199 research outputs found
A More Accurate Generalized Gradient Approximation for Solids
We present a new nonempirical density functional generalized gradient
approximation (GGA) that gives significant improvements for lattice constants,
crystal structures, and metal surface energies over the most popular
Perdew-Burke-Ernzerhof (PBE) GGA. The new functional is based on a diffuse
radial cutoff for the exchange-hole in real space, and the analytic gradient
expansion of the exchange energy for small gradients. There are no adjustable
parameters, the constraining conditions of PBE are maintained, and the
functional is easily implemented in existing codes.Comment: 5 pages, corrected the errors of the sublimation energy of Ih ic
Origin of the large phonon band-gap in SrTiO3 and the vibrational signatures of ferroelectricity in ATiO3 perovskite: First principles lattice dynamics and inelastic neutron scattering of PbTiO3, BaTiO3 and SrTiO3
We report first principles density functional perturbation theory
calculations and inelastic neutron scattering measurements of the phonon
density of states, dispersion relations and electromechanical response of
PbTiO3, BaTiO3 and SrTiO3. The phonon density-of-states of the quantum
paraelectric SrTiO3 is found to be fundamentally distinct from that of
ferroelectric PbTiO3 and BaTiO3 with a large 70-90 meV phonon band-gap. The
phonon dispersion and electromechanical response of PbTiO3 reveal giant
anisotropies. The interplay of covalent bonding and ferroelectricity, strongly
modulates the electromechanical response and give rise to spectacular
signatures in the phonon spectra. The computed charge densities have been used
to study the bonding in these perovskites. Distinct bonding characteristics in
the ferroelectric and paraelectric phases give rise to spectacular vibrational
signatures. While a large phonon band-gap in ATiO3 perovskites seems a
characteristic of quantum paraelectrics, anisotropy of the phonon spectra
correlates well with ferroelectric strength. These correlations between the
phonon spectra and ferroelectricity, can guide future efforts at custom
designing still more effective piezoelectrics for applications. These results
suggest that vibrational spectroscopy can help design novel materials.Comment: 11 pages, 4 color figures and 2 Table
Semiempirical Hartree-Fock calculations for KNbO3
In applying the semiempirical intermediate neglect of differential overlap
(INDO) method based on the Hartree-Fock formalism to a cubic perovskite-based
ferroelectric material KNbO3, it was demonstrated that the accuracy of the
method is sufficient for adequately describing the small energy differences
related to the ferroelectric instability. The choice of INDO parameters has
been done for a system containing Nb. Based on the parametrization proposed,
the electronic structure, equilibrium ground state structure of the
orthorhombic and rhombohedral phases, and Gamma-TO phonon frequencies in cubic
and rhombohedral phases of KNbO3 were calculated and found to be in good
agreement with the experimental data and with the first-principles calculations
available.Comment: 7 pages, 2 Postscript figures, uses psfig.tex. To be published in
Phys.Rev.B 54, No.4 (1996
Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations
The frequencies of transverse-optical phonons in KNbO and
KTaO are calculated in the frozen-phonon scheme making use of the
full-potential linearized muffin-tin orbital method. The calculated frequencies
in the cubic phase of KNbO and in the tetragonal ferroelectric phase are in
good agreement with experimental data. For KTaO, the effect of lattice
volume was found to be substantial on the frequency of the soft mode, but
rather small on the relative displacement patterns of atoms in all three modes
of the symmetry. The TO frequencies in KTaO are found to be of the
order of, but somehow higher than, the corresponding frequencies in cubic
KNbO.Comment: 8 pages + 1 LaTeX figure, Revtex 3.0, SISSA-CM-94-00
Born Effective Charges of Barium Titanate: band by band decomposition and sensitivity to structural features
The Born effective charge tensors of Barium Titanate have been calculated for
each of its 4 phases. Large effective charges of Ti and O, also predicted by
shell model calculations and made plausible by a simplified model, reflect the
partial covalent character of the chemical bond. A band by band decomposition
confirms that orbital hybridization is not restricted to Ti and O atoms but
also involves Ba which appears more covalent than generally assumed. Our
calculations reveal a strong dependence of the effective charges on the atomic
positions contrasting with a relative insensitivity on isotropic volume
changes.Comment: 13 page
Polarization Dependence of Born Effective Charge and Dielectric Constant in KNbO
The Born effective charge Z^{*} and dielectric tensor \epsilon_{\infty} of
KNbO_3 are found to be very sensitive to the atomic geometry, changing by as
much as 27% between the paraelectric cubic and ferroelectric tetragonal and
rhombohedral phases. Subtracting the bare ionic contribution reveals changes of
the dynamic component of Z^{*} as large as 50%, for atomic displacements that
are typically only a few percent of the lattice constant. Z^{*},
\epsilon_{\infty} and all phonon frequencies at the Brillouin zone center were
calculated using the {\it ab initio} linearized augmented plane-wave linear
response method with respect to the reference cubic, experimental tetragonal,
and theoretically determined rhombohedral ground state structures. The ground
state rhombohedral structure of KNbO_3 was determined by minimizing the forces
on the relaxed atoms. By contrast with the cubic structure, all zone center
phonon modes of the rhombohedral structure are stable and their frequencies are
in good agreement with experiment. In the tetragonal phase, one of the soft
zone center modes in the cubic phase is stablized. In view of the small atomic
displacements involved in the ferroelectric transitions, it is evident that not
only the soft mode frequencies but also the Born effective charge and
dielectric constants are very sensitive to the atomic geometry.Comment: 26 pages, revtex, no figures; to appear in Phys. Rev. B15 (Oct.),
199
Dynamic force microscopy for imaging of viruses under physiological conditions
Dynamic force microscopy (DFM) allows imaging of the structure and the assessment of the function of biological specimens in their physiological environment. In DFM, the cantilever is oscillated at a given frequency and touches the sample only at the end of its downward movement. Accordingly, the problem of lateral forces displacing or even destroying bio-molecules is virtually inexistent as the contact time and friction forces are reduced. Here, we describe the use of DFM in studies of human rhinovirus serotype 2 (HRV2) weakly adhering to mica surfaces. The capsid of HRV2 was reproducibly imaged without any displacement of the virus. Release of the genomic RNA from the virions was initiated by exposure to low pH buffer and snapshots of the extrusion process were obtained. In the following, the technical details of previous DFM investigations of HRV2 are summarized
Correlated local distortions of the TlO layers in TlBaCuO: An x-ray absorption study
We have used the XAFS (x-ray-absorption fine structure) technique to
investigate the local structure about the Cu, Ba, and Tl atoms in orthorhombic
Tl-2201 with a superconducting transition temperature T=60 K. Our results
clearly show that the O(1), O(2), Cu, and Ba atoms are at their ideal sites as
given by the diffraction measurements, while the Tl and O(3) atoms are more
disordered than suggested by the average crystal structure. The Tl-Tl distance
at 3.5 \AA{ } between the TlO layers does not change, but the Tl-Tl distance at
3.9 \AA{ } within the TlO layer is not observed and the Tl-Ba and Ba-Tl peaks
are very broad. The shorter Tl-O(3) distance in the TlO layer is about 2.33
\AA, significantly shorter than the distance calculated with both the Tl and
O(3) atoms at their ideal sites ( 0 or ). A model based
on these results shows that the Tl atom is displaced along the
directions from its ideal site by about 0.11 \AA; the displacements of
neighboring Tl atoms are correlated. The O(3) atom is shifted from the $4e$
site by about 0.53 \AA{ } roughly along the directions. A comparison of
the Tl L-edge XAFS spectra from three samples, with T=60 K, 76 K,
and 89 K, shows that the O environment around the Tl atom is sensitive to T
while the Tl local displacement is insensitive to T and the structural
symmetry. These conclusions are compared with other experimental results and
the implications for charge transfer and superconductivity are discussed. This
paper has been submitted to Phys. Rev. B.Comment: 20 pages plus 14 ps figures, REVTEX 3.
The physics of dynamical atomic charges: the case of ABO3 compounds
Based on recent first-principles computations in perovskite compounds,
especially BaTiO3, we examine the significance of the Born effective charge
concept and contrast it with other atomic charge definitions, either static
(Mulliken, Bader...) or dynamical (Callen, Szigeti...). It is shown that static
and dynamical charges are not driven by the same underlying parameters. A
unified treatment of dynamical charges in periodic solids and large clusters is
proposed. The origin of the difference between static and dynamical charges is
discussed in terms of local polarizability and delocalized transfers of charge:
local models succeed in reproducing anomalous effective charges thanks to large
atomic polarizabilities but, in ABO3 compounds, ab initio calculations favor
the physical picture based upon transfer of charges. Various results concerning
barium and strontium titanates are presented. The origin of anomalous Born
effective charges is discussed thanks to a band-by-band decomposition which
allows to identify the displacement of the Wannier center of separated bands
induced by an atomic displacement. The sensitivity of the Born effective
charges to microscopic and macroscopic strains is examined. Finally, we
estimate the spontaneous polarization in the four phases of barium titanate.Comment: 25 pages, 6 Figures, 10 Tables, LaTe
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