195 research outputs found
CaSiO3 perovskite at lower mantle pressures
We investigate by first-principles the structural behavior of CaSiO3 perovskite up to lower mantle pressures. We confirm that the cubic perovskite modification is unstable at all pressures. The zero Kelvin structure is stabilized by SiO6 octahedral rotations that lower the symmetry to tetragonal, orthorhombic, rhombohedral, or to a cubic supercell. The resulting structures have comparable energies and equation of state parameters. This suggests that relatively small deviatoric/ shear stresses might induce phase transformations between these various structures softening some elastic moduli, primarily the shear modulus. The seismic signature accompanying a local increase in CaSiO3 content should be a positive density anomaly and a negative V-S anomaly
Ferromagnetic insulating state in tensile-strained LaCoO thin films
With local density approximation + Hubbard (LDA+) calculations, we
show that the ferromagnetic (FM) insulating state observed in tensile-strained
LaCoO epitaxial thin films is most likely a mixture of low-spin (LS) and
high-spin (HS) Co, namely, a HS/LS mixture state. Compared with other FM
states, including the intermediate-spin (IS) state (\textit{metallic} within
LDA+), which consists of IS Co only, and the insulating IS/LS mixture state,
the HS/LS state is the most favorable one. The FM order in HS/LS state is
stabilized via the superexchange interactions between adjacent LS and HS Co. We
also show that Co spin state can be identified by measuring the electric field
gradient (EFG) at Co nucleus via nuclear magnetic resonance (NMR) spectroscopy
Velocity and density characteristics of subducted oceanic crust and the origin of lower-mantle heterogeneities
Seismic heterogeneities detected in the lower mantle were proposed to be related to subducted oceanic crust. However, the velocity and density of subducted oceanic crust at lower-mantle conditions remain unknown. Here, we report ab initio results for the elastic properties of calcium ferrite‐type phases and determine the velocities and density of oceanic crust along different mantle geotherms. We find that the subducted oceanic crust shows a large negative shear velocity anomaly at the phase boundary between stishovite and CaCl2-type silica, which is highly consistent with the feature of mid-mantle scatterers. After this phase transition in silica, subducted oceanic crust will be visible as high-velocity heterogeneities as imaged by seismic tomography. This study suggests that the presence of subducted oceanic crust could provide good explanations for some lower-mantle seismic heterogeneities with different length scales except large low shear velocity provinces (LLSVPs)
Metric tensor as the dynamical variable for variable cell-shape molecular dynamics
We propose a new variable cell-shape molecular dynamics algorithm where the
dynamical variables associated with the cell are the six independent dot
products between the vectors defining the cell instead of the nine cartesian
components of those vectors. Our choice of the metric tensor as the dynamical
variable automatically eliminates the cell orientation from the dynamics.
Furthermore, choosing for the cell kinetic energy a simple scalar that is
quadratic in the time derivatives of the metric tensor, makes the dynamics
invariant with respect to the choice of the simulation cell edges. Choosing the
densitary character of that scalar allows us to have a dynamics that obeys the
virial theorem. We derive the equations of motion for the two conditions of
constant external pressure and constant thermodynamic tension. We also show
that using the metric as variable is convenient for structural optimization
under those two conditions. We use simulations for Ar with Lennard-Jones
parameters and for Si with forces and stresses calculated from first-principles
of density functional theory to illustrate the applications of the method.Comment: 10 pages + 6 figures, Latex, to be published in Physical Review
Adaptive Genetic Algorithm for Crystal Structure Prediction
We present a genetic algorithm (GA) for structural search that combines the
speed of structure exploration by classical potentials with the accuracy of
density functional theory (DFT) calculations in an adaptive and iterative way.
This strategy increases the efficiency of the DFT-based GA by several orders of
magnitude. This gain allows considerable increase in size and complexity of
systems that can be studied by first principles. The method's performance is
illustrated by successful structure identifications of complex binary and
ternary inter-metallic compounds with 36 and 54 atoms per cell, respectively.
The discovery of a multi-TPa Mg-silicate phase with unit cell containing up to
56 atoms is also reported. Such phase is likely to be an essential component of
terrestrial exoplanetary mantles.Comment: 14 pages, 4 figure
Evidence for a Peierls phase-transition in a three-dimensional multiple charge-density waves solid
The effect of dimensionality on materials properties has become strikingly
evident with the recent discovery of graphene. Charge ordering phenomena can be
induced in one dimension by periodic distortions of a material's crystal
structure, termed Peierls ordering transition. Charge-density waves can also be
induced in solids by strong Coulomb repulsion between carriers, and at the
extreme limit, Wigner predicted that crystallization itself can be induced in
an electrons gas in free space close to the absolute zero of temperature.
Similar phenomena are observed also in higher dimensions, but the microscopic
description of the corresponding phase transition is often controversial, and
remains an open field of research for fundamental physics. Here, we photoinduce
the melting of the charge ordering in a complex three-dimensional solid and
monitor the consequent charge redistribution by probing the optical response
over a broad spectral range with ultrashort laser pulses. Although the
photoinduced electronic temperature far exceeds the critical value, the
charge-density wave is preserved until the lattice is sufficiently distorted to
induce the phase transition. Combining this result with it ab initio}
electronic structure calculations, we identified the Peierls origin of multiple
charge-density waves in a three-dimensional system for the first time.Comment: Accepted for publication in Proc. Natl. Acad. Sci. US
Ensemble density-functional theory for ab-initio molecular dynamics of metals and finite-temperature insulators
A new method is presented for performing first-principles molecular-dynamics
simulations of systems with variable occupancies. We adopt a matrix
representation for the one-particle statistical operator Gamma, to introduce a
``projected'' free energy functional G that depends on the Kohn-Sham orbitals
only and that is invariant under their unitary transformations. The Liouville
equation [ Gamma , H ] = 0 is always satisfied, guaranteeing a very efficient
and stable variational minimization algorithm that can be extended to
non-conventional entropic formulations or fictitious thermal distributions.Comment: 5 pages, two-column style with 2 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#nm_meta
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