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ABSTRACT: Structural and electronic properties, including deformation, magnetic moment, Mulliken population, bond order, as well as electronic transport properties, of zigzag graphene nanoribbon (ZGNR) with Co adatoms on hollow sites are investigated by quasi-atomic minimal basis orbits (QUAMBOs), a first-principles tight binding (TB) scheme based on density functional theory (DFT), combined with a non-equilibrium Green's function. For electronic transport, below the Fermi level the transmission is strongly suppressed and spin dependent as a result of magnetism by Co adatom adsorption, while above the Fermi level the transmission is slightly distorted and spin independent. Due to the local environment dependence of QUAMBOs-TB parameters, we construct QUAMBOs-TB parameters of ZGNR leads and ZGNR with Co adatoms on hollow center sites by a divide-and-conquer approach, and accurately reproduce the electronic transmission behavior. Our QUAMBO-NEGF method is a new and promising way of examining electronic transport in large-scale systems.
Journal of Physics Condensed Matter 03/2013; 25(10):105302. · 2.55 Impact Factor
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F. Hardy,
A. E. Boehmer,
D. Aoki,
P. Burger,
T. Wolf,
P. Schweiss,
R. Heid,
P. Adelmann, Y. X. Yao,
G. Kotliar,
J. Schmalian,
C. Meingast
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ABSTRACT: Using resistivity, heat-capacity, thermal-expansion, and susceptibility
measurements we study the normal-state behavior of KFe2As2. We find that both
the Sommerfeld coefficient gamma = 103 mJ mol-1 K-2 and the Pauli
susceptibility chi = 4x10-4 are strongly enhanced, which confirm the existence
of heavy quasiparticles inferred from previous de Haas-van Alphen and ARPES
experiments. We discuss this large enhancement using a Gutzwiller slave-boson
mean-field calculation, which reveals the proximity of KFe2As2 to an
orbital-selective Mott transition. The temperature dependence of the magnetic
susceptibility and the thermal expansion provide strong experimental evidence
for the existence of a coherence-incoherence crossover, similar to what is
found in heavy fermion and ruthenate compounds, due to Hund's coupling between
orbitals.
02/2013;
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ABSTRACT: We present a systematic study of metal adatom adsorption on graphene by ab initio calculations. The calculations cover alkali metals, sp-simple metals, 3d and group 10 transition metals, noble metals, as well as rare earth metals. The correlation between the adatom adsorption properties and the growth morphology of the metals on graphene is also investigated. We show that the growth morphology is related to the ratio of the metal adsorption energy to its bulk cohesive energy (E(a)/E(c)) and the diffusion barrier (ΔE) of the metal adatom on graphene. Charge transfer, electric dipole and magnetic moments, and graphene lattice distortion induced by metal adsorption would also affect the growth morphologies of the metal islands. We also show that most of the metal nanostructures on graphene would be thermally stable against coarsening.
Physical Chemistry Chemical Physics 05/2012; 14(25):9157-66. · 3.57 Impact Factor
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ABSTRACT: To elucidate the role played by the transition-metal ion in the pnictide materials, we compare the electronic and magnetic properties of BaFe2As2 with BaMn2As2. To this end we employ the LDA + Gutzwiller method to analyze the mass renormalizations and the size of the ordered magnetic moment of the two systems. We study a model that contains all five transition-metal 3d orbitals together with the Ba 5d and As 4p states (ddp-model) and compare these results with a downfolded model that consists of Fe/Mn d states only (d-model). Electronic correlations are treated using the multiband Gutzwiller approximation. The paramagnetic phase has also been investigated using the LDA + Gutzwiller method with electron density self-consistency. The renormalization factors for the correlated Mn 3d orbitals in the paramagnetic phase of BaMn2As2 are shown to be generally smaller than those of BaFe2As2, which indicates that BaMn2As2 has stronger electron correlation effect than BaFe2As2. The screening effect of the main As 4p electrons to the correlated Fe/Mn 3d electrons is evident by the systematic shift of the results to the larger Hund's rule coupling J side from the ddp-model compared with those from the d-model. A gradual transition from paramagnetic state to the antiferromagnetic ground state with increasing J is obtained for the models of BaFe2As2 which has a small experimental magnetic moment, while a rather sharp jump occurs for the models of BaMn2As2, which has a large experimental magnetic moment. The key difference between the two systems is shown to be the d-level occupation. BaMn2As2, with approximately five d electrons per Mn atom, is for the same values of the electron correlations closer to the transition to a Mott insulating state than BaFe2As2. Here an orbitally selective Mott transition, required for a system with close to six electrons, only occurs at significantly larger values for the Coulomb interactions.
Phys. Rev. B. 12/2011; 84(24).
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ABSTRACT: The conductance of single-atom carbon chain (SACC) between two zigzag graphene nanoribbons (GNR) is studied by an efficient scheme utilizing tight-binding (TB) parameters generated via quasi-atomic minimal basis set orbitals (QUAMBOs) and non-equilibrium Green's function (NEGF). Large systems (SACC contains more than 50 atoms) are investigated and the electronic transport properties are found to correlate with SACC's parity. The SACCs provide a stable off or on state in broad energy region (0.1-1 eV) around Fermi energy. The off state is not sensitive to the length of SACC while the corresponding energy region decreases with the increase of the width of GNR.
Physics Letters A 10/2011; 375:3710. · 1.63 Impact Factor
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ABSTRACT: We introduce a scheme to include many-body screening processes explicitly
into a set of self-consistent equations for electronic structure calculations
using the Gutzwiller approximation. The method is illustrated by the
application to a tight-binding model describing the strongly correlated
{\gamma}-Ce system. With the inclusion of the 5d-electrons into the local
Gutzwiller projection subspace, the correct input Coulomb repulsion U_{ff}
between the 4f-electrons for {\gamma}-Ce in the calculations can be pushed far
beyond the usual screened value U_{ff}^{scr} and close to the bare atomic value
U_{ff}^{bare}. This indicates that the d-f many-body screening is the dominant
contribution to the screening of U_{ff} in this system. The method provides a
promising way towards the ab initio Gutzwiller density functional theory.
09/2011;
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ABSTRACT: To elucidate the role played by the transition metal ion in the pnictide
materials, we compare the electronic and magnetic properties of BaFe_{2}As_{2}
with BaMn_{2}As_{2}. To this end we employ the LDA+Gutzwiller method to analyze
the mass renormalizations and the size of the ordered magnetic moment of the
two systems. We study a model that contains all five transition metal 3d
orbitals together with the Ba-5d and As-4p states (ddp-model) and compare these
results with a downfolded model that consists of Fe/Mn d-states only (d-model).
Electronic correlations are treated using the multiband Gutzwiller
approximation. The paramagnetic phase has also been investigated using
LDA+Gutzwiller method with electron density self-consistency. The
renormalization factors for the correlated Mn 3d orbitals in the paramagnetic
phase of BaMn_{2}As_{2} are shown to be generally smaller than those of
BaFe_{2}As_{2}, which indicates that BaMn_{2}As_{2} has stronger electron
correlation effect than BaFe_{2}As_{2}. The screening effect of the main As 4p
electrons to the correlated Fe/Mn 3d electrons is evident by the systematic
shift of the results to larger Hund's rule coupling J side from the ddp-model
compared with those from the d-model. A gradual transition from paramagnetic
state to the antiferromagnetic ground state with increasing J is obtained for
the models of BaFe_{2}As_{2} which has a small experimental magnetic moment;
while a rather sharp jump occurs for the models of BaMn_{2}As_{2}, which has a
large experimental magnetic moment. The key difference between the two systems
is shown to be the d-level occupation. BaMn_{2}As_{2}, with approximately five
d-electrons per Mn atom, is for same values of the electron correlations closer
to the transition to a Mott insulating state than BaFe_{2}As_{2}. Here an
orbitally selective transition, required for a system with close to six
electrons only occurs at significantly larger values for the Coulomb
interactions.
09/2011;
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ABSTRACT: We report a collection of lowest-energy structures of hydrocarbon molecules
C_{m}H_{n} (m=1-18; n=0-2m+2). The structures are examined within a wide
hydrogen chemical potential range. The genetic algorithm combined with
Brenner's empirical potential is applied for the search. The resultant
low-energy structures are further studied by ab initio quantum chemical
calculations. The lowest-energy structures are presented with several
additional low-energy structures for comparison. The results are expected to
provide useful information for some unresolved astronomical spectra and the
nucleation of growth of nano-diamond film.
08/2011;
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ABSTRACT: An ab initio molecular dynamics (MD) simulation is performed to investigate the structural evolution in Al(90)Sm(10) liquid from 1500 to 900 K. Development of Al(11)Sm(3) local order upon rapid cooling is suggested by the Honeycutt-Anderson (HA) index analysis and the appearance of a predominant Sm-Sm-Sm bond angle around 90° when the liquid approaches the melting point (∼920 K). Direct structural evidence of Al(11)Sm(3) fragments at 900 K is obtained using an atomic cluster alignment method developed recently. Meanwhile, development of strong icosahedral short range order (ISRO) and a non-negligible amount of fcc-type clusters around Al in the system are also observed. These results suggest that fcc Al and Al(11)Sm(3) crystalline phases would compete strongly with the formation of an amorphous phase that exhibits ISRO in the diffusionless solidification limit upon rapid quenching.
Journal of Physics Condensed Matter 06/2011; 23(23):235104. · 2.55 Impact Factor
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ABSTRACT: The electronic structure and transport property of a carbon chain between two graphene nanoribbon leads are studied using an ab initio tight-binding (TB) model and Landauer's formalism combined with a non-equilibrium Green's function. The TB Hamiltonian and overlap matrices are extracted from first-principles density functional calculations through the quasi-atomic minimal basis orbital scheme. The accuracy of the TB model is demonstrated by comparing the electronic structure from the TB model with that from first-principles density functional theory. The results of electronic transport on a carbon atomic chain connected to armchair and zigzag graphene ribbon leads, such as different transport characters near the Fermi level and at most one quantized conductance, reveal the effect of the electronic structure of the leads and the scattering from the atomic chain. In addition, bond length alternation and an interesting transmission resonance are observed in the atomic chain connected to zigzag graphene ribbon leads. Our approach provides a promising route to quantitative investigation of both the electronic structure and transport property of large systems.
Journal of Physics Condensed Matter 01/2011; 23(2):025302. · 2.55 Impact Factor
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ABSTRACT: 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 Cu(64.5)Zr(35.5) 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 Bergman-type MRO from our order-mining technique provides unique insights into the topological ordering near the glass transition and the relationship between metallic glasses and quasicrystals.
Scientific Reports 01/2011; 1:194.
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ABSTRACT: We demonstrate an efficient and accurate first-principles method to calculate the electronic structure of a large system using a divide-and-conquer strategy based on localized quasi-atomic minimal basis set orbitals recently developed. Tight-binding Hamiltonian and overlap matrices of a large system can be constructed by extracting the matrix elements for a given pair of atoms from first-principles calculations of smaller systems that represent the local bonding environment of the particular atom pair. The approach is successfully applied to the studies of electronic structure in graphene nanoribbons. This provides a promising way to do the electronic simulation for large systems directly from first principles.
Journal of Physics Condensed Matter 06/2009; 21(23):235501. · 2.55 Impact Factor
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ABSTRACT: The conductance of single-atom carbon chain (SACC) between two zigzag graphene nanoribbons (GNR) is studied by an efficient scheme utilizing tight-binding (TB) parameters generated via quasi-atomic minimal basis set orbitals (QUAMBOs) and non-equilibrium Greenʼs function (NEGF). Large systems (SACC contains more than 50 atoms) are investigated and the electronic transport properties are found to correlate with SACCʼs parity. The SACCs provide a stable off or on state in broad energy region (0.1–1 eV) around Fermi energy. The off state is not sensitive to the length of SACC while the corresponding energy region decreases with the increase of the width of GNR.Highlights► Graphene has many superior electronic properties. ► First-principles calculation are accurate but limited to system size. ► QUAMBOs construct tight-binding parameters with spatial localization, and then use divide-and-conquer method. ► SACC (single carbon atom chain): structure and transport show even–odd parity, and long chains are studied.
Physics Letters A. 375(42):3710-3715.
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ABSTRACT: Ab initio molecular dynamics simulations are performed to study the liquid and undercooled liquid of zirconium. Local structure orders in the liquid at temperatures from 2500 K down to 1830 K are analyzed by using the recently developed cluster alignment method. Our study clearly shows the presence of body-centered-cube (BCC) short-range order in the undercooled liquid in addition to icosahedral order. A strong fluctuation and competition between the short-range icosahedra and BCC orders in the undercooled liquid at 1830 K are also demonstrated from the cluster alignment scheme. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3662189]
Journal of Applied Physics. 110(10):103518.
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ABSTRACT: A chemical bonding scheme is presented for the analysis of solid-state systems. The scheme is based on the intrinsic oriented quasiatomic minimal-basis-set orbitals (IO-QUAMBOs) previously developed by Ivanic and Ruedenberg for molecular systems. In the solid-state scheme, IO-QUAMBOs are generated by a unitary transformation of the quasiatomic orbitals located at each site of the system with the criteria of maximizing the sum of the fourth power of interatomic orbital bond order. Possible bonding and antibonding characters are indicated by the single particle matrix elements, and can be further examined by the projected density of states. We demonstrate the method by applications to graphene and (6,0) zigzag carbon nanotube. The oriented-orbital scheme automatically describes the system in terms of sp2 hybridization. The effect of curvature on the electronic structure of the zigzag carbon nanotube is also manifested in the deformation of the intrinsic oriented orbitals as well as a breaking of symmetry leading to nonzero single particle density matrix elements. In an additional study, the analysis is performed on the Al3V compound. The main covalent bonding characters are identified in a straightforward way without resorting to the symmetry analysis. Our method provides a general way for chemical bonding analysis of ab initio electronic structure calculations with any type of basis sets.
Phys. Rev. B. 81(23).
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ABSTRACT: Adsorption of the alkali-, group-III, and 3d-transition-metal adatoms (Na, K, Al, In, V, Fe, Co, and Ni) on graphene was studied systematically by first-principles calculations. The bonding character and electron transfer between the metal adatoms and graphene were analyzed using the recently developed quasi-atomic minimal basis set orbitals (QUAMBOs) approach. The calculations showed that the interaction between alkali-metal adatoms and graphene is ionic and has minimal effects on the lattice and electronic states of the graphene layer, in agreement with previous calculations. For group-III metal adatom adsorptions, mixed covalent and ionic bonding is demonstrated. In comparison, 3d-transition-metal adsorption on graphene exhibits strong covalent bonding with graphene. The majority of the contributions to the covalent bonds are from strong hybridization between the dx2-y2 and dyz orbitals of the 3d-transition-metal adatoms and pz orbitals of the carbon atoms. The strong covalent bonds cause large in-plane lattice distortions in the graphene layer. Charge redistributions upon adsorptions also induce significant electric dipole moments and affect the magnetic moments.
Phys. Rev. B. 83(23).
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ABSTRACT: Adsorption of rare-earth (RE) adatoms (Nd, Gd, Eu, and Yb) on graphene was studied by first-principles calculations based on the density-functional theory. The calculations show that the hollow site of graphene is the energetically favorable adsorption site for all the RE adatoms studied. The adsorption energies and diffusion barriers of Nd and Gd on graphene are found to be larger than those of Eu and Yb. Comparison with scanning tunneling microscopy experiments for Gd and Eu epitaxially grown on graphene confirms these calculated adsorption and barrier differences, since fractal-like islands are observed for Gd and flat-topped crystalline islands for Eu. The formation of flat Eu islands on graphene can be attributed to its low diffusion barrier and relatively larger ratio of adsorption energy to its bulk cohesive energy. The interactions between the Nd and Gd adatoms and graphene cause noticeable in-plane lattice distortions in the graphene layer. Adsorption of the RE adatoms on graphene also induces significant electric dipole and magnetic moments.
Phys. Rev. B. 82(24).
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ABSTRACT: A jellium-passivated cluster model is developed to study the energetics of short-range ordering in supercooled liquid and glass systems. Calculations for single atoms embedded in jellium yield results in good agreement with bulk values for the cohesive energy, atomic volume, as well as angular-momentum-projected electronic density of states. The energy difference between icosahedral clusters and fcc embryos in jellium is found to correlate with the glass-forming ability of liquid Al alloys. The model will be useful for studying the short-range order tendency with minor chemical additions in metallic glass formation, without the use of large unit cell calculations.
Phys. Rev. B. 76(17).
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ABSTRACT: An atomistic cluster alignment method is developed to identify and characterize the local atomic structural order in liquids and glasses. With the “order mining” idea for structurally disordered systems, the method can detect the presence of any type of local order in the system and can quantify the structural similarity between a given set of templates and the aligned clusters in a systematic and unbiased manner. Moreover, population analysis can also be carried out for various types of clusters in the system. The advantages of the method in comparison with other previously developed analysis methods are illustrated by performing the structural analysis for four prototype systems (i.e., pure Al, pure Zr, Zr35Cu65, and Zr36Ni64). The results show that the cluster alignment method can identify various types of short-range orders (SROs) in these systems correctly while some of these SROs are difficult to capture by most of the currently available analysis methods (e.g., Voronoi tessellation method). Such a full three-dimensional atomistic analysis method is generic and can be applied to describe the magnitude and nature of noncrystalline ordering in many disordered systems.
Phys. Rev. B. 82(18).
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ABSTRACT: A minimal basis set of localized quasiatomic orbitals for Mo is constructed using the fully converged eigenstates from first-principles calculations with a large basis set. The orbitals, although similar in shape to those of a free atom, are slightly deformed such that it can reproduce all the occupied-state electronic properties of the system. They are very useful for analyzing chemical bonding by calculating the Mulliken overlap population and bond order index between atoms. In addition, the transferability of tight-binding parametrizations can be evaluated, for example, the effect of the two-center approximation.
Phys. Rev. B. 76(20).
