283 research outputs found
Theoretical calculations of magnetic order and anisotropy energies in molecular magnets
We present theoretical electronic structure calculations on the nature of electronic states and the magnetic coupling in the Mn12O12 free cluster and the Mn12O12(RCOO)16(H2O)4 molecular magnetic crystal. The calculations have been performed with the all-electron full-potential NRLMOL code. We find that the free Mn12O12cluster relaxes to an antiferromagneticcluster with no net moment. However, when coordinated by sixteen HCOO ligands and four H2O groups, as it is in the molecular crystal, we find that the ferrimagnetic ordering and geometrical and magnetic structure observed in the experiments is restored. Local Mn moments for the free and ligandated molecular magnets are presented and compared to experiment. We identify the occupied and unoccupied electronic states that are most responsible for the formation of the large anisotropy barrier and use a recently developed full-space and full-potential method for calculating the spin–orbit coupling interaction and anisotropy energies. Our calculated second-order anisotropy energy is in excellent agreement with experiment
Coupling to haloform molecules in intercalated C60?
For field-effect-doped fullerenes it was reported that the superconducting
transition temperature Tc is markedly larger for C60.2CHX_3 (X=Cl, Br)
crystals, than for pure C60. Initially this was explained by the expansion of
the volume per C60-molecule and the corresponding increase in the density of
states at the Fermi level in the intercalated crystals. On closer examination
it has, however, turned out to be unlikely that this is the mechanism behind
the increase in Tc. An alternative explanation of the enhanced transition
temperatures assumes that the conduction electrons not only couple to the
vibrational modes of the C60-molecule, but also to the modes of the
intercalated molecules. We investigate the possibility of such a coupling. We
find that, assuming the ideal bulk structure of the intercalated crystal, both
a coupling due to hybridization of the molecular levels, and a coupling via
dipole moments should be very small. This suggests that the presence of the
gate-oxide in the field-effect-devices strongly affects the structure of the
fullerene crystal at the interface.Comment: 4 pages, 1 figure, to be published in PRB (rapid communication
A new multi-center approach to the exchange-correlation interactions in ab initio tight-binding methods
A new approximate method to calculate exchange-correlation contributions in
the framework of first-principles tight-binding molecular dynamics methods has
been developed. In the proposed scheme on-site (off-site) exchange-correlation
matrix elements are expressed as a one-center (two-center) term plus a {\it
correction} due to the rest of the atoms. The one-center (two-center) term is
evaluated directly, while the {\it correction} is calculated using a variation
of the Sankey-Niklewski \cite{Sankey89} approach generalized for arbitrary
atomic-like basis sets. The proposed scheme for exchange-correlation part
permits the accurate and computationally efficient calculation of corresponding
tight-binding matrices and atomic forces for complex systems. We calculate bulk
properties of selected transition (W,Pd), noble (Au) or simple (Al) metals, a
semiconductor (Si) and the transition metal oxide Ti with the new method
to demonstrate its flexibility and good accuracy.Comment: 17 pages, 5 figure
Quasiparticle energies for large molecules: a tight-binding GW approach
We present a tight-binding based GW approach for the calculation of
quasiparticle energy levels in confined systems such as molecules. Key
quantities in the GW formalism like the microscopic dielectric function or the
screened Coulomb interaction are expressed in a minimal basis of spherically
averaged atomic orbitals. All necessary integrals are either precalculated or
approximated without resorting to empirical data. The method is validated
against first principles results for benzene and anthracene, where good
agreement is found for levels close to the frontier orbitals. Further, the size
dependence of the quasiparticle gap is studied for conformers of the polyacenes
() up to n = 30.Comment: 10 pages, 5 eps figures submitted to Phys. Rev.
Predicted Infrared and Raman Spectra for Neutral Ti_8C_12 Isomers
Using a density-functional based algorithm, the full IR and Raman spectra are
calculated for the neutral Ti_8C_12 cluster assuming geometries of Th, Td, D2d
and C3v symmetry. The Th pentagonal dodecahedron is found to be dynamically
unstable. The calculated properties of the relaxed structure having C3v
symmetry are found to be in excellent agreement with experimental gas phase
infrared results, ionization potential and electron affinity measurements.
Consequently, the results presented may be used as a reference for further
experimental characterization using vibrational spectroscopy.Comment: 6 pages, 5 figures. Physical Review A, 2002 (in press
Gauge field for edge state in graphene
By considering the continuous model for graphene, we analytically study a
special gauge field for the edge state. The gauge field explains the properties
of the edge state such as the existence only on the zigzag edge, the partial
appearance in the -space, and the energy position around the Fermi energy.
It is demonstrated utilizing the gauge field that the edge state is robust for
surface reconstruction, and the next nearest-neighbor interaction which breaks
the particle-hole symmetry stabilizes the edge state.Comment: 9 pages, 5 figure
Vibrational signatures for low-energy intermediate-sized Si clusters
We report low-energy locally stable structures for the clusters Si20 and Si21. The structures were obtained by performing geometry optimizations within the local density approximation. Our calculated binding energies for these clusters are larger than any previously reported for this size regime. To aid in the experimental identification of the structures, we have computed the full vibrational spectra of the clusters, along with the Raman and IR activities of the various modes using a recently developed first-principles technique. These represent, to our knowledge, the first calculations of Raman and IR spectra for Si clusters of this size
Systematic generation of finite-range atomic basis sets for linear-scaling calculations
Basis sets of atomic orbitals are very efficient for density functional
calculations but lack a systematic variational convergence.
We present a variational method to optimize numerical atomic orbitals using a
single parameter to control their range.
The efficiency of the basis generation scheme is tested and compared with
other schemes for multiple zeta basis sets.
The scheme shows to be comparable in quality to other widely used schemes
albeit offering better performance for linear-scaling computations
Density-functional-based predictions of Raman and IR spectra for small Si clusters
We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared (IR) intensities for all of the vibrational modes of several clusters (SiN with N=3-8, 10, 13, 20, and 21) using the local density approximation (LDA). For the smaller clusters (N=3-8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si10 and for the lowest-energy structure of Si13 found to date. For the static response, we compare our calculated polarizabilities for N=10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems
Theory of superconductivity of carbon nanotubes and graphene
We present a new mechanism of carbon nanotube superconductivity that
originates from edge states which are specific to graphene. Using on-site and
boundary deformation potentials which do not cause bulk superconductivity, we
obtain an appreciable transition temperature for the edge state. As a
consequence, a metallic zigzag carbon nanotube having open boundaries can be
regarded as a natural superconductor/normal metal/superconductor junction
system, in which superconducting states are developed locally at both ends of
the nanotube and a normal metal exists in the middle. In this case, a signal of
the edge state superconductivity appears as the Josephson current which is
sensitive to the length of a nanotube and the position of the Fermi energy.
Such a dependence distinguishs edge state superconductivity from bulk
superconductivity.Comment: 5 pages, 2 figure
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