomplete description of the layered CTS and are thus inappropriate for such systems. [Preview Abstract] |
Friday, October 26, 2012 4:54PM - 5:06PM |
E4.00003: Surface Plasmon Induced Zero-Point Image Repulsion at the Nanoscale Charles Cherqui, Dave Dunlap, Andrei Piryatinski Quantum corrections to the classical image potential of a charge moving towards a metal surface has a long history of interest in physics. This has lead to some interesting effects, namely the existence of electron recoil at the surface of a planar interface. We present a new approach of dealing with the problem and in particular examine the correction to the classical image force for the case of a charge moving towards a metal sphere. We analyze the problem classically to show that this effect can be interpreted as the well known ponderomtive repulsive force for a charge in an fast oscillating field. Quantum mechanically the effect is present, even in the ground state of the parametrically displaced plasmon oscillator (i.e., plasmon coherent state). Based on this observation we propose a new type of field effect transistor based on a carbon nanotube-metal nanoparticle hybrid system. [Preview Abstract] |
Friday, October 26, 2012 5:06PM - 5:30PM |
E4.00004: Exitonic relaxation and coupling in semiconductor nanostructures studied with optical 2D Fourier transform spectroscopy Invited Speaker: Mark Siemens Many next-generation photovoltaic schemes are built on nanoscale confinement effects, but successful implementation of these schemes requires efficient harvesting of energy from the confined states, which depends on fast carrier extraction or exciton diffusion to reaction centers. Understanding how nanoscale environment influences exciton diffusion dynamics is critical to the long-term goal of being able to direct excitons to optimal sites. This electronic structure and dynamics can be captured by optical 2D-Fourier-transform spectroscopy (2DFTS), which tracks the phase of the nonlinear signal during two time delays of a multi-pulse excitation sequence. We used optical 2DFTS to study the coherent response of an ensemble of interfacial ``natural'' GaAs quantum dots (QD), found within the monolayer fluctuations of a quantum well (QW). The QD and the QW are excited simultaneously and homogenous and inhomogeneous linewidths of both are measured. The absence of a phonon-activation peak in the 2D spectra reveals that elastic exciton-phonon scattering is the primary dephasing mechanism. Upon variation of the population time delay and lattice temperature, 2D spectra clearly reveal a coupling from the QW states to the lower energy QD mediated by incoherent phonon interactions. [Preview Abstract] |
Friday, October 26, 2012 5:30PM - 5:42PM |
E4.00005: Finding the Alloy Genome Gus L.W. Hart, Lance J. Nelson, Fei Zhou, Vidvuds Ozolins First-principles codes can nowadays provide hundreds of high-fidelity enthalpies on thousands of alloy systems with a modest investment of a few tens of millions of CPU hours. But a mere database of enthalpies provides only the starting point for uncovering the ``alloy genome.'' What one needs to fundamentally change alloy discovery and design are complete searches over candidate structures (not just hundreds of known experimental phases) and models that can be used to simulate both kinetics and thermodynamics. Despite more than a decade of effort by many groups, developing robust models for these simulations is still a human-time-intensive endeavor. Compressive sensing solves this problem in dramatic fashion by automatically extracting the ``sparse model'' of an alloy in only minutes. This new paradigm to model building has enabled a new framework that will uncover, automatically and in a general way across the periodic table, the important components of such models and reveal the underlying ``genome'' of alloy physics. [Preview Abstract] |
Friday, October 26, 2012 5:42PM - 5:54PM |
E4.00006: Efficient Construction of Robust Materials Models Using Compressive Sensing and Bayesian Inference Lance Nelson, Gus L.W. Hart, Fei Zhou, Vidvuds Ozolins Recently, a technique from the field of signal processing, compressive sensing, has emerged as an efficient and robust way to construct models for describing materials' properties. Compressive sensing exploits the widely-held intuition that the properties of materials can be expressed using a small number of variables. Using this assumption to restrict the solution search results in an efficient way for building very robust models. One way to restrict the model space is through the use of Bayesian inference. In a natural way, Bayesian methods provide error bars on predictions made, a systematic approach for adding data, and noise quantification. We demonstrate Bayesian compressive sensing applied to a cluster expansion model, but the approach is general and could be used in many other model building approaches. This new technique for building materials models, combined with high-throughput {\emph ab-initio} databases, will allow the fast construction of alloy models for hundreds of systems, representing a major step forward in the endeavor to discover the underlying ``genome'' of alloy physics. [Preview Abstract] |
Friday, October 26, 2012 5:54PM - 6:06PM |
E4.00007: Modified EFG Components and Their Joint pdf for Use in Modeling ihb in PAC M. Adams, P. Matheson, T. Park, M. Stufflebeam, J. Hodges, W.E. Evenson, M.O. Zacate Spectra of hyperfine interactions involving the electric field gradient tensor (EFG) are subject to broadening by statistical variations of EFG components. In perturbed angular correlation (PAC) experiments, the inhomogeneous broadening (ihb) of the $G_{2}$(c,$t)$ spectrum is produced by randomly distributed lattice defects of concentration, $c$. The EFG tensor has two independent components. The concentration dependence of ihb is determined by the joint probability distribution function (pdf) of these components. In typical PAC analyses, the independent coordinates are assumed to be$ V_{zz}$ and the asymmetry parameter $\eta =$ $(2V_{xx}+V_{zz})/V_{zz}$. However, the pdf $P(c,V_{zz}$,$\eta )$ is not known, and in any case it is easy to show that $V_{zz}$ and $\eta $ are highly correlated, and not independent. We have found that the application of the Czjzek transformation [1], followed by a simple conformal mapping produces two, nearly independent EFG coordinates $W_{1}(c,V_{zz},\eta )$ and $W_{2}(c,V_{zz},\eta )$. The pdfs of each coordinate are readily characterized, and their product $P(c,W_{1},W_{2})=P_{1}(c,W_{1})P_{2}(c,W_{2})$ forms an appropriate joint pdf that can be used to model ihb in a variety of situations. We show the application of this method by reporting results modeling the concentration dependence of ihb in various PAC models, for simple cubic (sc), face-centered cubic (fcc) and body-centered cubic (bcc) lattices. \\[4pt] [1] Czjzek, G. Hyperfine Interactions 14(1983) 189-194. [Preview Abstract] |
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