Publications (86)184.72 Total impact

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ABSTRACT: A combination of density functional theory (DFT), kinetic Monte Carlo and meanfield rate theory is applied to analyze point defect migration and its effect on the observed growth of hexagonal closepacked (hcp) Zr under 1 MeV electron irradiation. DFT is used to study stability of various configurations of vacancies and selfinterstitial atoms (SIAs) and migration barriers. The data are used in kinetic Monte Carlo modeling of defect diffusion at different temperatures. It is found that both defects exhibit anisotropic diffusion, predominantly parallel to the basal planes. The ratio of diffusion coefficients parallel and perpendicular to the basal planes is found to be higher for vacancies as compared to SIAs at temperatures below ∼600 K. This raises doubts that the observed radiation growth in Zr irradiated with 1 MeV electrons, namely positive strains in prismatic and negative strains in basal directions, and void alignment along basal planes, can be accounted for by the anisotropy of point defect diffusion, which predicts opposite strain signs. It is speculated that formation of small SIA clusters with higher diffusion anisotropy may be responsible for the experimental observations.Acta Materialia 10/2014; 78:173–180. DOI:10.1016/j.actamat.2014.06.024 · 3.94 Impact Factor 
Dataset: Paper 171 2013

Article: Magnetic states of the fiveorbital Hubbard model for onedimensional ironbased superconductors
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ABSTRACT: The magnetic phase diagrams of models for quasi onedimensional compounds belonging to the ironbased superconductors family are presented. The fiveorbital Hubbard model and the realspace HartreeFock approximation are employed, supplemented by density functional theory to obtain the hopping amplitudes. Phase diagrams are constructed varying the Hubbard $U$ and Hund $J$ couplings and at zero temperature. The study is carried out at electronic density (electrons per iron) $n = 5.0$, which is of relevance for the already known material TlFeSe$_2$, and also at $n = 6.0$, where representative compounds still need to be synthesized. At $n = 5.0$ there is a clear dominance of staggered spin order along the chain direction. At $n = 6.0$ and the realistic Hund coupling $J/U = 0.25$, the phase diagram is far richer including a variety of ``block'' states involving ferromagnetic clusters that are antiferromagnetically coupled, in qualitative agreement with recent Density Matrix Renormalization Group calculations for the threeorbital Hubbard model in a different context. These block states arise from the competition between ferromagnetic order (induced by double exchange, and prevailing at large $J/U$) and antiferromagnetic order (dominating at small $J/U$). The density of states and orbital compositions of the many phases are also provided.Physical Review B 03/2014; 90(3). DOI:10.1103/PhysRevB.90.035128 · 3.66 Impact Factor 
Conference Paper: First Principles Studies of the Magnetic Properties of Alnico Permanent Magnet Materials
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ABSTRACT: Until the advent of rare earth based magnets Alnico was one of the highest energy product hard magnets available. Recently, interest in this system has been rekindled as system whose properties and utility may be further enhanced but does not contain rare earth elements. Recent experiments on Alnico alloy suggest that there is no sharp interface between the disordered bcc FeCo magnetic phase and the ordered B2 NiAl nonmagnetic phase; thereby undermining our understanding of the large coercivity of this material. By utilizing several electronic structure methods we first study the issue of the effect of substitutions of additional elements into B2 NiAl phase. We also calculate the magnetic moment distribution across the interface and examine the magnetic ground state. These calculations suggest that the magnetic structure of the B2phase as well as the interface in much more complex than previously thought.American Physical Society March Meeting; 03/2013 
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ABSTRACT: Mobility of screw dislocations controls low temperature plasticity in bcc metals including ferritic alloys. Density functional theory (DFT) is an effective tool in providing parameterfree information on the energetic and magnetic properties of defects including screw dislocations. We summarize DFT calculations on atomic properties of 1/2<111> screw dislocations in FeCr system. The periodic quadrupole approach was applied to model the core dislocation structure, core interaction with Cr solute atoms and to estimate their effect on Peierls stress and barrier. The binding energy of Cr impurity atoms with a screw dislocation and its effect on the dislocation core structure are discussed and the importance of magnetism in the effects of Cr on screw dislocation mobility is demonstrated. This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences. 
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ABSTRACT: Classical Molecular Dynamic (MD) simulations characterizing extended defects typically require millions of atoms. First principles calculations employed to understand these defect systems at an electronic level cannot, and should not deal with such large numbers of atoms. We present an efficient coarse graining (CG) approach to calculate local electronic properties of large MDgenerated structures from the first principles. We used the Locally Selfconsistent Multiple Scattering (LSMS) method for two types of iron defect structures 1) screwdislocation dipoles and 2) radiation cascades. The multiple scattering equations are solved at fewer sites using the CG. The atomic positions were determined by MD with an embedded atom force field. The local moments in the neighborhood of the defect cores are calculated with firstprinciples based on full local structure information, while atoms in the rest of the system are modeled by representative atoms with approximated properties. This CG approach reduces computational costs significantly and makes largescale structures amenable to first principles study. Work is sponsored by the USDoE, Office of Basic Energy Sciences, ``Center for Defect Physics,'' an Energy Frontier Research Center. This research used resources of the Oak Ridge Leadership Computing Facility at the ORNL, which is supported by the Office of Science of the USDoE under Contract No. DEAC0500OR22725. 
Conference Paper: Calculated magnetic structure of mobile defects in Fe
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ABSTRACT: Mobile defects such as dislocations and crowdions respond to gradients of strain, temperature, concentration, and applied field, thereby, determining a material's viability in particular applications. In Fe, defects affect the magnetic state of the surrounding atoms. We discuss the defectinduced changes in magnetic moment magnitude and orientation, magnetic anisotropy and magnetic interactions. These quantities are calculated (density functional theory (DFT)) for defect models ranging in size from a few hundred to a few thousand. Comparisons are made between different DFT methods. The importance of magnetism to the response of defects to gradients is discussed.American Physical Society March Meeting; 03/2013 
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ABSTRACT: The alignment of vacancy loops and voids along basal planes observed in irradiated Zr and Zr alloys requires anisotropic pointdefect transport with a dominant contribution along the basal plane. For neutron irradiation, this can be explained by onedimensional mobility of selfinterstitial atom (SIA) clusters, but experiments with electron irradiation indicate unambiguously that even single SIA should exhibit anisotropic diffusion. No experimental information is available on SIA properties in Zr and the previous ab initio calculations did not provide any evidence of anisotropic diffusion mechanisms. An extensive investigation of SIAs in Zr has been performed from first principles using two different codes. It was demonstrated that the simulation cell size, type of pseudopotential, exchangecorrelation functional and the c/a ratio are crucially important for determining the properties of interstitials in hcp Zr. The most stable SIA configurations lie in the basal plane, which should lead to SIA diffusion mainly along basal planes.Philosophical Magazine Letters 02/2013; 93(2):93100. DOI:10.1080/09500839.2012.745653 · 1.27 Impact Factor 
Article: Coarse Grained Approach to First Principles Modeling of Radiation Cascade in Large Fe Supercells
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ABSTRACT: Classical Molecular Dynamics (MD) simulations characterizing dislocations and radiation damage typically treat 105107 atoms. First principles techniques employed to understand systems at an atomistic level are not practical for such large systems consisting of millions of atoms. We present an efficient coarse grained (CG) approach to calculate local electronic and magnetic properties of large MDgenerated structures from the first principles. Local atomic magnetic moments in crystalline Fe are perturbed by the presence of radiation generated vacancies and interstitials. The effects are most pronounced near the defect cores and decay slowly as the strain field of the defects decrease with distance. We develop the CG technique based on the Locally Selfconsistent Multiple Scattering (LSMS) method that exploits the nearsightedness of the electron Green function. The atomic positions were determined by MD with an embedded atom force field. The local moments in the neighborhood of the defect cores are calculated with firstprinciples based on full local structure information. Atoms in the rest of the system are modeled by representative atoms with approximated properties. The calculations result in local moments near the defect centers with firstprinciples accuracy, while capturing coarsegrained details of local moments at greater length scales. This CG approach makes these large scale structures amenable to first principles study.Journal of Physics Conference Series 12/2012; 402. DOI:10.1088/17426596/402/1/012011 
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ABSTRACT: Several transition metals were examined to evaluate their potential for improving the ductility of tungsten. The dislocation core structure and Peierls stress and barrier of 1/2〈111〉 screw dislocations in binary tungstentransition metal alloys (W(1x)TM(x)) were investigated using density functional theory calculations. The periodic quadrupole approach was applied to model the structure of the 1/2〈111〉 dislocation. Alloying with transition metals was modeled using the virtual crystal approximation and the applicability of this approach was assessed by calculating the equilibrium lattice parameter and elastic constants of the tungsten alloys. Reasonable agreement was obtained with experimental data and with results obtained from the conventional supercell approach. Increasing the concentration of a transition metal from the VIIIA group, i.e. the elements in columns headed by Fe, Co and Ni, leads to reduction of the C' elastic constant and increase of the elastic anisotropy A = C(44)/C'. Alloying W with a group VIIIA transition metal changes the structure of the dislocation core from symmetric to asymmetric, similarly to results obtained for W(1x)Re(x) alloys in the earlier work of Romaner et al (2010 Phys. Rev. Lett. 104 195503). In addition to a change in the core symmetry, the values of the Peierls stress and barrier are reduced. The latter effect could lead to increased ductility in a tungstenbased alloy. Our results demonstrate that alloying with any of the transition metals from the VIIIA group should have a similar effect to alloying with Re.Journal of Physics Condensed Matter 11/2012; 25(2):025403. DOI:10.1088/09538984/25/2/025403 · 2.22 Impact Factor 
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ABSTRACT: Under the DOE Deep Burn program TRISO fuel is being investigated as a fuel form for consuming plutonium and minor actinides, and for greater efficiency in uranium utilization. The result will thus be to drive TRISO particulate fuel to very high burnups. In the current effort the various phenomena in the TRISO particle are being modeled using a variety of techniques. The chemical behavior is being treated utilizing thermochemical analysis to identify phase formation/transformation and chemical activities in the particle, including kernel migration. Density functional theory is being used to understand fission product diffusion within the plutonia oxide kernel, the fission product's attack on the SiC coating layer, as well as fission product diffusion through an alternative coating layer, ZrC. Finally, a multiscale approach is being used to understand thermal transport, including the effect of radiation damage induced defects, in a model SiC material. (C) 2012 Elsevier B.V. All rights reserved.Journal of Nuclear Materials 11/2012; 430(1313):181189. DOI:10.1016/j.jnucmat.2012.06.041 · 2.02 Impact Factor 
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ABSTRACT: The vibrational excitations of crystalline solids corresponding to acoustic or optic onephonon modes appear as sharp features in measurements such as neutron spectroscopy. In contrast, manyphonon excitations generally produce a complicated, weak and featureless response. Here we present timeofflight neutron scattering measurements for the binary solid uranium nitride, showing welldefined, equally spaced, highenergy vibrational modes in addition to the usual phonons. The spectrum is that of a single atom, isotropic quantum harmonic oscillator and characterizes independent motions of light nitrogen atoms, each found in an octahedral cage of heavy uranium atoms. This is an unexpected and beautiful experimental realization of one of the fundamental, exactly solvable problems in quantum mechanics. There are also practical implications, as the oscillator modes must be accounted for in the design of generation IV nuclear reactors that plan to use uranium nitride as a fuel.Nature Communications 10/2012; 3:1124. DOI:10.1038/ncomms2117 · 10.74 Impact Factor 
Article: Erratum: Magnetic field effects on transport properties of PtSn4 [Phys. Rev. B 85, 035135 (2012)]
Physical review. B, Condensed matter 08/2012; 86(7). DOI:10.1103/PhysRevB.86.079909 · 3.66 Impact Factor 
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ABSTRACT: We present the growths and detailed thermodynamic and transport measurements on single crystals of the recently discovered binary intermetallic superconductors, SrSn4 and BaSn5. Their superconducting transition temperatures Tc are found to be 4.8 K and 4.4 K respectively. Both materials are stronglycoupled, possibly multiband superconductors. Hydrostatic pressure causes a decrease in the superconducting transition temperature at the rate of 0.068 K/kbar for SrSn4, and 0.053 K/kbar for BaSn5. Band structure and upper superconducting critical field anisotropy of SrSn4 suggest complex, multisheet Fermi surface formed by four bands. De Hassvan Alphen oscillations are observed in BaSn5, which indicates a more complex topology of Fermi surface. 
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ABSTRACT: The anisotropic physical properties of single crystals of orthorhombic PtSn4 are reported for magnetic fields up to 140 kOe, applied parallel and perpendicular to the crystallographic b axis. The magnetic susceptibility has an approximately temperatureindependent behavior and reveals an anisotropy between the ac plane and b axis. Clear de Haasvan Alphen oscillations in fields as low as 5 kOe and at temperatures as high as 30 K were detected in magnetization isotherms. The thermoelectric power and resistivity of PtSn4 show the strong temperature and magnetic field dependencies. A change of the thermoelectric power at H=140 kOe is observed as high as ≃50 μV/K. Single crystals of PtSn4 exhibit very large transverse magnetoresistance of ≃5×105% for the ac plane and of ≃1.4×105% for the b axis resistivity at 1.8 K and 140 kOe, as well as pronounced Shubnikov de Haas oscillations. The magnetoresistance of PtSn4 appears to obey Kohler's rule in the temperature and field range measured. The Hall resistivity shows a linear temperature dependence at high temperatures followed by a sign reversal around 25 K which is consistent with thermoelectric power measurements. The observed quantum oscillations and band structure calculations indicate that PtSn4 has threedimensional Fermi surfaces.Physical review. B, Condensed matter 01/2012; 85(3). DOI:10.1103/PhysRevB.85.035135 · 3.66 Impact Factor 
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ABSTRACT: We present detailed thermodynamic and transport measurements on single crystals of the recently discovered binary intermetallic superconductor, SrSn(4). We find this material to be a slightly anisotropic threedimensional, strongly coupled, possibly multiband, superconductor. Hydrostatic pressure causes a decrease in the superconducting transition temperature at the rate of ≈  0.068 K kbar(1). Band structure calculations are consistent with experimental data on the Sommerfeld coefficient and upper superconducting critical field anisotropy, and suggest a complex, multisheet Fermi surface formed by four bands.Journal of Physics Condensed Matter 11/2011; 23(45):455703. DOI:10.1088/09538984/23/45/455703 · 2.22 Impact Factor 
Article: Molecular dynamics study of influence of vacancy types defects on thermal conductivity of βSiC
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ABSTRACT: A molecular dynamics technique has been used to study the impact of single vacancies and small vacancy clusters/microvoids on thermal conductivity of Si and βSiC. It is found that single vacancies reduce thermal conductivity more significantly than do microvoids with the same total number of vacancies in the crystal. The vacancy concentration dependence of the relative change of thermal resistivity of both Si and SiC changes from linear at low concentrations to squareroot at higher values. In contrast, the dependence on the volume fraction of microvoids switches from squareroot at small swelling values to nearly linear dependence at higher swelling. In the case of SiC the results obtained for vacancies and microvoids agree reasonably well with experimental values. The computational results are compared with the commonly used Debye–Callaway model.Highlights► Green–Kubo MD method was used to study thermal conductivity in Si and βSiC. ► The impact of defects on thermal conductivity was determined for both Si and βSiC. ► Good agreement between calculations and experimental data was obtained. ► MD provides better agreement with data than predictions of Debye–Callaway model.Journal of Nuclear Materials 11/2011; 418(13):174181. DOI:10.1016/j.jnucmat.2011.06.036 · 2.02 Impact Factor 
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ABSTRACT: Local atomic magnetic moments in crystalline Fe are perturbed by the presence of dislocations. The effects are most pronounced near the dislocation core and decay slowly as the strain field of the dislocation decreases with distance. We have calculated local moments using the locally selfconsistent multiple scattering (LSMS) method for a supercell containing a screwdislocation quadrupole. Finite size effects are found to be significant indicating that dislocation cores affect the electronic structure and magnetic moments of neighboring dislocations. The influence of neighboring dislocations points to a need to study individual dislocations from first principles just as they appear amid surrounding atoms in largescale classical force field simulations. An approach for the use of the LSMS to calculate local moments in subvolumes of large atomic configurations generated in the course of classical molecular dynamics simulation of dislocation dynamics is discussed.Journal of Applied Physics 04/2011; 109(7):07E15907E1593. DOI:10.1063/1.3562217 · 2.19 Impact Factor 
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ABSTRACT: Modern ab initio theories of the magnetic phase transition (Curie Temperature, TC) of Fe and Ni based on the Disordered Local Moment (DLM) type models generally rely on (constrained) density functional theory calculations performed at 0K and assume that the atoms occupy their equilibrium lattice sites. Here we point out that finite temperature lattice vibrations can result in large fluctuations in the local moments associated with individual site beyond those already accounted for in these approaches. These conclusions are based on large cell (˜10^4  atoms) ab initio calculations of the magnetic state of Fe and Ni based on the O[N] Locally Selfconsistent Multiple Scattering (LSMS) method. Atom positions are obtained from freezes of individual time steps of molecular dynamics simulations based on classical interaction potentials. Calculations are performed for a range of temperatures up and beyond TC that illustrate the extent of the moment fluctuations. We discuss the consequences of these findings for the adequacy of existing theories TC. 
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ABSTRACT: The high ferromagnetic ordering temperature of the dilute, rareearthbearing, intermetallic compound GdFe2Zn20 has been understood as being the consequence of the Gd3+ moment being embedded in a nearly ferromagnetic Fermi liquid. To test this understanding in detail, single crystals of the pseudoternary series GdFe2(AlxZn1x)(20) (x <= 0.122) and YFe2(AlxZn1x)(20) (x <= 0.121) were grown out of Znrich solution. Magnetization, heat capacity, and resistivity measurements show that, with Al doping, the ferromagnetic phase transition temperatures of the GdFe2(AlxZn1x)(20) compounds decrease from 86 K (x = 0) to 10 K (x = 0.122); for the nonmagnetic analog, the YFe2(AlxZn1x)(20) series, the Stoner enhancement factor Z decreases from 0.88 (x = 0) to 0.35 (x = 0.121) in a similar manner. Tightbinding linearmuffintin orbital atomicsphere approximation band structure calculations are used to rationalize this trend. These results, together with the earlier studies of the R(Fe1xCox)(2)Zn20 (R = Gd and Y) series, clearly highlight the importance of band filling and the applicability of even a simple, rigidband model to these compounds.Physical review. B, Condensed matter 02/2011; 83(5). DOI:10.1103/PhysRevB.83.054416 · 3.66 Impact Factor
Publication Stats
1k  Citations  
184.72  Total Impact Points  
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Institutions

2011–2014

Oak Ridge National Laboratory
 • Materials Science and Technology Division
 • Physical Sciences Directorate
Oak Ridge, Florida, United States


2003–2012

Iowa State University
 • Department of Physics and Astronomy
 • Ames Laboratory
 • Department of Chemistry
Ames, IA, United States


1998

Kurchatov Institute
Moskva, Moscow, Russia
