68 research outputs found
Ab-initio study of structure and dynamics properties of crystalline ice
We investigated the structural and dynamical properties of a tetrahedrally
coordinated crystalline ice from first principles based on density functional
theory within the generalized gradient approximation with the projected
augmented wave method. First, we report the structural behaviour of ice at
finite temperatures based on the analysis of radial distribution functions
obtained by molecular dynamics simulations. The results show how the ordering
of the hydrogen bonding breaks down in the tetrahedral network of ice with
entropy increase in agreement with the neutron diffraction data. We also
calculated the phonon spectra of ice in a 3x1x1 supercell by using the direct
method. So far, due to the direct method used in this calculation, the phonon
spectra is obtained without taking into account the effect of polarization
arising from dipole-dipole interactions of water molecules which is expected to
yield the splitting of longitudinal and transverse optic modes at the
Gamma-point. The calculated longitudinal acoustic velocities from the initial
slopes of the acoustic mode is in a reasonable agreement with the neutron
scatering data. The analysis of the vibrational density of states shows the
existence of a boson peak at low energy of translational region a
characteristic common to amorphous systems.Comment: International symposium on structure and dynamics of heterogeneous
system SDHS'0
Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces
Using a combination of first-principles theory and experiments, we provide a
quantitative explanation for chemical contributions to surface-enhanced Raman
spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on
planar Au(111) surfaces. With density functional theory calculations of the
static Raman tensor, we demonstrate and quantify a strong mode-dependent
modification of benzene thiol Raman spectra by Au substrates. Raman active
modes with the largest enhancements result from stronger contributions from Au
to their electron-vibron coupling, as quantified through a deformation
potential, a well-defined property of each vibrational mode. A straightforward
and general analysis is introduced that allows extraction of chemical
enhancement from experiments for specific vibrational modes; measured values
are in excellent agreement with our calculations.Comment: 5 pages, 4 figures and Supplementary material included as ancillary
fil
First-principles study of lattice instabilities in the ferromagnetic martensite NiMnGa
The phonon dispersion relations and elastic constants for ferromagnetic
NiMnGa in the cubic and tetragonally distorted Heusler structures are
computed using density-functional and density-functional perturbation theory
within the spin-polarized generalized-gradient approximation. For
, the TA tranverse acoustic branch along and
symmetry-related directions displays a dynamical instability at a wavevector
that depends on . Through examination of the Fermi-surface nesting and
electron-phonon coupling, this is identified as a Kohn anomaly. In the parent
cubic phase the computed tetragonal shear elastic constant,
C=(CC)/2, is close to zero, indicating a marginal
elastic instability towards a uniform tetragonal distortion. We conclude that
the cubic Heusler structure is unstable against a family of energy-lowering
distortions produced by the coupling between a uniform tetragonal distortion
and the corresponding modulation. The computed relation between the
ratio and the modulation wavevector is in excellent agreement with
structural data on the premartensitic ( = 1) and martensitic ( =
0.94) phases of NiMnGa.Comment: submitted to Phys. Rev.
Lattice dynamics and structural stability of ordered Fe3Ni, Fe3Pd and Fe3Pt alloys
We investigate the binding surface along the Bain path and phonon dispersion
relations for the cubic phase of the ferromagnetic binary alloys Fe3X (X = Ni,
Pd, Pt) for L12 and DO22 ordered phases from first principles by means of
density functional theory. The phonon dispersion relations exhibit a softening
of the transverse acoustic mode at the M-point in the L12-phase in accordance
with experiments for ordered Fe3Pt. This instability can be associated with a
rotational movement of the Fe-atoms around the Ni-group element in the
neighboring layers and is accompanied by an extensive reconstruction of the
Fermi surface. In addition, we find an incomplete softening in [111] direction
which is strongest for Fe3 Ni. We conclude that besides the valence electron
density also the specific Fe-content and the masses of the alloying partners
should be considered as parameters for the design of Fe-based functional
magnetic materials.Comment: Revised version, accepted for publication in Physical Review
Effects of Magnetovolume and Spin-orbit Coupling in the Ferromagnetic Cubic Perovskite BaRuO3
BaRuO3 having five different crystal structures has been synthesized by
varying the pressure while sintering. Contrary to the other phases being
nonmagnetic, the cubic perovskite phase synthesized recently shows an itinerant
ferromagnetic character. We investigated this ferromagnetic BaRuO3 using first
principles calculations. A few van Hove singularities appear around the Fermi
energy, causing unusually high magnetovolume effects of ~
4.3 /\AA as well as a Stoner instability [IN(0) ~ 1.2]. At the optimized
lattice parameter a, the magnetic moment M is 1.01 in the local spin
density approximation. When spin-orbit coupling is included, the topologies of
some Fermi surfaces are altered, and the net moment is reduced by 10% to a
value very close to the experimentally observed value of ~ 0.8 . Our
results indicate that this ferromagnetism is induced by the Stoner instability,
but the combined effects of the p-d hybridization, the magnetovolume, and the
spin-orbit coupling determine the net moment. In addition, we briefly discuss
the results of the tight-binding Wannier function technique.Comment: 5 pages and 5 embedded figures; proceedings of ICM 201
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