Open Access. Powered by Scholars. Published by Universities.®

Physics Commons

Open Access. Powered by Scholars. Published by Universities.®

Articles 1 - 22 of 22

Full-Text Articles in Physics

Spatially Resolved Distribution Function And The Medium-Range Order In Metallic Liquid And Glass, X. W. Fang, Cai-Zhuang Wang, S. G. Hao, Matthew J. Kramer, Yongxin Yao, Mikhail I. Mendelev, Z. J. Ding, Ralph E. Napolitano, Kai-Ming Ho Dec 2011

Spatially Resolved Distribution Function And The Medium-Range Order In Metallic Liquid And Glass, X. W. Fang, Cai-Zhuang Wang, S. G. Hao, Matthew J. Kramer, Yongxin Yao, Mikhail I. Mendelev, Z. J. Ding, Ralph E. Napolitano, Kai-Ming Ho

Materials Science and Engineering Publications

The structural description of disordered systems has been a longstanding challenge in physical science. We propose an atomic cluster alignment method to reveal the development of three-dimensional topological ordering in a metallic liquid as it undercools to form a glass. By analyzing molecular dynamic (MD) simulation trajectories of a Cu64.5Zr35.5 alloy, we show that medium-range order (MRO) develops in the liquid as it approaches the glass transition. Specifically, around Cu sites, we observe “Bergman triacontahedron” packing (icosahedron, dodecahedron and icosahedron) that extends out to the fourth shell, forming an interpenetrating backbone network in the glass. The discovery of ...


Anisotropic Magnetoelastic Coupling In Single-Crystalline Cefeaso As Seen Via High-Resolution X-Ray Diffraction, H.-F. Li, J.-Q. Yan, J. W. Kim, R. William Mccallum, Thomas A. Lograsso, David Vaknin Dec 2011

Anisotropic Magnetoelastic Coupling In Single-Crystalline Cefeaso As Seen Via High-Resolution X-Ray Diffraction, H.-F. Li, J.-Q. Yan, J. W. Kim, R. William Mccallum, Thomas A. Lograsso, David Vaknin

Ames Laboratory Publications

Single-crystal synchrotron x-ray diffraction studies of CeFeAsO reveal strong anisotropy in the charge-correlation lengths along or perpendicular to the in-plane antiferromagnetic (AFM) wave vector at low temperatures, indicating an anisotropic two-dimensional magnetoelastic coupling. The high-resolution setup allows to distinctly monitor each of the twin domains by virtue of a finite misfit angle between them that follows the order parameter. In addition, we find that the in-plane correlations, above the orthorhombic (O)-to-tetragonal (T) transition, are shorter than those in each of the domains in the AFM phase, indicating a distribution of the in-plane lattice constants. This strongly suggests that the ...


Accurate And Fast Numerical Solution Of Poisson's Equation For Arbitrary, Space-Filling Voronoi Polyhedra: Near-Field Corrections Revisited, Aftab Alam, Brian G. Wilson, Duane D. Johnson Nov 2011

Accurate And Fast Numerical Solution Of Poisson's Equation For Arbitrary, Space-Filling Voronoi Polyhedra: Near-Field Corrections Revisited, Aftab Alam, Brian G. Wilson, Duane D. Johnson

Ames Laboratory Publications

We present an accurate and rapid solution of Poisson's equation for space-filling, arbitrarily shaped, convex Voronoi polyhedra (VP); the method is O(NVP), where NVP is the number of distinct VP representing the system. In effect, we resolve the long-standing problem of fast but accurate numerical solution of the near-field corrections, contributions to the potential due to near VP—typically those involving multipole-type conditionally convergent sums, or use of fast Fourier transforms. Our method avoids all ill-convergent sums, is simple, accurate, efficient, and works generally, i.e., for periodic solids, molecules, or systems with disorder or imperfections. We demonstrate ...


Chemically Mediated Quantum Criticality In Nbfe2, Aftab Alam, Duane D. Johnson Nov 2011

Chemically Mediated Quantum Criticality In Nbfe2, Aftab Alam, Duane D. Johnson

Ames Laboratory Publications

Laves-phase Nb1+cFe2−c is a rare itinerant intermetallic compound exhibiting magnetic quantum criticality at ccr∼1.5%Nb excess; its origin, and how alloying mediates it, remains an enigma. For NbFe2, we show that an unconventional band critical point above the Fermi level EF explains most observations and that chemical alloying mediates access to this unconventional band critical point by an increase in EF with decreasing electrons (increasing %Nb), counter to rigid-band concepts. We calculate that EF enters the unconventional band critical point region for ccr>1.5%Nb and by 1.74%Nb there is no Nb site-occupation ...


Magnetic And Thermal Properties Of Er75dy25 Single Crystals, Mahmud Khan, Deborah L. Schlagel, Thomas A. Lograsso, Karl A. Gschneidner Jr., Vitalij K. Pecharsky Oct 2011

Magnetic And Thermal Properties Of Er75dy25 Single Crystals, Mahmud Khan, Deborah L. Schlagel, Thomas A. Lograsso, Karl A. Gschneidner Jr., Vitalij K. Pecharsky

Ames Laboratory Publications

We report on an experimental study of the thermal and magnetic properties of Er75Dy25 single crystals with magnetic fields applied parallel to the [0001] and [101̅ 0] directions. The [0001] direction is the easy axis of magnetization, while the [101̅ 0] direction is the hard axis of magnetization. Three major transitions are observed in the heat capacity, magnetization, and alternating current magnetic susceptibility data. A first-order transition is observed at ∼30 K, and two second-order transitions occur at ∼45 and ∼110 K. The H-T phase diagrams constructed from the experimental data reveal that several magnetic phases, including ...


Dynamic Arrest And Glass Formation Induced By Self-Aggregation Of Icosahedral Clusters In Zr1−Xcux Alloys, S. G. Hao, Cai-Zhuang Wang, M. Z. Li, Ralph E. Napolitano, Kai-Ming Ho Aug 2011

Dynamic Arrest And Glass Formation Induced By Self-Aggregation Of Icosahedral Clusters In Zr1−Xcux Alloys, S. G. Hao, Cai-Zhuang Wang, M. Z. Li, Ralph E. Napolitano, Kai-Ming Ho

Materials Science and Engineering Publications

Using ab initio molecular dynamics simulations, we find direct evidence that icosahedral clusters have a strong tendency to aggregate in Zr1−xCux liquids. Remarkably, the dynamic arrest effect is much more significant in aggregated icosahedral clusters than in nonaggregated ones by a factor of 2 or more. Simulations using a lattice model demonstrate that the dynamic arrest effect resulting from the aggregation of icosahedral clusters may be a fundamental characteristic of the glass transition. Our studies provide an atomistic structural mechanism for dynamic arrest and glass formation.


Phase-Field Investigation Of Rod Eutectic Morphologies Under Geometrical Confinement, Melis Şerefoğlu, Ralph E. Napolitano, Mathis Plapp Jul 2011

Phase-Field Investigation Of Rod Eutectic Morphologies Under Geometrical Confinement, Melis Şerefoğlu, Ralph E. Napolitano, Mathis Plapp

Materials Science and Engineering Publications

Three-dimensional phase-field simulations are employed to investigate rod-type eutectic growth morphologies in confined geometry. Distinct steady-state solutions are found to depend on this confinement effect with the rod array basis vectors and their included angle (α) changing to accommodate the geometrical constraint. Specific morphologies are observed, including rods of circular cross sections, rods of distorted (elliptical) cross sections, rods of peanut-shaped cross-sections, and lamellar structures. The results show that, for a fixed value of α>10∘, the usual (triangular) arrays of circular rods are stable in a broad range of spacings, with a transition to the peanut-shaped cross sectioned rods ...


Efficient Isoparametric Integration Over Arbitrary Space-Filling Voronoi Polyhedra For Electronic Structure Calculations, Aftab Alam, Suffian N. Khan, Brian G. Wilson, Duane D. Johnson Jul 2011

Efficient Isoparametric Integration Over Arbitrary Space-Filling Voronoi Polyhedra For Electronic Structure Calculations, Aftab Alam, Suffian N. Khan, Brian G. Wilson, Duane D. Johnson

Ames Laboratory Publications

A numerically efficient, accurate, and easily implemented integration scheme over convex Voronoi polyhedra (VP) is presented for use in ab initio electronic-structure calculations. We combine a weighted Voronoi tessellation with isoparametric integration via Gauss-Legendre quadratures to provide rapidly convergent VP integrals for a variety of integrands, including those with a Coulomb singularity. We showcase the capability of our approach by first applying it to an analytic charge-density model achieving machine-precision accuracy with expected convergence properties in milliseconds. For contrast, we compare our results to those using shape-functions and show our approach is greater than 105 times faster and 107 times ...


Contamination From Magnetic Starting Materials In Flux-Grown Single Crystals Of Rfeaso Superconductors, J.-Q. Yan, Qingfeng Xing, Brandt A. Jensen, H. Xu, Kevin W. Dennis, R. William Mccallum, Thomas A. Lograsso Jul 2011

Contamination From Magnetic Starting Materials In Flux-Grown Single Crystals Of Rfeaso Superconductors, J.-Q. Yan, Qingfeng Xing, Brandt A. Jensen, H. Xu, Kevin W. Dennis, R. William Mccallum, Thomas A. Lograsso

Ames Laboratory Publications

While the flux growth often provides the best method for obtaining single crystals of new-novel materials, the process is prone to producing crystals with trapped second-phase inclusions. In the case of NdFeAsO single crystals grown out of NaAs flux under ambient pressure, some crystals show a lambda anomaly in the specific heat curve at ~12 K. This anomaly is missing in other crystals from the same batch. We show that the lambda anomaly at 12 K is from residual NdAs starting material which is mainly on the edges of the crystals. The isothermal magnetization of LaFeAsO at 300 K clearly ...


Soft Holographic Interference Lithography Microlens For Enhanced Organic Light Emitting Diode Light Extraction, Joong Mok Park, Zhengqing Gan, Wai Y. Leung, Rui Liu, Zhuo Ye, Kristen P. Constant, Joseph Shinar, Ruth Shinar, Kai-Ming Ho Jul 2011

Soft Holographic Interference Lithography Microlens For Enhanced Organic Light Emitting Diode Light Extraction, Joong Mok Park, Zhengqing Gan, Wai Y. Leung, Rui Liu, Zhuo Ye, Kristen P. Constant, Joseph Shinar, Ruth Shinar, Kai-Ming Ho

Materials Science and Engineering Publications

Very uniform 2 μm-pitch square microlens arrays (μLAs), embossed on the blank glass side of an indium-tin-oxide (ITO)-coated 1.1 mm-thick glass, are used to enhance light extraction from organic light-emitting diodes (OLEDs) by ~100%, significantly higher than enhancements reported previously. The array design and size relative to the OLED pixel size appear to be responsible for this enhancement. The arrays are fabricated by very economical soft lithography imprinting of a polydimethylsiloxane (PDMS) mold (itself obtained from a Ni master stamp that is generated from holographic interference lithography of a photoresist) on a UV-curable polyurethane drop placed on the ...


Ternary Tetradymite Compounds As Topological Insulators, Lin-Lin Wang, Duane D. Johnson Jun 2011

Ternary Tetradymite Compounds As Topological Insulators, Lin-Lin Wang, Duane D. Johnson

Ames Laboratory Publications

Ternary tetradymites Bi2Te2S, Bi2Te2Se, and Bi2Se2Te are found to be stable, bulk topological insulators via theory, showing band inversion between group V and VI pz orbitals. We identify Bi2Se2Te as a good candidate to study massive Dirac fermions, with a (111) cleavage-surface-derived Dirac point (DP) isolated in the bulk-band gap at the Fermi energy (Ef)-like Bi2Se3 but with a spin texture alterable by layer chemistry. In contrast, Bi2Te2S and Bi2Te2Se (111) behave like Bi2Te3, with a DP below Ef buried in bulk bands. Bi2Te2S offers large bulk resistivity needed for devices.


Electron Localization Into Spin-Polaron State In Mnsi, Vyacheslav G. Storchak, Jess H. Brewer, Roger L. Lichti, Thomas A. Lograsso, Deborah L. Schlagel Apr 2011

Electron Localization Into Spin-Polaron State In Mnsi, Vyacheslav G. Storchak, Jess H. Brewer, Roger L. Lichti, Thomas A. Lograsso, Deborah L. Schlagel

Ames Laboratory Publications

Strong electron localization into a bound state has been found in both paramagnetic and ferromagnetic states of the transition metal compound MnSi by muon spin-rotation spectroscopy in magnetic fields up to 7 T and from 2 K to room temperature. This bound state, with a characteristic radius R≈0.4 nm and net spin S=24±2, is consistent with confinement of the electron’s wave function within roughly one lattice cell of MnSi and is suggested to be a spin polaron. Such spin polarons may form due to a strong exchange interaction between itinerant electrons and the magnetic electrons ...


Topologically Correct Phase Boundaries And Transition Temperatures For Ising Hamiltonians Via Self-Consistent Coarse-Grained Cluster-Lattice Models, Teck L. Tan, Duane D. Johnson Apr 2011

Topologically Correct Phase Boundaries And Transition Temperatures For Ising Hamiltonians Via Self-Consistent Coarse-Grained Cluster-Lattice Models, Teck L. Tan, Duane D. Johnson

Ames Laboratory Publications

We derive a cluster mean-field theory for an Ising Hamiltonian using a cluster-lattice Fourier transform with a cluster of size Nc and a coarse-grained (CG) lattice into cells of size Ncell. We explore forms with Ncell⩾Nc, including a non-CG (NCG) version with Ncell→∞. For Nc=Ncell, the set of static, self-consistent equations relating cluster and CG lattice correlations is analogous to that in dynamical cluster approximation and cellular dynamical mean-field theory used in correlated electron physics. A variational Nc-site cluster grand potential based on Nc=Ncell CG lattice maintains thermodynamic consistency and improves predictions, recovering Monte Carlo and series ...


Magnetostriction, Elasticity, And D03 Phase Stability In Fe–Ga And Fe–Ga–Ge Alloys, G. Petculescu, K. L. Ledet, Mianliang Huang, Thomas A. Lograsso, Y. N. Zhang, R. Q. Wu, M. Wun-Fogle, J. B. Restorff, A. E. Clark, K. B. Hathaway Mar 2011

Magnetostriction, Elasticity, And D03 Phase Stability In Fe–Ga And Fe–Ga–Ge Alloys, G. Petculescu, K. L. Ledet, Mianliang Huang, Thomas A. Lograsso, Y. N. Zhang, R. Q. Wu, M. Wun-Fogle, J. B. Restorff, A. E. Clark, K. B. Hathaway

Ames Laboratory Conference Papers, Posters, and Presentations

The contrast between the saturation tetragonal magnetostriction, λγ,2 = (3/2)λ100, of Fe1−xGax and Fe1−yGey, at compositions where both alloys exhibit D03 cubic symmetry (second peak region), was investigated. This region corresponds to x = 28 at. % Ga and y = 18 at. % Ge or, in terms of e/a = 2 x + 3 y + 1, to an e/a value of ∼1.55 for each of the alloys. Single crystal, slow-cooled, ternary Fe1−xy GaxGey alloys with e/a ∼1.55 and gradually increasing y ...


High-Pressure Study Of The Phase Transition In The Itinerant Ferromagnet Cos2, V. A. Sidorov, V. N. Krashnorussky, Alla E. Petrova, A. N. Utyuzh, W. M. Yuhasz, Thomas A. Lograsso, J. D. Thompson, Sergei M. Stishov Feb 2011

High-Pressure Study Of The Phase Transition In The Itinerant Ferromagnet Cos2, V. A. Sidorov, V. N. Krashnorussky, Alla E. Petrova, A. N. Utyuzh, W. M. Yuhasz, Thomas A. Lograsso, J. D. Thompson, Sergei M. Stishov

Ames Laboratory Publications

Electrical resistivity and magnetic susceptibility measurements of the itinerant ferromagnet CoS2 at high pressures reveal that its magnetic ordering temperature is tuned to zero at a critical pressure 4.8 GPa, indicating the existence of a quantum-phase transition. The ambient pressure continuous magnetic-phase transition in CoS2 becomes first order on a very slight pressure increase, with a tricritical point probably located almost at ambient pressure. No deviations from Fermi-liquid behavior were found in the vicinity of the quantum-phase transition in CoS2, implying its strong first-order nature.


Flux Requirements For The Growth Of Rfeaso (R = Rare Earth) Superconductors, J.-Q. Yan, Brandt A. Jensen, Kevin W. Dennis, R. William Mccallum, Thomas A. Lograsso Feb 2011

Flux Requirements For The Growth Of Rfeaso (R = Rare Earth) Superconductors, J.-Q. Yan, Brandt A. Jensen, Kevin W. Dennis, R. William Mccallum, Thomas A. Lograsso

Ames Laboratory Publications

Millimeter-sized LaFeAsO single crystals have been successfully grown out of NaAs flux starting with (LaAs+1/2Fe2O3):20NaAs. The factors which allow the growth of these crystals out of NaAs but not out of many other fluxes, such as FeAs, have been investigated. X-ray powder diffraction found that NaAs synthesized for the growth ofRFeAsO superconductors has monoclinic LiAs structure. Thermal analysis confirmed that NaAs melts congruently at about 600 °C. The ability to extract RFeAsO crystals from this NaAs flux suggests that NaAs has a significant oxygen solubility, possibly due to the formation of ...


Fabrication Of Submicron Metallic Grids With Interference And Phase-Mask Holography, Joong Mok Park, Tae Geun Kim, Kristen P. Constant, Kai-Ming Ho Jan 2011

Fabrication Of Submicron Metallic Grids With Interference And Phase-Mask Holography, Joong Mok Park, Tae Geun Kim, Kristen P. Constant, Kai-Ming Ho

Materials Science and Engineering Publications

Complex, submicron Cu metallic mesh nanostructures are made by electrochemical deposition using polymer templates made from photoresist. The polymer templates are fabricated with photoresist using two-beam interference holography and phase mask holography with three diffracted beams. Freestanding metallic mesh structures are made in two separate electrodepositions with perpendicular photoresist grating templates. Cu mesh square nanostructures having large (52.6%) open areas are also made by single electrodeposition with a photoresist template made with a phase mask. These structures have potential as electrodes in photonic devices.


Magnetic Field Dependence Of The Maximum Magnetic Entropy Change, Julia Lyubina, Michael D. Kuz'min, Konstantin Nenkov, Oliver Gutfleisch, Manuel Richter, Deborah L. Schlagel, Thomas A. Lograsso, Karl A. Gschneidner Jr. Jan 2011

Magnetic Field Dependence Of The Maximum Magnetic Entropy Change, Julia Lyubina, Michael D. Kuz'min, Konstantin Nenkov, Oliver Gutfleisch, Manuel Richter, Deborah L. Schlagel, Thomas A. Lograsso, Karl A. Gschneidner Jr.

Ames Laboratory Publications

The maximum isothermal entropy change in a magnetic refrigerant with a second-order phase transition is shown to depend on applied magnetic field H as follows: (-ΔS)max = A(H + H0)2/3 – AH02/3 + BH4/3. Here A and B are intrinsic parameters of the cooling material and H0 is an extrinsic parameter determined by the purity and homogeneity of the sample. This theoretical prediction is confirmed by measurements on variously pure poly- and single-crystalline samples of Gd. The Curie point of pure Gd is found to be 295(1) K; however, the maximum of -ΔS ...


Temperature-Dependent Growth Shapes Of Ni Nanoclusters On Nial(110), Yong Han, Baris Unal, Dapeng Jing, Patricia A. Thiel, James W. Evans Jan 2011

Temperature-Dependent Growth Shapes Of Ni Nanoclusters On Nial(110), Yong Han, Baris Unal, Dapeng Jing, Patricia A. Thiel, James W. Evans

Physics and Astronomy Publications

Scanning tunneling microscopy studies reveal that two-dimensional nanoscale Ni islands formed by deposition of Ni on NiAl(110) between 200–400 K exhibit far-from-equilibrium growth shapes which change systematically with temperature. Island structure reflects the two types of adsorption sites available for Ni adatoms, and island shapes are controlled by the details of adatom diffusion along island edges accounting for numerous local configurations. The temperature dependence of the island shapes is captured and elucidated by kinetic Monte Carlo simulation of a realistic atomistic-level multisite lattice-gas model incorporating precise diffusion barriers. These barriers are obtained by utilizing density functional theory to ...


Piezoelectric Ceramics With Compositions At The Morphotropic Phase Boundary In The Bifeo3–Pbzro3–Pbtio3 Ternary System, Wei Hu, Xiaoli Tan, Krishna Rajan Jan 2011

Piezoelectric Ceramics With Compositions At The Morphotropic Phase Boundary In The Bifeo3–Pbzro3–Pbtio3 Ternary System, Wei Hu, Xiaoli Tan, Krishna Rajan

Materials Science and Engineering Publications

Since ceramics in the PbZrO3–PbTiO3 binary system display excellent piezoelectric properties and those in BiFeO3–PbTiO3 exhibit high Curie temperatures, morphotropic phase boundary (MPB) compositions in the BiFeO3–PbZrO3–PbTiO3 ternary solid solution system are investigated for the development of piezoelectric ceramics for high temperature applications. It is found that the MPB compositions in the ternary system deviate away from the mixture of two binary MPB compositions 0.70BiFeO3–0.30PbTiO3 and 0.52PbZrO3–0.48PbTiO3. With decreasing amount of BiFeO3 in the MPB compositions in the ...


Destabilization Of Ag Nanoislands On Ag(100) By Adsorbed Sulfur, Mingmin Shen, Selena M. Russell, Da-Jiang Liu, Patricia A. Thiel Jan 2011

Destabilization Of Ag Nanoislands On Ag(100) By Adsorbed Sulfur, Mingmin Shen, Selena M. Russell, Da-Jiang Liu, Patricia A. Thiel

Chemistry Publications

Sulfur accelerates coarsening of Ag nanoislands on Ag(100) at 300 K, and this effect is enhanced with increasing sulfur coverage over a range spanning a few hundredths of a monolayer, to nearly 0.25 monolayers. We propose that acceleration of coarsening in this system is tied to the formation of AgS2 clusters primarily at step edges. These clusters can transport Ag more efficiently than can Ag adatoms (due to a lower diffusion barrier and comparable formation energy). The mobility of isolated sulfur on Ag(100) is very low so that formation of the complex is kinetically limited at ...


Fe-Fe Adatom Interaction And Growth Morphology On Graphene, Xiaojie Liu, Cai-Zhuang Wang, Myron Hupalo, Wen-Cai Lu, Patricia A. Thiel, Kai-Ming Ho, Michael C. Tringides Jan 2011

Fe-Fe Adatom Interaction And Growth Morphology On Graphene, Xiaojie Liu, Cai-Zhuang Wang, Myron Hupalo, Wen-Cai Lu, Patricia A. Thiel, Kai-Ming Ho, Michael C. Tringides

Chemistry Publications

The nucleation and growth of Fe on graphene is highly unusual. A constantly increasing island density indicates the presence of strong, predominantly repulsive, adatom interactions. We study these interactions by first-principles calculations to identify their origin. We find that the interactions consist of a short-range attraction and longer-range repulsion. We show that electric dipole-dipole interaction can contribute only part of the repulsive force (∼30%) between Fe adatoms, and the repulsion due to the elastic interaction is small. We suggest that the dominant contribution to the repulsive energy would originate from the indirect (or RKKY) interactions mediated through the delocalized electrons ...