Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q43: Metals II 
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Chair: David Parker, Oak Ridge National Laboratory Room: Mile High Ballroom 4B 
Wednesday, March 5, 2014 2:30PM  2:42PM 
Q43.00001: Possible Superconductivity in the 2Dimensional Electride Ca$_{2}$N Jeonghoon Ha, Hongwoo Baek, Duming Zhang, Yeji Kim, SungWng Kim, Young Jae Song, Young Kuk, Fred Sharifi, Joseph A. Stroscio An electride is an ionic compound in which electrons take the place of negative charged ions and the topology of the cavities confining these anionic electrons determines the physical properties of the material. A recent study reported Ca$_{2}$N to have a layered structure with anionic electrons confined to 2D cavities between the cationic crystal layers. Magnetoresistance measurements confirmed diffusive 2D transport in electron layers. In the present work, we use an ultralow temperature STM to investigate the local electronic structure of a cleaved surface of a Ca$_{2}$N crystal. A small energy gap was observed in the tunneling spectrum with a main gap of 0.4 meV. The spectra contain multiple coherencelike peaks suggestive of possible multijunction superconductivity. Temperaturedependent measurements show a gradual reduction of the gap up to 2 K, but the gap is not suppressed in the presence of a perpendicular magnetic field up to 14.5T, suggesting that if the material is superconducting, then the upper critical field is extremely large compared to the transition temperatue. This can be understood in the context of recent reports on unconventional superconductivity of chalcogenide compounds. [Preview Abstract] 

Q43.00002: ABSTRACT WITHDRAWN 
Wednesday, March 5, 2014 2:54PM  3:06PM 
Q43.00003: Flux Growth of Large Single Crystals of YFe$_{2}$Al$_{10}$ by Nucleation Site Reduction Jedediah KistnerMorris, Liusou Wu, William Gannon, Meigan Aronson The metallic delectron compound YFe$_{2}$Al$_{10}$ is near a quantum critical point. Large single crystals of this compound are required for inelastic neutron scattering experiments. We synthesized high quality single crystals via aluminum flux growth. A number of adjustments to the growth procedure were required to optimize crystal quality and size. First, the cooling rate of the flux growth was adjusted to produce a thermodynamically favorable environment for YFe$_{2}$Al$_{10}$ growth, which was found to grow around 920$^{\circ}$C. Second, initial composition of the growths were then optimized to avoid the growth of the binary phases, YAl$_{3}$ and Fe$_{4}$Al$_{13}$, as well as to maximize crystal size and reduce site nucleation. Third, site nucleation was further reduced by polishing the alumina growth crucibles with sandpaper and then etching them with aqua regia. The result after optimization is that individual growths produced three to five polyhedral crystals with single facets up to 9mm in width, and mass of about 700mg. The implemented nucleation site reduction techniques can be applied to other flux systems to increase crystal size and mass. [Preview Abstract] 
Wednesday, March 5, 2014 3:06PM  3:18PM 
Q43.00004: Structural Evolution in NiZr Liquids and Glasses Cooper SinaiYunker Metallic alloys have been shown to possess a widerange of glassforming abilities (GFA). While multicomponent alloys tend to have higher GFA than simpler alloys, the underlying principles that govern GFA are relatively unknown. While the atomic arrangements in metallic glasses do not possess welldefined longrange atomic order that characterizes crystalline metals they do demonstrate short (SRO) and mediumrange (MRO) atomic order. Previous studies suggest a link between structural evolution and GFA. In this talk we discuss recent results on the temperature evolution of the atomic structures of NiZr liquids and glasses using \textit{in situ} highenergy synchrotron Xray diffraction. NiZr is an excellent system to explore due to its simplicity but also because it forms the basis for high GFA multicomponent alloys. By utilizing the beamline electrostatic levitation (BESL) technique, several compositions were prepared in both the equilibrium and supercooled state. Reverse Monte Carlo (RMC) fits were conducted whereby 3D atomic configurations consistent with experimental data were be generated and subsequently analyzed. The fits were constrained by partial paircorrelation functions generated from \textit{ab initio} molecular dynamics simulations. Quantitative results will be discussed which suggest that NiZr has a high degree of MRO, chemical ordering and is a highly \textit{fragile} system. [Preview Abstract] 
Wednesday, March 5, 2014 3:18PM  3:30PM 
Q43.00005: Pair interaction model based Kinetic Monte Carlo simulation of oxygen and metal diffusion in Nibased alloys Dominic Alfonso, DeNyago Tafen Investigation of oxygen and metal diffusion processes in Nibased alloy is a problem of high relevance in the area of understanding corrosion behavior. We explored the use of combined approach consisting of density functional theory to compute migration barriers and kinetic Monte Carlo method to evaluate longtime diffusivities of oxygen and metals in Ni containing Al and Fe. Pair interaction model was used to evaluate the influence of the local environment on the kinetic parameters. For reference, vacancy mediated selfdiffusion in pure Ni was examined. A diffusion prefactor and barrier of Do $=$ 2.4 x 10$^{5}$ m$^{2}$/sec and Q $=$2.90 eV were predicted in very good agreement with experimental values of 9.8 x 10$^{5}$ m$^{2}$/sec and Q $=$2.88 eV. Results for (i) Al impurity, (ii) Fe impurity and (iii) concentrated Fe diffusion exhibit good agreement with available experimental data. The model was employed for oxygen diffusion in Ni and it was found that the inclusion of vacancy improves the level of agreement with compilation of experimental studies. The presence of either Al or Fe reduces the diffusivity of oxygen. This was attributed to the increase in the excess energy of oxygen in Ni due to these metals. [Preview Abstract] 
Wednesday, March 5, 2014 3:30PM  3:42PM 
Q43.00006: High through put computation of defect properties in metals Bharat Medasani, Maciej Haranczyk, Mark Asta We combine first principles density functional method and MPWorks, a high throughput framework, to compute the vacancy formation energies in metals. Three different exchange correlation (xc) functionals, PBE, PW91 and LDA are evaluated with respect to the computed formation energies. Bulk and defect structures are relaxed using a mesh of 108000 kpoints X atoms to achieve an accuracy of 10 meV or better. Of the three functionals, LDA gives better results compared to the PBE and PW91 (GGA) functionals due to the cancellation of exchange and correlation errors arising due to the presence of internal void surface. PBE and PW91 predict noticeably different vacancy formation energy values even though the lattice constants and cohesive energies predicted by them are very close. Applying surface error correction brings the formation energies computed with the three functionals closer to the experimental values. The surface correction is in general small for LDA and bigger for GGA functionals. MetaGGA functionals are expected to predict better surface energies and hence better vacancy formation energies. We report the performance of one such metaGGA functional, revTPSS. [Preview Abstract] 
Wednesday, March 5, 2014 3:42PM  3:54PM 
Q43.00007: Magnetic and structural properties of transition metals and their alloys Matteo Cococcioni, Koichiro Umemoto, Burak Himmetoglu In this talk I will discuss the relationship between magnetic and structural properties emerged from recent DFT calculations on transitionmetals and their alloys. In the first part I will present results on Ni$_2$MnGa, a prototype magnetic Heusler alloy. I will demonstrate that improving the description of localized electrons on the d states of Mn is crucial to fix both the magnetization and the relative stability of the austenite and martensite phases. The larger energy difference between Hubbard bands obtained from DFT+U also proves fundamental to capture the stabilization of the non distorted austenite in alloys with eccess Mn, in agreement with experiments. In the second part I will report our recent discovery of a new phase of bulk Fe. The new allotrope is characterized by a unit cell of six atoms and a crystal structure based on a ``wavy'' pattern of distorted Fe octahedra. Although always metastable, it is more stable than other known phases (e.g., HCP) and transforms into FCC Fe under pressure. In addition, the distorted crystal structure results in a magnetization density about 10\% higher than that of other allotropes which could disclose interesting applications for this materials, including magnetic steels and rareearthfree permanent magnets. [Preview Abstract] 
Wednesday, March 5, 2014 3:54PM  4:06PM 
Q43.00008: The elastic properties and martensitic transformation behaviour of Ti$_{50}$Pt$_{\mathrm{50x}}$Co$_{\mathrm{x}}$ shape memory alloy Hasani Chauke, Rosinah Mahlangu, Phuti Ngoepe Tibased shape memory alloys (SMAs) such as PdTi, AuTi and PtTi are important in the design for high temperature alloys due to their high martensitic transformation (Ms) of above 673 K. PtTi is the most attractive for the development of high temperature shape memory alloys (HTSMAs) since it has the highest Ms of about 1273 K. Above 1273 K The crystal structure is the ordered cubic B2 phase, and transforms to an orthorhombic B19 phase at lower temperature. The supercell approach method was used to investigate the effect of partial substitution of Pt with Co on the TiPt potential shape memory alloy. The firstprinciples calculations were carried out within the generalized gradient approximation to determine the stability of the T$_{\mathrm{i5}}$0P$_{\mathrm{t50}}$xCo for x=6.25, 18.75 and 25. We found that the calculated heats of formation and density of states predicted the 6.25 at.{\%} Co to be the most stable structures compared. The elastic properties and the phonon dispersion results suggest that the partial substitution of Pt with Co increases the Ms of TiPt with the softening of the ' shear moduli. [Preview Abstract] 
Wednesday, March 5, 2014 4:06PM  4:18PM 
Q43.00009: Calculating the titanium $\alpha$ to $\beta$ transition using tightbinding molecular dynamics: a comparison of methods Jon Williams, Daniel Finkenstadt, A. Shabaev, N. Bernstein, S.G. Lambrakos, M.J. Mehl In this we analyze the Titanium phase transition between HCP and BCC structures. TightBinding Molecular Dynamics Simulations were run with 64 and 216 atom lattices over temperatures ranging from 50 to 1500 K. Analysis of the data included the phonon Density of States, time evolution of the lattice structure and temperature, and the Vibrational Free Energy of the system. We also develop a theoretical model of the transition based on a perturbation in the Harmonic Oscillator using the coherent states representation. In addition to the tightbinding simulations, we use density functional theory with variable cell shape molecular dynamics at a range of temperatures to study the transition. Direct simulations shows the transition between the two structures, and we investigate the use of constrained simulation to calculate their free energy differences. [Preview Abstract] 
Wednesday, March 5, 2014 4:18PM  4:30PM 
Q43.00010: Anisotropic magnetoelastic coupling and magnetocaloric effect in the Fe$_{5}$Si$_{3}$type hexagonal single crystal Guixin Cao, Satoshi Okamoto, M.A. McGuire, Junjie Guo, Ling Li, Jieyu Yi, David Mandrus, Matthew F. Chisholm, Brian C. Sales, Zheng Gai The structural, magnetic properties, and the magnetocaloric effect (MCE) of Fe$_{5}$Si$_{3}$ single crystal with trace of Mn and P doping (Fe$_{4.83}$Mn$_{0.16}$Si$_{2.91}$P$_{0.09})$ are investigated. A firstorder magnetoelastic transition was found at the magnetic transition temperature $T_{C}$, with the magnetic easy axis lying in the ab plane. While the trace of Mn and P doping in the Fe$_{5}$Si$_{3}$ single crystal was found to increase both the maximum magnetic entropy change and relative cooling power from those in polycrystalline Fe$_{5}$Si$_{3}$ compound, indicating the intrinsic broaden entropy change in larger temperature span. The anisotropy in the MCE between H//ab and H//c is observed, which originates from the anisotropic spinlattice coupling between the ab plane and the c axis. The density functional theory calculations were performed to gain microscopic insights into the experimental findings. Our results suggest hexagonal Fe$_{5}$Si$_{3}$ system may become a new candidate of giant MCE as LaFeSi and FeMnPSi systems. [Preview Abstract] 
Wednesday, March 5, 2014 4:30PM  4:42PM 
Q43.00011: Barocaloric effect in rare earth compounds Nilson Antunes de Oliveira The magnetic barocaloric effect, which is characterized by the isothermal entropy change and adiabatic temperature change upon pressure variation can be a very useful to improve the performance of magnetic refrigerator. In this work, we discuss the barocaloric effect in rare earth compounds. To this end we use a model of interacting localized magnetic moments [1]. In the first part of the work we make a systematic analysis in terms of the model parameters, considering the simplest case whose angular momentum is 1/2. Our calculations show that the behavior of the barocaloric quantities can be normal, inverse or anomalous. In the second part of the work, we apply the model to describe the barocaloric effect in the compounds RCo$_{2}$, Gd$_{5}$Si$_{2}$Ge$_{2}$ and Tb$_{5}$Si$_{2}$Ge$_{2}$. Our theoretical calculations for Gd$_{5}$Si$_{2}$Ge$_{2}$ is in a reasonable agreement with the available experimental data [2]. Our calculations for the other compounds, need experimental data to be confirmed. \\[4pt] [1] N. A. de Oliveira, P. J. von Ranke Phys. Rep. 489, 89 (2010).\\[0pt] [2] Yuce et al . Appl. Phys. Lett., 101, 071906 (2012). [Preview Abstract] 
Wednesday, March 5, 2014 4:42PM  4:54PM 
Q43.00012: Greaterthanbulk melting temperatures explained: Gallium melts Gangnam style Nicola Gaston, Krista Steenbergen The experimental discovery of superheating in gallium clusters [1] contradicted the clear and welldemonstrated paradigm that the melting temperature of a particle should decrease with its size. However the extremely sensitive dependence of melting temperature on size also goes to the heart of cluster science, and the interplay between the effects of electronic and geometric structure. We have performed extensive firstprinciples molecular dynamics calculations, incorporating parallel tempering for an efficient exploration of configurational phase space. This is necessary, due to the complicated energy landscape of gallium. In the nanoparticles, melting is preceded by a transitions between phases. A structural feature, referred to here as the Gangnam motif, is found to increase with the latent heat and appears throughout the observed phase changes of this curious metal. We will present our detailed analysis of the solidstate isomers, performed using extensive statistical sampling of the trajectory data for the assignment of cluster structures to known phases of gallium. Finally, we explain the greaterthanbulk melting through analysis of the factors that stabilise the liquid structures.\\[4pt] [1] G. A. Breaux, et al, Phys. Rev. Lett. 91, 215508 (2003) [Preview Abstract] 
Wednesday, March 5, 2014 4:54PM  5:06PM 
Q43.00013: GW quasiparticle calculations with spinorbit coupling for the light actinides Towfiq Ahmed, Robert C. Albers, A.V. Balatsky, Christoph Friedrich, JianXin Zhu We report on the importance of GW selfenergy corrections for the electronic structure of light actinides in the weaktointermediate coupling regime. Our study is based on calculations of the band structure and total density of states of Np, U, and Pu within a oneshot and spinorbit coupling enabled formulation of the GW approximation within a full potential LAPWframework. We also present RPA screened effective Coulomb interactions for the f electron orbitals for different lattice constants, and show that there is an increased contribution from electronelectron correlation in these systems for expanded lattices. We find a significant amount of electronic correlation in these highly localized electronic systems. [Preview Abstract] 
Wednesday, March 5, 2014 5:06PM  5:18PM 
Q43.00014: First principles calculations on Ga stabilized $\delta $Pu alloys Sarah C. Hernandez, Daniel S. Schwartz, Christopher D. Taylor, Asok K. Ray The high temperature facecenteredcubic phase ($\delta )$ of plutonium (Pu) may be stabilized at room temperature with the inclusion of impurities, such as gallium (Ga) or aluminum. The addition of Ga within the $\delta $phase influences the structural and electronic properties of Pu. Using the fullpotential linearized augmented planewave density functional theory based method we present a systematic study of bulk PuGa alloys. The goals of these calculations are to understand the evolution of the structural and electronic properties of Ga in a $\delta $Pu lattice. A 32atom $\delta $Pu supercell was used to study PuGa alloys at 3.125, 6.25, and 9.375 at. {\%} Ga concentrations. We observe that regardless of the Ga concentration, the magnetic ground state predicted was antiferromagnetic, which is contrary to experimental results. However, the equilibrium lattice constants decrease with increasing Ga concentration, which is in agreement with experimental observations. Furthermore, when more than one Ga impurity is present within the supercell, the Ga atoms prefer to be at third nearest neighbor distance. The local effects of the bond lengths around the Ga atom, formation energies and partial density of states (PDOS) in the lowest energy structures will be discussed in detail. PDOS illustrates a Pu \textit{6d} and Ga \textit{4p} hybridization. Finally, we will also discuss the effects of a hydrogenvacancy complex within a bulk PuGa alloy. [Preview Abstract] 
Wednesday, March 5, 2014 5:18PM  5:30PM 
Q43.00015: \textit{Ab }initio study of the electronic and structural properties of PuAl alloys Jaime L. Sterrett, Sarah C. Hernandez Existing only at higher temperatures, $\delta $plutonium (Pu) can be stabilized at room temperature when doped with the socalled $\delta $stabilizers, which are impurities from the IIIA group, such as gallium or aluminum (Al). This is ideal since the $\delta $phase is the preferred phase used for technological applications. Using density functional theory, we modeled a 32 atom Pu supercell doped with Al concentrations of 3.125, 6.25, and 9.375 atomic percent. The results to be presented will include energy versus volume curves (EV) for fixed atomic positions performed at the nonmagnetic, ferromagnetic, and antiferromagnetic spin structures. Further optimizations of the internal positions of the lowest energy EV structures will be discussed. The effects of Al within the $\delta $Pu lattice, particularly changes in the lattice constants, bulk modulus, preferred location of Al, and the bond lengths between Al and the first nearest neighbor Pu atoms will be discussed in detail. The electronic interactions, specifically the PuAl hybridizations, will also be discussed by analyzing the partial electronic density of states. Finally, we will the present relaxation and formation energies of the PuAl systems. [Preview Abstract] 
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