1,375 research outputs found
Accurate nonrelativistic ground-state energies of 3d transition metal atoms
We present accurate nonrelativistic ground-state energies of the transition
metal atoms of the 3d series calculated with Fixed-Node Diffusion Monte Carlo
(FN-DMC). Selected multi-determinantal expansions obtained with the CIPSI
method (Configuration Interaction using a Perturbative Selection made
Iteratively) and including the most prominent determinants of the full CI
expansion are used as trial wavefunctions. Using a maximum of a few tens of
thousands determinants, fixed-node errors on total DMC energies are found to be
greatly reduced for some atoms with respect to those obtained with Hartree-Fock
nodes. The FN-DMC/(CIPSI nodes) ground-state energies presented here are, to
the best of our knowledge, the most accurate values reported so far. Thanks to
the variational property of FN-DMC total energies, the results also provide
lower bounds for the absolute value of all-electron correlation energies,
.Comment: 5 pages, 3 table
Interface optical phonons in spheroidal dots: Raman selection rules
The contribution of interface phonons to the first order Raman scattering in
nanocrystals with non spherical geometry is analyzed. Interface optical phonons
in the spheroidal geometry are discussed and the corresponding Frohlich-like
electron-phonon interaction is reported in the framework of the dielectric
continuum approach. It is shown that the interface phonon modes are strongly
dependent on the nanocrystal geometry, particularly on the ellipsoid's
semi-axis ratio. The new Raman selection rules have revealed that solely
interface phonon modes with even angular momentum are allowed to contribute to
the first order phonon-assisted scattering of light. On this basis we are able
to give an explanation for the observed low frequency shoulders present in the
Raman cross-section of several II-VI semiconductor nanostructures.Comment: 8 pages, 2 figure
Several new catalysts for reduction of oxygen in fuel cells
Test results prove nickel carbide or nitride, nickel-cobalt carbide, titanium carbide or nitride, and intermetallic compounds of the transition or noble metals to be efficient electrocatalysts for oxygen reduction in alkaline electrolytes in low temperature fuel cells
Resonant hyper-Raman scattering in spherical quantum dots
A theoretical model of resonant hyper-Raman scattering by an ensemble of
spherical semiconductor quantum dots has been developed. The electronic
intermediate states are described as Wannier-Mott excitons in the framework of
the envelope function approximation. The optical polar vibrational modes of the
nanocrystallites (vibrons) and their interaction with the electronic system are
analized with the help of a continuum model satisfying both the mechanical and
electrostatic matching conditions at the interface. An explicit expression for
the hyper-Raman scattering efficiency is derived, which is valid for incident
two-photon energy close to the exciton resonances. The dipole selection rules
for optical transitions and Fr\"ohlich-like exciton-lattice interaction are
derived: It is shown that only exciton states with total angular momentum
and vibrational modes with angular momentum contribute to the
hyper-Raman scattering process. The associated exciton energies, wavefunctions,
and vibron frequencies have been obtained for spherical CdSe zincblende-type
nanocrystals, and the corresponding hyper-Raman scattering spectrum and
resonance profile are calculated. Their dependence on the dot radius and the
influence of the size distribution on them are also discussed.Comment: 12 pages REVTeX (two columns), 2 tables, 8 figure
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