Bulletin of the American Physical Society
Annual Meeting of the APS Four Corners Section
Volume 60, Number 11
Friday–Saturday, October 16–17, 2015; Tempe, Arizona
Session E6: Materials V: Materials Theory and Analysis |
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Chair: Quan Qing, Arizona State University Room: MU246 |
Friday, October 16, 2015 3:17PM - 3:29PM |
E6.00001: Determination of E$_{\mathrm{0}}$ band gaps of Ge-rich GeSi films using UV-Vis ellipsometry Chi Xu, James Gallagher, Charutha Senaratne, John Kouvetakis, Jose Menendez Ge-rich Ge$_{\mathrm{1-x}}$Si$_{\mathrm{x}}$ (x$=$0.003-0.132) films were grown in a gas-source molecular epitaxy reactor on Si(100) by using new-generation group-IV gaseous reactants Ge4H10 and Si4H10. Films were around 1.5 micron thick with excellent crystallinity as shown by sharp and symmetric XRD peaks. UV-Vis ellipsometry data were taken in the range of 0.6-1.5 eV with 5 meV intervals. Dielectric functions were obtained from point-by-point fits, and two data analysis methods were employed to extract fundamental band gap E$_{\mathrm{0}}$ values. The 1st method fits the imaginary part $\varepsilon $2 with a theoretical expression consolidating all contributions to the dielectric function. The 2nd method first numerically smoothes and differentiates the experimental $\varepsilon $1 and $\varepsilon $2 to obtain second derivatives with respect to energy, which are then fitted together using an expression of a three-dimensional critical point. Effects of small residual strains were corrected to obtain band gap values for strain-free materials. Excellent agreement between these two methods has been achieved. Analysis of the compositional dependence of E$_{\mathrm{0}}$ revealed a negative bowing parameter which is greater compared to literature. [Preview Abstract] |
Friday, October 16, 2015 3:29PM - 3:41PM |
E6.00002: From Database to Materials Seyedayat Ghazisaeed, Boris Kiefer Designing new materials with specific features is one of the main scientific and technological challenges, at present. Large experimental and computational efforts are devoted to reach this goal. Still, these efforts are largely based on trial and error strategies and the question arises how to guide and how to accelerate this design process. An approach that addresses both questions is based on the realization that large databases of material structures have been compiled since the advent of diffraction in the early 20$^{\mathrm{th}}$ century. In particular, the detailed analysis of these databases gives access to local structure and geometrical distortions. These distortions in turn, are at least partly responsible for magnetic and electronic properties of transition metal bearing compound. We will present results of our effort to develop and employ database search strategies that extend the ability of available crystal structure search engines. We will also discuss sample search results and how they may guide and accelerate material design. [Preview Abstract] |
Friday, October 16, 2015 3:41PM - 3:53PM |
E6.00003: Understanding Integration Schemes in DFT Matthew Burbidge, Spencer Hart, Jeremy Jorgensen, Gus Hart The 1998 Nobel prize was given to Kohn and Pople for their development of Density Functional Theory. DFT allows one to do quantum-mechanical calculations for materials and has been developed into a powerful computational tool. Typical DFT calculations require a numerical integral over the electron occupied states in the material. Even though this integral is a small piece of the overall calculation, it is a primary source of error. Through the use of a simple toy problem, we will explain the fundamentals of the integration problem. We will introduce some of the attempts at resolving it and explore their effectiveness in current DFT codes. The resolution of this integration problem for metals will result in millions of CPU hours saved for a typical computational materials scientist. [Preview Abstract] |
Friday, October 16, 2015 3:53PM - 4:05PM |
E6.00004: Electronic and Magnetic Properties of TMPO$_{\mathrm{4}}$ (TM$=$Fe, Mn, Cr) Boris Kiefer Advances in experiment and theory continue to increase our understanding of macroscopic and atomistic structure/property relationships in materials. However, solving the property to structure problem, and hence rational materials design remains challenging at present. Here we report on the use of a few simple rules to design a structural template for a new class of half-metals. In the proposed structure tetrahedral TMO$_{\mathrm{4}}$ (TM$=$Fe, Mn, Cr) groups share corners with intermittent PO$_{\mathrm{4}}$ groups to form a 3d periodic bond topology. All computations are based on spin-polarized Density-Functional-Theory (DFT) computations at the GGA-PBE level using all-electron like PAW interaction potentials. FePO$_{\mathrm{4}}$ is antiferromagnetic consistent with experimental observations. In contrast, the DFT results for TM$=$ Mn and Cr predict half-metallicity: a spin-gap in the minority spin channel and integer magnetic moments of 3 $\mu_{\mathrm{B}}$/fu and 4 $\mu _{\mathrm{B}}$/fu for the Cr and Mn compound, respectively. Furthermore, in both compounds the half-metallic state is energetically more favorable as compared to the competing antiferromagnetic state. We will also discuss our DFT$+$U results that allow assessing the reliability of the DFT predictions that MnPO$_{\mathrm{4}}$ and CrPO$_{\mathrm{4}}$ are half-metals. [Preview Abstract] |
Friday, October 16, 2015 4:05PM - 4:29PM |
E6.00005: French Fries or Onions? Improving integration in DFT calculations Invited Speaker: Gus Hart The amount of recent cpu time ($>100$ mega cpu hours) spent in our group on high-throughput materials prediction led us to re-examine convergence issues in standard DFT calculations. Calculations for metals typically take 100 times longer than calculations for semiconductors. By improving the integration technique can we shrink the``metal deficit''? Conceptually one can attack the problem in the typical ``rectangle" fashion (french fries) or by integrating in energy space (onions). The standard approach converges very slowly in the case of metals. Revisiting the k-point integration issue in light of modern DFT practice, we demonstrate that this ``metal deficit'' can be reduced to only a factor of 5--10 worse than semiconductors. [Preview Abstract] |
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