1,105 research outputs found
Interplay between Bonding and Magnetism in the Adsorption of NO on Rh Clusters
We have studied the adsorption of NO on small Rh clusters, containing one to
five atoms, using density functional theory in both spin-polarized and
non-spin-polarized forms. We find that NO bonds more strongly to Rh clusters
than it does to Rh(100) or Rh(111); however, it also quenches the magnetism of
the clusters. This (local) effect results in reducing the magnitude of the
adsorption energy, and also washes out the clear size-dependent trend observed
in the non-magnetic case. Our results illustrate the competition present
between the tendencies to bond and to magnetize, in small clusters.Comment: Submitted to J. of Chem. Phy
Structure maps for hcp metals from first principles calculations
The ability to predict the existence and crystal type of ordered structures
of materials from their components is a major challenge of current materials
research. Empirical methods use experimental data to construct structure maps
and make predictions based on clustering of simple physical parameters. Their
usefulness depends on the availability of reliable data over the entire
parameter space. Recent development of high throughput methods opens the
possibility to enhance these empirical structure maps by {\it ab initio}
calculations in regions of the parameter space where the experimental evidence
is lacking or not well characterized. In this paper we construct enhanced maps
for the binary alloys of hcp metals, where the experimental data leaves large
regions of poorly characterized systems believed to be phase-separating. In
these enhanced maps, the clusters of non-compound forming systems are much
smaller than indicated by the empirical results alone.Comment: 7 pages, 4 figures, 1 tabl
Half-Heusler Topological Insulators: A First-Principle Study with the Tran-Blaha Modified Becke-Johnson Density Functional
We systematically investigate the topological band structures of half-Heusler
compounds using first-principles calculations. The modified Becke-Johnson
exchange potential together with local density approximation for the
correlation potential (MBJLDA) has been used here to obtain accurate band
inversion strength and band order. Our results show that a large number of
half-Heusler compounds are candidates for three-dimensional topological
insulators. The difference between band structures obtained using the local
density approximation (LDA) and MBJLDA potential is also discussed.Comment: 5 figures, 1 tabl
Ab initio study of the thermodynamic properties of rare-earthmagnesium intermetallics MgRE (RE=Y, Dy, Pr, Tb)
We have performed an ab initio study of the thermodynamical properties of
rare-earth-magnesium intermetallic compounds MgRE (RE=Y, Dy, Pr, Tb) with
CsCl-type B2-type structures. The calculations have been carried out the
density functional theory and density functional perturbation theory in
combination with the quasiharmonic approximation. The phonon-dispersion curves
and phonon total and partial density of states have been investigated. Our
results show that the contribution of RE atoms is dominant in phonon frequency,
and this character agrees with the previous discussion by using atomistic
simulations. The temperature dependence of various quantities such as the
thermal expansions, bulk modulus, and the heat capacity are obtained. The
electronic contributions to the specific heat are discussed, and found to be
important for the calculated MgRE intermetallics.Comment: 12 pages, 6 figure
Topology of the Spin-polarized Charge Density in bcc and fcc Iron
We investigate the topology of the spin-polarized charge density in bcc and
fcc iron. While the total spin-density is found to possess the topology of the
non-magnetic prototypical structures, in some cases the spin-polarized
densities are characterized by unique topologies; for example, the
spin-polarized charge densities of bcc and high-spin fcc iron are atypical of
any known for non-magnetic materials. In these cases, the two spin-densities
are correlated: the spin-minority electrons have directional bond paths with
deep minima in the minority density, while the spin-majority electrons fill
these holes, reducing bond directionality. The presence of two distinct spin
topologies suggests that a well-known magnetic phase transition in iron can be
fruitfully reexamined in light of these topological changes. We show that the
two phase changes seen in fcc iron (paramagnetic to low-spin and low-spin to
high-spin) are different. The former follows the Landau symmetry-breaking
paradigm and proceeds without a topological transformation, while the latter
also involves a topological catastrophe.Comment: 5 pages, 3 figures. Phys. Rev. Lett. (in press
Superconductivity without Fe or Ni in the phosphides BaIr2P2 and BaRh2P2
Heat capacity, resistivity, and magnetic susceptibility measurements confirm
bulk superconductivity in single crystals of BaIrP (T=2.1K) and
BaRhP (T = 1.0 K). These compounds form in the ThCrSi (122)
structure so they are isostructural to both the Ni and Fe pnictides but not
isoelectronic to either of them. This illustrates the importance of structure
for the occurrence of superconductivity in the 122 pnictides. Additionally, a
comparison between these and other ternary phosphide superconductors suggests
that the lack of interlayer bonding favors superconductivity. These
stoichiometric and ambient pressure superconductors offer an ideal playground
to investigate the role of structure for the mechanism of superconductivity in
the absence of magnetism.Comment: Published in Phys Rev B: Rapid Communication
Electrons and phonons in the ternary alloy CaAlSi} as a function of composition
We report a detailed first-principles study of the structural, electronic and
vibrational properties of the superconducting C phase of the ternary
alloy CaAlSi, both in the experimental range ,
for which the alloy has been synthesised, and in the theoretical limits of high
aluminium and high silicon concentration. Our results indicate that, in the
experimental range, the dependence of the electronic bands on composition is
well described by a rigid-band model, which breaks down outside this range.
Such a breakdown, in the (theoretical) limit of high aluminium concentration,
is connected to the appearance of vibrational instabilities, and results in
important differences between CaAl and MgB. Unlike MgB, the
interlayer band and the out-of-plane phonons play a major role on the stability
and superconductivity of CaAlSi and related C intermetallic compounds
Correlation energies by the generator coordinate method: computational aspects for quadrupolar deformations
We investigate truncation schemes to reduce the computational cost of
calculating correlations by the generator coordinate method based on mean-field
wave functions. As our test nuclei, we take examples for which accurate
calculations are available. These include a strongly deformed nucleus, 156Sm, a
nucleus with strong pairing, 120Sn, the krypton isotope chain which contains
examples of soft deformations, and the lead isotope chain which includes the
doubly magic 208Pb. We find that the Gaussian overlap approximation for angular
momentum projection is effective and reduces the computational cost by an order
of magnitude. Cost savings in the deformation degrees of freedom are harder to
realize. A straightforward Gaussian overlap approximation can be applied rather
reliably to angular-momentum projected states based on configuration sets
having the same sign deformation (prolate or oblate), but matrix elements
between prolate and oblate deformations must be treated with more care. We
propose a two-dimensional GOA using a triangulation procedure to treat the
general case with both kinds of deformation. With the computational gains from
these approximations, it should be feasible to carry out a systematic
calculation of correlation energies for the nuclear mass table.Comment: 11 pages revtex, 9 eps figure
First-principles prediction of a decagonal quasicrystal containing boron
We interpret experimentally known B-Mg-Ru crystals as quasicrystal
approximants. These approximant structures imply a deterministic decoration of
tiles by atoms that can be extended quasiperiodically. Experimentally observed
structural disorder corresponds to phason (tile flip) fluctuations.
First-principles total energy calculations reveal that many distinct tilings
lie close to stability at low temperatures. Transfer matrix calculations based
on these energies suggest a phase transition from a crystalline state at low
temperatures to a high temperature state characterized by tile fluctuations. We
predict BMgRu forms a decagonal quasicrystal that is
metastable at low temperatures and may be thermodynamically stable at high
temperatures.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
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