John W. Wilkins

The Ohio State University, Columbus, Ohio, United States

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Publications (191)430.96 Total impact

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    ABSTRACT: The computational cost of quantum Monte Carlo (QMC) calculations of realistic periodic systems depends strongly on the method of storing and evaluating the many-particle wave function. Previous work [A. J. Williamson et al., Phys. Rev. Lett. 87, 246406 (2001); D. Alf\`e and M. J. Gillan, Phys. Rev. B 70, 161101 (2004)] has demonstrated the reduction of the O(N^3) cost of evaluating the Slater determinant with planewaves to O(N^2) using localized basis functions. We compare four polynomial approximations as basis functions -- interpolating Lagrange polynomials, interpolating piecewise-polynomial-form (pp-) splines, and basis-form (B-) splines (interpolating and smoothing). All these basis functions provide a similar speedup relative to the planewave basis. The pp-splines have eight times the memory requirement of the other methods. To test the accuracy of the basis functions, we apply them to the ground state structures of Si, Al, and MgO. The polynomial approximations differ in accuracy most strongly for MgO and smoothing B-splines most closely reproduce the planewave value for of the variational Monte Carlo energy. Using separate approximations for the Laplacian of the orbitals increases the accuracy sufficiently to justify the increased memory requirement, making smoothing B-splines, with separate approximation for the Laplacian, the preferred choice for approximating planewave-represented orbitals in QMC calculations.
    09/2013;
  • Amita Wadehra, John W. Wilkins
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    ABSTRACT: Perovskites have been a focus of considerable research attention due to exhibiting a variety of interesting and unique physical properties such as magnetism, ferroelectricity, superconductivity and multiferroicity. Accurate computations are needed to gain insights into the underlying physics of these complex materials. We present a systematic computational study of a series of titanates (ATiO3; A=Sr, Ba, Ca, La, Sn, Pb) using the hybrid functional HSE in density functional theory. HSE surpasses standard DFT and computes properties such as lattice constants, band gaps, structural and magnetic phases in excellent agreement with available experimental data. We also discuss the importance of spin-orbit interaction in determining the electronic structure and magnetic properties of these complex oxides.
    03/2013;
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    ABSTRACT: Density-functional theory energies, forces, and elastic constants determine the parametrization of an empirical, modified embedded-atom method potential for molybdenum. The accuracy and transferability of the potential are verified by comparison to experimental and density-functional data for point defects, phonons, thermal expansion, surface and stacking fault energies, and ideal shear strength. Searching the energy landscape predicted by the potential using a genetic algorithm verifies that it reproduces not only the correct bcc ground state of molybdenum but also all low-energy metastable phases. The potential is also applicable to the study of plastic deformation and used to compute energies, core structures, and Peierls stresses of screw and edge dislocations.
    Physical review. B, Condensed matter 06/2012; 85(21). · 3.77 Impact Factor
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    ABSTRACT: We show that carbon-doped hexagonal boron nitride (h-BN) has extraordinary properties with many possible applications. We demonstrate that the substitution-induced impurity states, associated with carbon atoms, and their interactions dictate the electronic structure and properties of C-doped h-BN. Furthermore, we show that stacking of localized impurity states in small C clusters embedded in h-BN forms a set of discrete energy levels in the wide gap of h-BN. The electronic structures of these C clusters have a plethora of applications in optics, magneto-optics, and opto-electronics.
    Applied Physics Letters 05/2012; 100(25). · 3.52 Impact Factor
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    ABSTRACT: Advances in development of atomic-layer crystals with a plethora of new materials are greatly extending the range of possible applications of these two-dimensional (2D) materials. One of these materials is the hexagonal structure of boron nitride (h-BN). Hexagonal BN has a wide band gap and a lattice constant similar to that of graphene. We show that even small quantities of C atoms can offer new functionalities and transform h-BN to be an amazing playground for 2D physics. Large-scale accurate density-functional-theory calculations with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional reveal the electronic and the magnetic properties of h-BN with substitutionally embedded carbon atoms. Results of local magnetic moments induced by substitution and their interactions are presented for low C concentrations. We also show the electronic structures of quantum dots made of carbon nano-domains for applications in optics and opto-electronics.
    02/2012;
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    ABSTRACT: Graphene has attracted enormous research interest in the last few years because of its intriguing physics as well as application potential. Recent synthesis of BCN nanostructures by doping graphene with a wide bandgap insulator boron nitride (BN) has unveiled new possibilities for this material [1]. BCN nanostructures are semiconductors and possess interesting properties that are distinct from the parent compounds. Reliable theoretical estimates can predict the feasibility and usefulness of still largely unexplored BCN nanostructures, and provide a route to engineer their properties. We study electronic structures of a variety of 2D BCN nanostructures using hybrid functional HSE in density functional theory (DFT). We show that their properties can be gradually tuned and are sensitive to composition and the type of configurations. In agreement with experimental observation, a strong tendency to phase-segregate exists for low concentration of BN in graphene. We also investigate magnetic properties of graphene containing substitutional nitrogen atoms, and their suitability for magnetic devices.[4pt] [1]. L. Ci et al., Nature Materials 9, 430 (2010).
    02/2012;
  • Jeremy W. Nicklas, John W. Wilkins
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    ABSTRACT: The HgCdTe alloy is used in high-performance infrared detection applications with a band gap range extending across the infrared spectrum. HgTe in particular has sparked interest for its topological insulating behavior in quantum well devices due to its band inverted nature. We test the quality of the newer hybrid screened functional, HSE, on the two contrasting materials: HgTe (semimetal) and CdTe (semiconductor) to see how well it performs under a range of computational setups [1]. A direct comparison of HSE with the standard DFT functional PBE to experiment for the HgCdTe alloy reveals HSE is able to reproduce the experimental crossover composition of 17% Cd concentration when the alloy goes from a semimetal to semiconductor, whereas PBE overestimates this composition at 67% Cd concentration. HSE also predicts a higher valence band offset of 0.53 eV in the HgTe/CdTe heterostructure than previous first-principle and early experimental results, but in good agreement with the more recent experimental results. Supported by DOE-Basic Energy Science DOE-BES-DMS (DEFG02-99ER45795). Computing resources are provided by NERSC and OSC. [4pt] [1] Jeremy W. Nicklas and John W. Wilkins, Phys. Rev. B 84, 121308(R) (2011)
    02/2012;
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    ABSTRACT: A fundamental understanding of transformation and deformation processes in the bcc refractory metals (V, Nb, Ta, Mo, and W) is vital for designing new bcc-based commercial alloys with desired properties. Such an understanding is aided by computational methods capable of reaching length and time scales needed for meaningful simulations of phase transformations and extended defects responsible for plastic deformation. Classical interatomic potentials are indispensable for simulating such phenomena inaccessible to first-principles methods. We develop accurate and robust embedded-atom method (EAM) [1] and modified-EAM (MEAM) [2] potentials for the bcc metals by fitting the model parameters to accurate first-principles data. The potentials are applicable for studying mechanical and thermodynamic properties, yielding excellent agreement with both experiments and first-principles calculations. [1] M. S. Daw and M. I. Baskes, Phys. Rev. Lett. 50, 1285 (1983). [2] M. I. Baskes, Phys. Rev. Lett. 59, 2666 (1987).
    02/2012;
  • Jeremy W. Nicklas, Amita Wadehra, John W. Wilkins
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    ABSTRACT: We present a density functional study of the magnetic properties of Fe adatoms on Cu2N/Cu(100) surface. The magnetic anisotropy energies of a single Fe atom are in excellent agreement with the available experiments. Our results for the spin densities and exchange coupling strengths for Fe dimer and trimer establish antiferromagnetic configuration to be the ground state due to predominant superexchange interaction mediated by nitrogen atoms in this system.
    Journal of Applied Physics 12/2011; 110(12). · 2.21 Impact Factor
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    ABSTRACT: Mobile single interstitials can grow into extended interstitial defect structures during thermal anneals following ion implantation. The silicon tetra-interstitials present an important intermediate structure that can either provide a chain-like nucleation site for extended structures or form a highly stable compact interstitial cluster preventing further growth. In this paper, dimer searches using the tight-binding (TB) model by Lenosky et al. and density functional calculations show that the compact ground-state and the I4-chain are surrounded by high-lying neighboring local minima.To furthermore explore the phase space of tetra-interstitial structures an empirical potential is optimized to a database of silicon defect structures. The minima hopping method combined with this potential extensively searches the energy landscape of tetra-interstitials and discovers several new low-energy I4 structures. The second lowest-energy I4 structure turns out to be a distorted ground-state tri-interstitial bound with a single interstitial, which confirms that the ground-state tri-interstitial may serve as a nucleation center for the extended defects in silicon.
    physica status solidi (b) 06/2011; 248(9):2050 - 2055. · 1.49 Impact Factor
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    Hyoungki Park, John W. Wilkins
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    ABSTRACT: Clustering and annihilation of atomic-scale bond defects dominate nucleation and evolution of submicron-scale extended interstitial defects in irradiated silicon. Molecular dynamics simulations reveal the role of the bond defect in the thermal evolution of extended defects and identify the atomistic evolution paths. Accurate density functional theory calculations establish formation energies, activation barriers, and electronic structures of the bond defect and its clusters, and extended interstitial defects.
    Applied Physics Letters 04/2011; 98(17):171915-171915-3. · 3.52 Impact Factor
  • Amita Wadehra, Jeremy Nicklas, John Wilkins
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    ABSTRACT: Semiconductor alloy heterostructures are the backbone of optoelectronic devices. Among the most important parameters that determine the utility of heterostructure devices are the valence and conduction band offsets. Although DFT with standard functionals such as LDA or PBE does an acceptable job for valence band offsets, it fails to predict accurate conduction band offsets on its own due to the well-known band gap problem. We demonstrate the accuracy of HSE (Heyd-Scuseria-Ernzerhof) hybrid functional for computing the band gaps and band offsets of a broad selection of technologically important semiconductor alloys and their heterostructures, e.g., AlInAs/GaInAs, GaInP/AlGaAs, AlInP/GaInP [1]. The highlight of this study is the computation of conduction band offsets with a reliability that has eluded standard density functional theory. These results demonstrate predictive power of HSE for band engineering of relevant devices. [4pt] [1]. A. Wadehra, J. W. Nicklas and J. W. Wilkins, Appl. Phys. Lett. 97, 092119 (2010)
    03/2011;
  • Jeremy Nicklas, John Wilkins
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    ABSTRACT: The screened hybrid functional, HSE, used in density functional theory (DFT) has been gaining traction recently for its predictive powers of the band structure in bulk semiconductors. It is natural to assume that these accurate results would carry over to alloy semiconductors, but little work has been done to confirm this. We recently investigated the compositional dependence on the electronic band structure for a range of III-V semiconducting alloys (AlGaAs, InAlAs, AlInP, InGaP, and GaAsP) [1]. These alloys have a critical composition where the band gap crosses over from a direct band gap (having optoelectronic uses) to an indirect band gap (window layers in solar cells). A direct comparison of this critical composition is made between HSE and the standard density functional, PBE, revealing crossover compositions within 12% atomic composition when compared to experiment while PBE overestimates by as much as 39% atomic composition. Such results give merit that HSE is a reliable functional for tuning the electronic properties of semiconducting alloys.[4pt] [1] Jeremy W. Nicklas and John W. Wilkins, Appl. Phys. Lett. 97, 091902 (2010)
    03/2011;
  • physica status solidi (b) 02/2011; 248(2). · 1.49 Impact Factor
  • MRS Online Proceeding Library 01/2011; 731.
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    Jeremy W. Nicklas, John W. Wilkins
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    ABSTRACT: Hybrid screened density functional theory better describes the electronic structure of HgTe, CdTe, and HgCdTe systems in comparison with standard density functional theory. The unique hybrid functional reproduces the band inversion in the popular HgCdTe alloy, justifying it as a better method than standard density functional theory in the search for different topological insulators. In addition, the 0.53-eV valence-band offset obtained using the hybrid functional supports the recently observed higher band offset in the HgTe/CdTe heterostructure.
    Physical review. B, Condensed matter 01/2011; 84. · 3.77 Impact Factor
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    Jeremy W. Nicklas, John W. Wilkins
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    ABSTRACT: We report the compositional dependence of the electronic band structure for a range of III–V alloys. Standard density functional theory is insufficient to mimic the electronic gap energies at different symmetry points of the Brillouin zone. The Heyd–Scuseria–Ernzerhof hybrid functional with screened exchange accurately reproduces the experimental band gaps and, more importantly, the alloy concentration of the direct-indirect gap crossovers for the III–V alloys studied here: AlGaAs, InAlAs, AlInP, InGaP, and GaAsP.
    Applied Physics Letters 09/2010; · 3.52 Impact Factor
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    Amita Wadehra, Jeremy W. Nicklas, John W. Wilkins
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    ABSTRACT: We demonstrate the accuracy of the hybrid functional HSE06 for computing band offsets of semiconductor alloy heterostructures. The highlight of this study is the computation of conduction band offsets with a reliability that has eluded standard density functional theory. A high-quality special quasirandom structure models an infinite random pseudobinary alloy for constructing heterostructures along the (001) growth direction. Our excellent results for a variety of heterostructures establish HSE06's relevance to band engineering of high-performance electrical and optoelectronic devices. Comment: 3 pages, 2 figures
    Applied Physics Letters 07/2010; · 3.52 Impact Factor
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    ABSTRACT: Silicon undergoes a phase transition from the semiconducting diamond phase to the metallic beta-Sn phase under pressure. We use quantum Monte Carlo calculations to predict the transformation pressure and compare the results to density functional calculations employing the LDA, PBE, PW91, WC, AM05, PBEsol and HSE06 exchange-correlation functionals. Diffusion Monte Carlo predicts a transition pressure of 14.0 +- 1.0 GPa slightly above the experimentally observed transition pressure range of 11.3 to 12.6 GPa. The HSE06 hybrid functional predicts a transition pressure of 12.4 GPa in excellent agreement with experiments. Exchange-correlation functionals using the local-density approximation and generalized-gradient approximations result in transition pressures ranging from 3.5 to 10.0 GPa, well below the experimental values. The transition pressure is sensitive to stress anisotropy. Anisotropy in the stress along any of the cubic axes of the diamond phase of silicon lowers the equilibrium transition pressure and may explain the discrepancy between the various experimental values as well as the small overestimate of the quantum Monte Carlo transition pressure.
    Physical review. B, Condensed matter 07/2010; 82(1). · 3.77 Impact Factor
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    ABSTRACT: Quantum Monte Carlo approaches such as the diffusion Monte Carlo (DMC) method are among the most accurate many-body methods for extended systems. Their scaling makes them well suited for defect calculations in solids. We review the various approximations needed for DMC calculations of solids and the results of previous DMC calculations for point defects in solids. Finally, we present estimates of how approximations affect the accuracy of calculations for self-interstitial formation energies in silicon and predict DMC values of 4.4(1), 5.1(1) and 4.7(1) eV for the X, T and H interstitial defects, respectively, in a 16(+1)-atom supercell.
    physica status solidi (b) 06/2010; · 1.49 Impact Factor