The magnetocrystalline anisotropy ͑MAE͒ and spin and orbital moments of the ͑001͒ surface of bcc ... more The magnetocrystalline anisotropy ͑MAE͒ and spin and orbital moments of the ͑001͒ surface of bcc Co have been calculated by a first principles theory. The easy axis is found to be ͓100͔, in agreement with observations, and the size of the MAE is found to be substantially larger than the corresponding value of bulk bcc Co. The possibility of detecting this large surface MAE is discussed.
We investigate the lattice and electronic structures of the bulk and surface of the prototypical ... more We investigate the lattice and electronic structures of the bulk and surface of the prototypical layered topological insulators Bi2Se3 and Bi2Te3 using ab initio density functional methods, and systematically compare the results of different methods of including van der Waals (vdW) interactions. We show that the methods utilizing semi-empirical energy corrections yield accurate descriptions of these materials, with the most precise results obtained by properly accounting for the long-range tail of the vdW interactions. The bulk lattice constants, distances between quintuple layers and the Dirac velocity of the topological surface states (TSS) are all in excellent agreement with experiment. In Bi2Te3, hexagonal warping of the energy dispersion leads to complex spin textures of the TSS at moderate energies, while in Bi2Se3 these states remain almost perfectly helical away from the Dirac point, showing appreciable signs of hexagonal warping at much higher energies, above the minimum of...
The adsorption of a monolayer of methane upon the ͑100͒ surface of MgO was studied using a firstp... more The adsorption of a monolayer of methane upon the ͑100͒ surface of MgO was studied using a firstprinciples-based density functional approach employing a plane-wave basis set and periodic boundary conditions. Adsorption at both magnesium and oxygen sites was investigated, as were methane orientations where one, two, or three hydrogen atoms point toward the surface plane. In addition, the effect of one methane molecule on its neighbors was investigated by considering arrangements where translational symmetry across the surface was accompanied by appropriate rotations of the methane molecule. The minimum-energy configuration has the methane molecules located directly above a surface magnesium atom, the principal axis of the C 2v molecule is normal to the surface plane, and pairs of hydrogen atoms are oriented along the lattice lines that include adjacent ͑surface͒ oxygen atoms. Furthermore, neighboring methane molecules are rotated by 90°t o reduce the H-H steric interactions. This arrangement has direct ramifications for the stability of other proposed, similar arrangements.
Heterostructures made from topological and magnetic insulators promise to form excellent platform... more Heterostructures made from topological and magnetic insulators promise to form excellent platforms for new electronic and spintronic functionalities mediated by interfacial effects. We report the results of a first-principles density functional theory study of the geometric, electronic structure, and magnetic properties of EuS(111)/Bi2Se3 interface, including van der Waals and relativistic spinorbit effects. In contrast to previous theoretical studies, we find no appreciable magnetic anisotropy in such a heterostructure. We also do not see additional induced magnetization at the interface or the magnetic proximity effect on the topological states. This is due to the localized nature of Eu moments, and because of a partial charge transfer of ∼0.5 electron from Eu to Se. The formation of the surface dipole shifts the Dirac cone about 0.4 eV below the chemical potential, and the associated electrostatic screening moves the topological state from the first to the second quintuple layer of Bi2Se3.
Using detailed first principles calculations, we have found a new class of stable organic molecul... more Using detailed first principles calculations, we have found a new class of stable organic molecular magnets based on zwitterionic molecules possessing donor, π bridge, and acceptor groups. These molecules are organic molecules containing only C, H and N. The quantum ...
The physical properties of metamagnetic Fe3Ga4 single crystals are investigated to explore the se... more The physical properties of metamagnetic Fe3Ga4 single crystals are investigated to explore the sensitivity of the magnetic states to temperature, magnetic field, and sample history. The data reveal a moderate anisotropy in the magnetization and the metamagnetic critical field along with features in the specific heat at the magnetic transitions T1 = 68 K and T2 = 360 K. Both T1 and T2 are found to be sensitive to the annealing conditions of the crystals suggesting that disorder affects the competition between the ferromagnetic (FM) and antiferromagnetic (AFM) states. Resistivity measurements reveal metallic transport with a sharp anomaly associated with the transition at T2. The Hall effect is dominated by the anomalous contribution which rivals that of magnetic semiconductors in magnitude (−5µΩ cm at 2 T and 350 K) and undergoes a change of sign upon cooling into the low temperature FM state. The temperature and field dependence of the Hall effect indicate that the magnetism is likely to be highly itinerant in character and that a significant change in the electronic structure accompanies the magnetic transitions. We observe a contribution from the topological Hall effect in the AFM phase suggesting a non-coplanar contribution to the magnetism. Electronic structure calculations predict an AFM ground state with a wavevector parallel to the crystallographic c-axis preferred over the experimentally measured FM state by ≈ 50 meV per unit cell. However, supercell calculations with a small density of Fe-antisite defects introduced tend to stabilize the FM over the AFM state indicating that antisite defects may be the cause of the sensitivity to sample synthesis conditions.
Permalloy (Ni.sFe.2) is an important alloy for magnetic devices. We present calculations of the e... more Permalloy (Ni.sFe.2) is an important alloy for magnetic devices. We present calculations of the electronic and magnetic structure of permalloy and of an interdiffused interface between permalloy and copper. U' e also present the results of calculations of the transport properties of permalloy. We find that the magnetic moments in permalloy are collinear, however, there may be non-collinear moments near an interface between permalloy and copper. We find that the calculated electrical conductivity is very sensitive to the details of the electronic and magnetic sbructure, however calculations which include relativistic effects yield a value for t.he conductivity that is in reasonable agreement with experiment.
Ordered Intermetallics — Physical Metallurgy and Mechanical Behaviour, 1992
We review, briefly, the first principles KKR-CPA theory of the electronic structure and energetic... more We review, briefly, the first principles KKR-CPA theory of the electronic structure and energetics of alloys in which comp.ositional disorder plays a role. We also review the first principles theories of ordering and alloy phase stability that are built on the KKR-CPA description of the disordered state. We point to a number of underlying electronic driving mechanisms of ordering and clustering that have been uncovered. Specifically, we emphasize the important role that Fermi surface nesting plays in driving specific instabilities. Using a newly developed method for treating the effects of disorder in alloys that have complex lattices we present results for the ordering energies of beta phase NicAI(l_c) alloys. Finally, we show alloy "Fermi surfaces" in j3-phase Nio.625Alo.375 that support the notion that the pre-martensitic phenomena observed in this alloy are Fermi surface driven.
Computational chemistry at the petascale: Are we there yet?
Journal of Physics: Conference Series, 2009
ABSTRACT We have run computational chemistry calculations approaching the Petascale level of perf... more ABSTRACT We have run computational chemistry calculations approaching the Petascale level of performance (~ 0.5 PFlops). We used the Coupled Cluster CCSD(T) module of the computational chemistry code NWChem to evaluate accurate energetics of water clusters on a 1.4 PFlops Cray XT5 computer.
Proceedings of IEEE Scalable High Performance Computing Conference
We have developed a hybrid, parallel computer code to calculate the physical properties of both o... more We have developed a hybrid, parallel computer code to calculate the physical properties of both ordered for calculating the electronic structure of both ordered and substitutionally disordered materials using a firstand substitutionally disordered materials. By using principles approach. The amount of cpu time for one PVM$.3, we can integrate into our local computer en-iteration of a small system containing five atoms per vironment multiple parallel and vector supercomputers unit cell running at 90% peak performance (2.35 gias well as high performance workstations. Without gaflops) on a Cray YMP/8 is one hour. Running the this approach, calculations of materials properties of same size problem on a 128-node Intel iPSC/860 hylarge systems would be otherwise untenable due to a percube is slightly better at 2.5 gigafiops, but the limlack of computer resources. For ezample, we have de-iting factor on the iPSC]860 is the 8 Mbytes of memtermined the short-range order intensity and its elec-ory per node. Given that the problem size scales as j tronic origin for the Ag-Mg alloy system, including an N 3 (where N is the number of atoms in the unit cell), estimate of the order.disorder (spinodal} temperature, the amount of cpu time and memory required for calculations on medium size systems containing tens of atoms per unit cell would dramatically increase ren-This rvport was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or impli_l, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer-enc¢ herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the _/ United States Government or any agency thereof.
We present an overview of the locally self-consistent multiple scattering (LSMS) method. The meth... more We present an overview of the locally self-consistent multiple scattering (LSMS) method. The method is based on real space multiple scattering theory, is naturally highly parallel, and has been implemented on Intel Paragon parallel platforms within the Center for Computational Sciences at Oak Ridge National Laboratory. O[N]-scaling is demonstrated for unit cells as large as 1000 atoms. The LSMS method can be extended to treat noncollinear magnetic states of materials. We applied the method to calculating the ground state magnetic structure of Feo.65Nio.35 alloys. The result indicates the possible existence of noncollinear arrangements of magnetic moments in these alloys.
Proceedings of the National Academy of Sciences of the United States of America, Jun 13, 2017
The subject of topological materials has attracted immense attention in condensed-matter physics ... more The subject of topological materials has attracted immense attention in condensed-matter physics because they host new quantum states of matter containing Dirac, Majorana, or Weyl fermions. Although Majorana fermions can only exist on the surface of topological superconductors, Dirac and Weyl fermions can be realized in both 2D and 3D materials. The latter are semimetals with Dirac/Weyl cones either not tilted (type I) or tilted (type II). Although both Dirac and Weyl fermions have massless nature with the nontrivial Berry phase, the formation of Weyl fermions in 3D semimetals require either time-reversal or inversion symmetry breaking to lift degeneracy at Dirac points. Here we demonstrate experimentally that canted antiferromagnetic BaMnSb2 is a 3D Weyl semimetal with a 2D electronic structure. The Shubnikov-de Hass oscillations of the magnetoresistance give nearly zero effective mass with high mobility and the nontrivial Berry phase. The ordered magnetic arrangement (ferromagneti...
h i g h l i g h t s Generalized 3D computational model of an electric double layer supercapacitor... more h i g h l i g h t s Generalized 3D computational model of an electric double layer supercapacitor. 3D microstructural aspects do not have a significant impact on the performance. Specific capacitance, ionic conductivity, and tortuosity are critical. State-of-the-art numerical methods provide accurate and robust solutions. a b s t r a c t A generalized three dimensional computational model based on unified formulation of electrode eelectrolyte system of an electric double layer supercapacitor has been developed. This model accounts for charge transport across the electrode-electrolyte system. It is based on volume averaging, a widely used technique in multiphase flow modeling ([1,2]) and is analogous to porous media theory employed for electrochemical systems [3e5]. A single-domain approach is considered in the formulation where there is no need to model the interfacial boundary conditions explicitly as done in prior literature ([6]). Spatio-temporal variations, anisotropic physical properties, and upscaled parameters from lower length-scale simulations and experiments can be easily introduced in the formulation. Model complexities like irregular geometric configuration, porous electrodes, charge transport and related performance characteristics of the supercapacitor can be effectively captured in higher dimensions. This generalized model also provides insight into the applicability of 1D models ([6]) and where multidimensional effects need to be considered. A sensitivity analysis is presented to ascertain the dependence of the charge and discharge processes on key model parameters. Finally, application of the formulation to non-planar supercapacitors is presented.
ABSTRACT A generalized three dimensional computational model based on unified formulation of elec... more ABSTRACT A generalized three dimensional computational model based on unified formulation of electrode–electrolyte system of an electric double layer supercapacitor has been developed. This model accounts for charge transport across the electrode-electrolyte system. It is based on volume averaging, a widely used technique in multiphase flow modeling (1 and 2) and is analogous to porous media theory employed for electrochemical systems 3, 4 and 5. A single-domain approach is considered in the formulation where there is no need to model the interfacial boundary conditions explicitly as done in prior literature ([6]). Spatio-temporal variations, anisotropic physical properties, and upscaled parameters from lower length-scale simulations and experiments can be easily introduced in the formulation. Model complexities like irregular geometric configuration, porous electrodes, charge transport and related performance characteristics of the supercapacitor can be effectively captured in higher dimensions. This generalized model also provides insight into the applicability of 1D models ([6]) and where multidimensional effects need to be considered. A sensitivity analysis is presented to ascertain the dependence of the charge and discharge processes on key model parameters. Finally, application of the formulation to non-planar supercapacitors is presented.
ABSTRACT We have run computational chemistry calculations approaching the Petascale level of perf... more ABSTRACT We have run computational chemistry calculations approaching the Petascale level of performance (~ 0.5 PFlops). We used the Coupled Cluster CCSD(T) module of the computational chemistry code NWChem to evaluate accurate energetics of water clusters on a 1.4 PFlops Cray XT5 computer.
Changes in ionic composition as Na+, K+, Ca 2+ and Mg 2+, osmolality inseminal fluid, percentage ... more Changes in ionic composition as Na+, K+, Ca 2+ and Mg 2+, osmolality inseminal fluid, percentage of motile spermatozoaand velocity were investigated in response toCPP and different dosage of LHRHa. The lowestvelocity of sperm was observed after use ...
Adams LA (1942) Age determination and rate of growth in Polyodon spathula by means of the growth ... more Adams LA (1942) Age determination and rate of growth in Polyodon spathula by means of the growth rings of the otoliths and dentary bones American Wildlife Nature 28 617-630 Ballard WW and Needham RG (1964) Normal embryonic stages of Polyodon spathula (Walbaum) Journal of Morphology 114 465-478 Billard R (1978) Changes in structure and fertilizing ability of marine and fresh water fish spermatozoa diluted in media of various salinities Aquaculture 14 187-198 Billard R, Cosson J, Perchec G and Linhart O (1995) Biology of sperm and artificial reproduction in carp Aquaculture 129 95-112 Carlson DM and Bonislawsky PS (1981) The paddlefish (Polyodon spathula) fisheries of the midwestern Fisheries 6 (2) 17-22, 26-27 Christen R, Gatti JL and Billard R (1987) Trout sperm motility European Journal of Biochemistry 166 667-671 Cosson J and Linhart O (1996) Paddlefish (Polyodon spathula) spermatozoa: effects of potassium and pH on motility Folia Zoologica 45 361-370 Cosson MP, Billard R, Carre D, Letellier L, Christen R, Cosson J and Gatti JL (1986) Control of flagellar movement in '9 + 2' spermatozoa: calcium and cyclic AMP dependent chemotactic behavior Cell Motility and the Cytoskeleton 6 237 Cosson MP, Billard R and Letellier L (1989) Rise of internal Ca 2+ accompanies the initiation of trout sperm motility Cell Motility and the Cytoskeleton 14 424-434 Cosson J, Billard R, Cibert C, Dreanno C, Linhart O and Suquet M (1997) Movements of fish sperm flagella studied by high speed videomicroscopy coupled to computer assisted image analysis Poliskie Archiwum Hydrobiologii 44 103-113 Cosson J, Billard R, Cibert C, Dréanno C and Suquet M (1999) Ionic factors regulating the motility of fish sperm. In From Basic Science the Male Gamete to Clinical Application pp 161-186 Eds C Gagnon. Cache River Press, Vienna, IL Cosson J, Linhart O, Mims SD, Shelton WL and Rodina M (2000) Analysis of motility parameters from paddlefish (Polyodon spathula) and shovelnose sturgeon (Scaphirhynchus platorynchus) spermatozoa Journal of Fish Biology 56 1348-1367 Drabkina BM (1961) Effect of different water salinities on the survival rate of spermatozoa, eggs and larvae of sturgeon Doklady AN SSSR 138 492-495 (in Russian) Folz DJ and Mezers LS (1985) Management of the lake sturgeon, Acipenser fulvescens, population in the Lake Winnebago system, Wisconsin. In Natural American Sturgeons Biology Aquaculture and Potential pp 135-146 Eds FP Binkowski and SI Doroshov. Junk Publishers, Dordrecht
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