[Show abstract][Hide abstract] ABSTRACT: While studying magnetism of d- and f-electron systems has been consistently an active research area in physics, chemistry, and biology, there is an increasing interest in the novel magnetism of p-electron systems, especially in graphene and graphene-derived nanostructures. Bulk graphite is diamagnetic in nature, however, graphene is known to exhibit either a paramagnetic response or weak ferromagnetic ordering. Although many groups have attributed this magnetism in graphene to defects or unintentional magnetic impurities, there is a lack of compelling evidence to pinpoint its origin. To resolve this issue, we systematically studied the influence of entropically necessary intrinsic defects (e.g., vacancies, edges) and extrinsic dopants (e.g., S-dopants) on the magnetic properties of graphene. We found that the saturation magnetization of graphene decreased upon sulfur doping suggesting that S-dopants demagnetize vacancies and edges. Our density functional theory calculations provide evidence for: (i) intrinsic defect demagnetization by the formation of covalent bonds between S-dopant and edges/vacancies concurring with the experimental results, and (ii) a net magnetization from only zig-zag edges, suggesting that the possible contradictory results on graphene magnetism in the literature could stem from different defect-types. Interestingly, we observed peculiar local maxima in the temperature dependent magnetizations that suggest the coexistence of different magnetic phases within the same graphene samples.
Full-text · Article · Mar 2016 · Journal of Magnetism and Magnetic Materials
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
Full-text · Article · May 2012 · Applied Physics Letters
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 . 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] . L.
Ci et al., Nature Materials 9, 430 (2010).
[Show abstract][Hide abstract] 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 . 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]  Jeremy W. Nicklas and John W. Wilkins, Phys. Rev. B 84,
[Show abstract][Hide abstract] 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)  and
modified-EAM (MEAM)  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.  M. S. Daw and M. I. Baskes, Phys. Rev.
Lett. 50, 1285 (1983).  M. I. Baskes, Phys. Rev. Lett. 59,
[Show abstract][Hide abstract] 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. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3672444]
No preview · Article · Dec 2011 · Journal of Applied Physics
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
No preview · Article · Jun 2011 · physica status solidi (b)
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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) . 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]  Jeremy W. Nicklas and John W. Wilkins, Appl. Phys. Lett. 97, 091902 (2010)
[Show abstract][Hide abstract] 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 . 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] . A. Wadehra, J. W. Nicklas and J. W. Wilkins, Appl. Phys. Lett. 97, 092119 (2010)
[Show abstract][Hide abstract] 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.
Full-text · Article · Feb 2011 · physica status solidi (b)
[Show abstract][Hide abstract] ABSTRACT: The simulation of defect dynamics and evolution is a technologically relevant challenge for computational materials science. The diffusion of small defects in silicon unfolds as a sequence of structural transitions. The relative infrequency of transition events requires simulation over extremely long time scales. We simulate the diffusion of small interstitial clusters (I 1, I 2, I 3) for a range of temperatures using large-scale molecular dynamics (MD) simulations with a realistic tight-binding potential. A total of 0.25 μ sec of simulation time is accumulated for the study. A novel real-time multiresolution analysis (RTMRA) technique extracts stable structures directly from the dynamics without structural relaxation. The discovered structures are relaxed to confirm their stability.
No preview · Article · Jan 2011 · MRS Online Proceeding Library
[Show abstract][Hide abstract] ABSTRACT: We report the compositional dependence of the electronic band structure for a
range of III-V alloys. Density functional theory with the PBE functional is
insufficient to mimic the electronic gap energies at different symmetry points
of the Brillouin zone. The HSE 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.