Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P8: Electrons, Phonons, and Electron Phonon Scattering V |
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Sponsoring Units: DCOMP Chair: Francesco Mauri, University of Roma Room: 267 |
Wednesday, March 15, 2017 2:30PM - 2:42PM |
P8.00001: Importance of strong-correlation on the lattice dynamics of light-actinides Th-Pa alloy Omar De la Peña Seaman, Rolf Heid, Klaus-Peter Bohnen We have studied the structural, electronic, and lattice dynamics of the Th$_{1-x}$Pa$_x$ actinide alloy. This system have been analyzed within the framework of density functional perturbation theory, using a mixed-basis pseudopotential method and the virtual crystal approximation (VCA) for modeling the alloy. In particular, the energetics is analyzed as the ground-state crystal structure is changed form fcc to bct, as well as the electronic density of states (DOS), and the phonon frequencies. Such properties have been calculated with and without strong correlations effects through the LDA+U formalism. Although the strong-correlation does not influence on a great manner the Th properties, such effects are more important as the content increases towards Pa, affecting even the definition of the ground-state crystal structure for Pa (experimentally determined as bct). The evolution of the density of states at the Fermi level ($N(E_F)$) and the phonon frequencies as a function of Pa-content are presented and discussed in detail, aiming to understand their influence on the electron-phonon coupling for the Th-Pa alloy. [Preview Abstract] |
Wednesday, March 15, 2017 2:42PM - 2:54PM |
P8.00002: Configurational entropy of solid solutions and high entropy alloys Michael Widom Multicomponent solid solutions such as high entropy alloys deviate from the ideal entropy of mixing owing to short-range chemical order among the constituent elements. The "mutual information" contained in correlation functions represents the entropy loss relative to ideal mixing. The entropy approximation of the cluster variation method is an example of this principle. We apply this method to estimate the configurational entropy of multicomponent body-centered cubic, partially ordered B2, and C15 Laves phase alloys using correlation functions obtained from first principles Monte Carlo/molecular dynamics simulations. [Preview Abstract] |
Wednesday, March 15, 2017 2:54PM - 3:06PM |
P8.00003: Microstructure of high entropy alloy: CoCuFeNi Phillip Sprunger, David Plaisance, Boliang Zhang, Amitava Roy, Shengmin Guo The properties of CoCrFeNi powder alloy were prepared by ball milling methods were investigated and compared to arc melted samples. The electronic and microstructural properties of this high entropy alloy (HEA) were investigated via X-ray powder diffraction (XRD), as well as X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, X-ray and VUV photoemission (PES) spectroscopy. XRD reveals a single-phase fcc structure with a lattice constant consistent with previous studies from arc-melting preparation method. XANES reveals subtle changes in metallic edge structure compared to single-phase elemental samples. Moreover, in spite of EXAFS limited energy range of these\textit{ 3d} transition metals, generated $\chi $(k) and corresponding Fourier transforms of this solid solution HEA alloy reveals that the nearest neighbor distances are nearly equal for all four elements (Co, Cr, Fe, and Ni), although subtle differences from bulk phase are observed. XPS and PES results of surface properties, and relevance to corrosion resistance, will also be presented. [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P8.00004: Carrier-induced phonon stiffening and localization in rutile TiO2 Grigory Kolesov, Boris Kolesov, Efthimios Kaxiras Crystalline transition metal oxides such as TiO$_2$ possess attractive properties for many practical applications. Despite several decades of researching rutile TiO$_2$ carrier-ion interactions that account for many important properties of this material, such as carrier mobility, effective mass and photo-catalytic activity, are still the subject of controversy. Here we study carrier properties in rutile TiO$_2$ both experimentally, using Raman spectroscopy of photo-excited samples, and computationally, with the real-time time-dependent density functional theory (RT-TDDFT) quantum-classical method, which employs mean-field classical (Ehrenfest) dynamics to couple the electronic and ionic subsystems. From simulation we find that small polaron formation in rutile titania is a strongly non-adiabatic process with the characteristic time scale of about 55 fs. In both experiment and theory we observe an unexpected stiffening of the $A_{1g}$ phonon mode under UV illumination. We computationally analyze the polaron structure and explain the observed effect. The resulting form of the potential with respect to oxygen atoms and $t_{2g}$-orbitals of central Ti atom offers a possible explanation for an anomalous temperature-dependence of the Hall mobility in rutile titania. [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P8.00005: Spin-lattice relaxation in Diamond NV-Centers Johannes Gugler, Peter Mohn Nitrogen-Vacancy centers in Diamond exhibit a very long electronic spin-relaxation time T$_{1}$. Thus, they are a promising candidates for solid-state qubit implementation. The temperature dependence of T$_{1}$ at the mK-temperature scale was not understood up to now. We performed DFT-calculations to obtain the structural, electronic and phononic properties of the NV-center in Diamond to investigate on the spin-lattice relaxation rate. Using the numerically obtained wavefunctions and the phononic bandstructure we calculated the electron-phonon transition matrix element of first order in time dependent perturbation theory. Taking into account spin-orbit coupling and the electronic screening of the electron-ion potential, we obtain the temperature dependence and the relaxation rates in agreement with experiment. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P8.00006: Ab-initio computation of Raman spectra within the DFPT-PAW formalism Lucas Baguet, Marc Torrent The interpretation of experimental Raman spectra of materials is, in general, a difficult task. To compare these spectra with theoretical ones gives a deeper understanding of the underlying physics phenomena. The computation of theoretical Raman spectra require the third-order derivatives of the ground state energy, which can be done using the $2n+1$ theorem, as in the Density Functional Perturbation Theory (DFPT), or with finite difference methods. Existing works using the $2n+1$ theorem are limited to norm-conserving (NC) (as in Veithen \emph{et al}, Phys. Rev. B 71, 125107 (2005)) or ultra-soft (US) pseudo-potentials (Miwa, Phys. Rev. B 84, 094304 (2011)). We present a method adapted to the projector-augmented wave (PAW) formalism, which conciliate the small cost of NC pseudo-potentials with the precision of all-electron calculations. Contrary to Veithen \emph{et al}, the electric field perturbation is treated analytically, leading to the resolution of a second-order Sternheimer equation (as in the work of Miwa). Comparing the different approaches, our results show that the analytical one has the best k-point convergence. [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P8.00007: Atomistic Modeling of Interconnect Resistance in Emerging Semiconductor Technologies Oscar D. Restrepo, Eduardo C. Silva, Byoung Youp Kim, Craig Child, Murali Kota With extreme scaling in semiconductor devices, power consumption due to Back-End-Of-Line (BEOL) interconnect resistance is critical in the performance of semiconductor chips. A typical via structure consists of Cu/diffusion barrier/binder/Cu. Besides the intrinsic resistance of the Cu Via, the TaN diffusion barrier, Co binding layer, and defects such as voids contribute to the overall BEOL resistance. To overcome the TaN resistance at the via interface, a method was developed for selective nitridation of via dielectric side walls, avoiding the formation of a TaN layer between the two Cu layers. Here, we perform Ab-initio electronic structure and transport calculations to understand the impact of $\alpha$ and $\beta$ phases of Ta, as well as replacing Co with Ru. We find excellent agreement with the experimental measurements of 30-40\% reduction in resistance for $\beta$-Ta compared to TaN, and predict that $\alpha$-Ta will further reduce the interface resistivity by 15\%. We also found that replacing Co with Ru causes an increase in the resistivity, even though void formation is reduced, which agrees with experimental results. This work lays the foundation for future work in optimizing BEOL resistance that is critical to the performance of nano-device based semiconductor chips. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P8.00008: Static and dynamic structural evolution of Fe/Ni multilayers studied by an ultrafast optical pump/x-ray probe technique. Aaron Loether, Brian Kelly, Anthony DiChiara, Robert Henning, Matthew DeCamp, Karl Unruh The static and dynamic structural evolution of an Fe/Ni thin film multilayer has been studied utilizing the picosecond (ps) duration optical pump pulses and 100 ps duration x-ray pulses available at the BioCARS (sector 14) beamline at the Advanced Photon Source at Argonne National Laboratory. The time-resolved x-ray diffraction patterns of the as-prepared multilayers reflected the effects of both reversible thermal expansion and irreversible alloying of the Fe and Ni multilayers by solid-state diffusional mixing. On the other hand, the time-resolved diffraction patterns of a fully alloyed multilayer were only due to thermal expansion. Based on a these two sets of measurements we conclude that optical excitation produces a lattice strain corresponding to a temperature rise of about 1000 K and a decay constant of 10s of nanoseconds (ns). In comparison, diffusional mixing takes place on a time scale about 10 times longer (i.e. on the order of 100s of ns). These time scales have been confirmed by the results of numerical simulations. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P8.00009: Mode coupling in liquid Zn. An ab-initio molecular dynamics study. Beatriz Gonzalez del Rio, Luis E. Gonzalez, David J. Gonzalez The static and dynamic properties of liquid Zn have been studied using an ab initio molecular dynamics method. Results are reported for the thermodynamic states at 723 K for which inelastic neutron scattering data are available [1]. The calculated static structure shows very good agreement with experimental measurements, including an asymmetric second peak. The dynamic structure reveals the existence of a second propagating mode in both the longitudinal and transverse current correlation functions. We explain this anomaly, previously observed in other liquid metals under high pressure [2], using Sjögren and Sjölander mode coupling theory (MCT) [3]. The MCT provides a satisfactory description of the results although, it lacks accuracy in some wavevector regions. Nevertheless, it provides a first step in the understanding of an interesting phenomenom that is starting to be observed in liquid metals and that, up to now, was thought to only appear at high pressures. [1] M. Zanatta et al, Phys. Rev. Lett. 114, 187801 (2015) [2] T. Bryk et al, J. Chem. Phys. 143, 104502 (2015); M. Marqués et al, J. Phys. Condens. Matter 28, 075101 (2016) [3] L. Sjögren, Phys. Rev. A 22, 2883 (1980) [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P8.00010: Temperature dependence of the pressure-induced topological phase transition in BiTeI V\'eronique Brousseau-Couture, Michel C\^ot\'e Tunable topological insulators are materials in which the band inversion can be turned on and off through an experimentally controllable parameter. Understanding the effect of temperature on these phase transitions becomes crucial for real devices intended to work in various conditions. This temperature dependence is directly related to the electron-phonon interactions, which induce a shift in the electronic band energies that may promote or suppress the topologically non-trivial phase. We investigate the temperature dependence of the pressure-induced topological phase transition in three-dimensional crystalline topological insulator BiTeI through first principles methods. We compute the electron-phonon coupling and the electronic bandstructure using density functional perturbation theory (DFPT). [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P8.00011: Thermally Dependent Behavior of Bimetallic Shells and Comparison to the Wittrick Model Maureen Smith, Moira Foster, M. J. Moelter, T. D. Gutierrez, N. C. Keim, N. C. Heston The build-up of thermal stress in shallow bimetallic shells can result in a rapid transition between concave and convex equilibrium states. Experimental literature is lacking that documents this dynamic transition process. We present here our research which documents the thermally dependent shape of bimetallic shells, and the resulting hysteresis cycle. Additional work shows that thermal treatments can be used to shift the hysteresis cycle. We also share the results of our high speed photography in which we’ve captured for the first time the dynamic transition process between stable states. To our knowledge, our work provides the first experimental data which can be compared to the Wittrick Model for spherically symmetric bimetallic shells. We observe trends consistent with his predictions. [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P8.00012: Conductive Barrier Layers of TiN$_{\mathrm{x}}$ for Measurements of Silicon Films at High Temperature brian Kearney, battogtokh Jugdersuren, Xiao Liu Electrical measurements of Silicon-based thin films can be challenging at high temperatures due to silicide formation and inter-diffusion of silicon with metal leads. We examine thin sputtered titanium nitride (TiN$_{\mathrm{x}})$ films and demonstrate that by saturating the TiN$_{\mathrm{x}}$ with Nitrogen and using the proper deposition and annealing temperatures, a stable, conductive barrier is produced. Rutherford backscattering (RBS) is used to characterize the film density and composition, conductivity is measured up to 750 K. A sputtered platinum film is deposited on TiN$_{\mathrm{x}}$ to further test the effectiveness of the barrier versus a silicon substrate. These films show excellent stability and conductivity after repeated thermal cycling, making them ideal for high temperature electrical measurements of silicon-based thin films. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P8.00013: Raman scattering study of low-energy excitations in the metallic glass Ni$_{67}$Zr$_{33}$ Rudolf Hackl, Bernhard Muschler, Istv\'an T\"utt\H{o}, Alfr\'ed Zawadowski, Judit Balogh We present Raman scattering results on the metallic glass Ni$_{67}$Zr$_{33}$. The spectra were measured as a function of polarization and temperature and were analyzed theoretically. There are three relevant types of excitations which can be disentangled: (i) A temperature independent Drude-like response due to electron-hole excitations, as suggested earlier for dirty metals, extending to approximately half an eV. (ii) A strictly linear response in the range below 20\,cm$^{-1}$. (iii) A maximum which loses about 30\% of its spectral weight upon increasing the temperature, just opposite to what is expected from the occupation number of bosonic excitations. The temperature dependence of the spectral weight indicates that local modes contribute substantially to the cross section. The increase with decreasing temperature can be described in a qualitative way assuming that scattering of conduction electrons is dominated by the temperature dependence of the Debye-Waller factor rather than by the occupation number of the vibrations alone. None of our assumptions is material specific, and the results are expected to be relevant for disordered systems in general such as doped semiconductors, metallic functionalized carbon nanotubes or polymers. [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P8.00014: Analysis of the Temperature Dependence of the Thermal Conductivity of Insulating Single Crystal Oxides Francisco Rivadulla, Eric Langenberg, Elias Ferreiro Vila, Victor Leborán, Adolfo Otero-Fumega, Victor Pardo The application of epitaxial thin-films to a broad range of phenomena which require a detailed knowledge of heat transport (thermoelectricity, heat-assisted spin-transfer effects, thermal insulation and heat dissipation in nanostructures, etc) is becoming increasingly important. For the particular case of oxides, strain engineering by epitaxial growth of thin films is a powerful technique when it comes to gaining access to novel phases, and thus engineering new or enhanced functional properties. A systematic variation of the film properties requires therefore the use of different substrates, which may differ substantially in their lattice parameters and orientation. Here we report the temperature dependence of the thermal conductivity of 27 different single crystal oxides, from 20 K to 350 K. These crystals have been selected among the most common substrates for growing epitaxial thin-film oxides, spanning over a range of lattice parameters from 3.7 Å to 12.5 $\AA$. Different contributions to the phonon relaxation time are discussed on the basis of the Debye model. This work provides a database for the selection of appropriate substrates for thin-film growth according to their desired thermal properties, for applications in which heat management is important. [Preview Abstract] |
Wednesday, March 15, 2017 5:18PM - 5:30PM |
P8.00015: First-principles Investigation of the Structure, Mobility and Optical Properties of Self-Trapped Excitons in Alkali Metal, Lanthanum and Barium Halide Scintillators Gregory Bizarri, Mauro Del Ben, Edith Bourret, Andrew Canning The performance of new and improved materials for gamma ray scintillator detectors is dependant on multiple factors such as quantum efficiency, energy transport etc. In halide scintillator materials the energy transport is often impacted by self-trapped exciton (STE) formation and mobility. We present first-principles calculations at the hybrid density functional theory level for the structure, mobility and optical properties of STEs and their associated lattice defects ($V_K$ centers) in two important families of scintillator materials, alkali metal and lanthanum halides (AX and LaX). AX and LaX have been extensively characterized by experiments and serve as benchmark systems to assess the accuracy of our theoretical procedure. We show that hydrid functionals accurately predict the different types of self-trapped excitons (on and off-center) found in AX and LX materials in agreement with EPR experiments. We then applied this approach to perform preliminary studies on classes of new scintillator materials including the barium mixed halides and compared with our new experimental results. These studies have the potential to benefit the development of improved scintillator materials tailored for specific applications. [Preview Abstract] |
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