28 research outputs found
An ab initio path integral Monte Carlo simulation method for molecules and clusters: application to Li_4 and Li_5^+
A novel method for simulating the statistical mechanics of molecular systems
in which both nuclear and electronic degrees of freedom are treated quantum
mechanically is presented. The scheme combines a path integral description of
the nuclear variables with a first-principles adiabatic description of the
electronic structure. The electronic problem is solved for the ground state
within a density functional approach, with the electronic orbitals expanded in
a localized (Gaussian) basis set. The discretized path integral is computed by
a Metropolis Monte Carlo sampling technique on the normal modes of the
isomorphic ring-polymer. An effective short-time action correct to order
is used. The validity and performance of the method are tested in two
small Lithium clusters, namely Li and Li. Structural and electronic
properties computed within this fully quantum-mechanical scheme are presented
and compared to those obtained within the classical nuclei approximation.
Quantum delocalization effects are significant but tunneling turns out to be
irrelevant at low temperatures.Comment: 11 text pages, 7 figures, to be published in J. Chem. Phy
Electronic and magnetic properties of the different phases of Ti 4O7 from density functional theory
Ab initio calculations using the local spin-density approximation plus Hubbard U (LSDA+U) method have been performed for the three reported phases of Ti4O7. Using the experimental structural parameters, we find that the electronic and magnetic properties are qualitatively different for each phase. The low-temperature structure is an antiferromagnetic semiconductor, with bipolarons arranged symmetrically in chains, separated by other nonmagnetic ion chains. The intermediate-temperature structure also contains bipolarons, but in a much more complicated order, in addition to unpaired magnetic Ti3+ ions and nonmagnetic Ti4+ ions. It has a smaller band gap than the low-temperature one. The high-temperature structure is metallic, and different distributions of Ti3+ and Ti4+ ions can be found that are almost degenerate.Fil: Weissmann, Mariana Dorotea. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; ArgentinaFil: Weht, Ruben Oscar. Universidad Nacional de San Martín. Instituto Sabato; Argentina. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin
Semimetalic antiferromagnetism in the half-Heusler compound CuMnSb
The half-Heusler compound CuMnSb, the first antiferromagnet (AFM) in the
Mn-based class of Heuslers and half-Heuslers that contains several conventional
and half metallic ferromagnets, shows a peculiar stability of its magnetic
order in high magnetic fields. Density functional based studies reveal an
unusual nature of its unstable (and therefore unseen) paramagnetic state, which
for one electron less (CuMnSn, for example) would be a zero gap semiconductor
(accidentally so) between two sets of very narrow, topologically separate bands
of Mn 3d character. The extremely flat Mn 3d bands result from the environment:
Mn has four tetrahedrally coordinated Cu atoms whose 3d states lie well below
the Fermi level, and the other four tetrahedrally coordinated sites are empty,
leaving chemically isolated Mn 3d states. The AFM phase can be pictured
heuristically as a self-doped CuMnSb compensated semimetal
with heavy mass electrons and light mass holes, with magnetic coupling
proceeding through Kondo and/or antiKondo coupling separately through the two
carrier types. The ratio of the linear specific heat coefficient and the
calculated Fermi level density of states indicates a large mass enhancement
, or larger if a correlated band structure is taken as the
reference
Hybrid Quantum and Classical Mechanical Monte Carlo Simulations of the Interaction of Hydrogen Chloride with Solid Water Clusters
Monte Carlo simulations using a hybrid quantum and classical mechanical
potential were performed for crystal and amorphous-like HCl-water(n) clusters
The subsystem composed by HCl and one water molecule was treated within Density
Functional Theory, and a classical force field was used for the rest of the
system. Simulations performed at 200 K suggest that the energetic feasibility
of HCl dissociation strongly depends on its initial placement within the
cluster. An important degree of ionization occurs only if HCl is incorporated
into the surface. We observe that local melting does not play a crucial role in
the ionization process.Comment: 14 Latex pages with 4 postscript figures, to appear in Chem. Phys.
Let
Direct observation of the influence of the FeAs4 tetrahedron on superconductivity and antiferromagnetic correlations in Sr2VO3FeAs
International audienceWe measure the pressure dependence of the electrical resistivity and the crystal structure of iron superconductor Sr2VO3FeAs. Below ∼ 10GPa the structure compresses but remains undeformed, with regular FeAs4 tetrahedrons, and a constant Tc. Beyond 10GPa, the tetrahedron strongly distorts, while Tc goes gradually to zero. Band structure calculations of the undistorted structure show multiple-nesting features that hinder the development of an antiferromagnetic (AF) ground state, allowing the appearance of superconductivity. The deformation of the tetrahedra that breaks band degeneracy degrades multiple nesting, thus favouring one particular AF state at the expense of Tc
Electronic Fine Structure in the Electron-Hole Plasma in SrB6
Electron-hole mixing-induced fine structure in alkaline earth hexaborides
leads to lower energy (temperature) scales, and thus stronger tendency toward
an excitonic instability, than in their doped counterparts (viz.
Ca(1-x)La(x)B(6), x=0.005), which are high Curie temperature, small moment
ferromagnets. Comparison of Fermi surfaces and spectral distributions with de
Haas - van Alphen (dHvA), optical, transport, and tunneling data indicates that
SrB6 remains a fermionic semimetal down to (at least) 5 K, rather than forming
an excitonic condensate. For the doped system the Curie temperature is higher
than the degeneracy temperature.Comment: Four two-column pages, three postscript figures. Phys. Rev. Lett.
(April 2000, in press
Molecular dynamics study of the fragmentation of silicon doped fullerenes
Tight binding molecular dynamics simulations, with a non orthogonal basis
set, are performed to study the fragmentation of carbon fullerenes doped with
up to six silicon atoms. Both substitutional and adsorbed cases are considered.
The fragmentation process is simulated starting from the equilibrium
configuration in each case and imposing a high initial temperature to the
atoms. Kinetic energy quickly converts into potential energy, so that the
system oscillates for some picoseconds and eventually breaks up. The most
probable first event for substituted fullerenes is the ejection of a C2
molecule, another very frequent event being that one Si atom goes to an
adsorbed position. Adsorbed Si clusters tend to desorb as a whole when they
have four or more atoms, while the smaller ones tend to dissociate and
sometimes interchange positions with the C atoms. These results are compared
with experimental information from mass abundance spectroscopy and the products
of photofragmentation.Comment: Seven two-column pages, six postscript figures. To be published in
Physical Review
Half-Metallic Ferrimagnetism in Mn_2VAl
We show that Mn_2VAl is a compound for which the generalized gradient
approximation (GGA) to the exchange-correlation functional in density
functional theory makes a qualitative change in predicted behavior compared to
the usual local density approximation (LDA). Application of GGA leads to
prediction of Mn_2VAl being a half-metallic ferrimagnet, with the minority
channel being the conducting one. The electronic and magnetic structure is
analyzed and contrasted with the isostructural enhanced semimetal Fe_2VAl.Comment: 5 pages, Latex, 6 postscript figures. Description and figures of the
(minority) Fermi surfaces have been adde