2,686 research outputs found
Effective-Hamiltonian modeling of external pressures in ferroelectric perovskites
The phase-transition sequence of a ferroelectric perovskite such as BaTiO_3
can be simulated by computing the statistical mechanics of a first-principles
derived effective Hamiltonian [Zhong, Vanderbilt and Rabe, Phys. Rev. Lett. 73,
1861 (1994)]. Within this method, the effect of an external pressure (in
general, of any external field) can be studied by considering the appropriate
"enthalpy" instead of the effective Hamiltonian itself. The legitimacy of this
approach relies on two critical assumptions that, to the best of our knowledge,
have not been adequately discussed in the literature to date: (i) that the
zero-pressure relevant degrees of freedom are still the only relevant degrees
of freedom at finite pressures, and (ii) that the truncation of the Taylor
expansion of the energy considered in the effective Hamiltonian remains a good
approximation at finite pressures. Here we address these issues in detail and
present illustrative first-principles results for BaTiO_3. We also discuss how
to construct effective Hamiltonians in cases in which these assumptions do not
hold.Comment: 5 pages, with 2 postscript figures embedded. Proceedings of
"Fundamental Physics of Ferroelectrics, 2002", R. Cohen and T. Egami, eds.
(AIP, Melville, New York, 2002). Also available at
http://www.physics.rutgers.edu/~dhv/preprints/ji_effp/index.htm
Predicting polarization enhancement in multicomponent ferroelectric superlattices
Ab initio calculations are utilized as an input to develop a simple model of
polarization in epitaxial short-period CaTiO3/SrTiO3/BaTiO3 superlattices grown
on a SrTiO3 substrate. The model is then combined with a genetic algorithm
technique to optimize the arrangement of individual CaTiO3, SrTiO3 and BaTiO3
layers in a superlattice, predicting structures with the highest possible
polarization and a low in-plane lattice constant mismatch with the substrate.
This modelling procedure can be applied to a wide range of layered
perovskite-oxide nanostructures providing guidance for experimental development
of nanoelectromechanical devices with substantially improved polar properties.Comment: 4 pages, submitted to PR
Compositional Inversion Symmetry Breaking in Ferroelectric Perovskites
Ternary cubic perovskite compounds of the form A_(1/3)A'_(1/3)A''_(1/3)BO_3
and AB_(1/3)B'_(1/3)B''_(1/3)O_3, in which the differentiated cations form an
alternating series of monolayers, are studied using first-principles methods.
Such compounds are representative of a possible new class of materials in which
ferroelectricity is perturbed by compositional breaking of inversion symmetry.
For isovalent substitution on either sublattice, the ferroelectric double-well
potential is found to persist, but becomes sufficiently asymmetric that
minority domains may no longer survive. The strength of the symmetry breaking
is enormously stronger for heterovalent substitution, so that the double-well
behavior is completely destroyed. Possible means of tuning between these
behaviors may allow for the optimization of resulting materials properties.Comment: 4 pages, two-column style with 3 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#sai_is
Influence of interface structure on electronic properties and Schottky barriers in Fe/GaAs magnetic junctions
The electronic and magnetic properties of Fe/GaAs(001) magnetic junctions are
investigated using first-principles density-functional calculations. Abrupt and
intermixed interfaces are considered, and the dependence of charge transfer,
magnetization profiles, Schottky barrier heights, and spin polarization of
densities of states on interface structure is studied. With As-termination, an
abrupt interface with Fe is favored, while Ga-terminated GaAs favors the
formation of an intermixed layer with Fe. The Schottky barrier heights are
particularly sensitive to the abruptness of the interface. A significant
density of states in the semiconducting gap arises from metal interface states.
These spin-dependent interface states lead to a significant minority spin
polarization of the density of states at the Fermi level that persists well
into the semiconductor, providing a channel for the tunneling of minority spins
through the Schottky barrier. These interface-induced gap states and their
dependence on atomic structure at the interface are discussed in connection
with potential spin-injection applications.Comment: 9 pages, 9 figures, to appear in PR
Comment on "Weyl fermions and the anomalous Hall effect in metallic ferromagnets"
We point out that, contrary to an assertion by Chen, Bergman and Burkov
[Phys. Rev. B 88, 125110 (2013)], the non-quantized part of the intrinsic
anomalous Hall conductivity can indeed be expressed as a Fermi-surface property
even when Weyl points are present in the bandstructure.Comment: Submitted to Physical Review
First-principles study of epitaxial strain in perovskites
Using an extension of a first-principles method developed by King-Smith and
Vanderbilt [Phys. Rev. B {\bf 49}, 5828 (1994)], we investigate the effects of
in-plane epitaxial strain on the ground-state structure and polarization of
eight perovskite oxides: BaTiO, SrTiO, CaTiO, KNbO, NaNbO,
PbTiO, PbZrO, and BaZrO. In addition, we investigate the effects of
a nonzero normal stress. The results are shown to be useful in predicting the
structure and polarization of perovskite oxide thin films and superlattices.Comment: 10 page
Spin-dependent electronic structure of transition-metal atomic chains adsorbed on single-wall carbon nanotubes
We present a systematic study of the electronic and magnetic properties of
transition-metal (TM) atomic chains adsorbed on the zigzag single-wall carbon
nanotubes (SWNTs). We considered the adsorption on the external and internal
wall of SWNT and examined the effect of the TM coverage and geometry on the
binding energy and the spin polarization at the Fermi level. All those adsorbed
chains studied have ferromagnetic ground state, but only their specific types
and geometries demonstrated high spin polarization near the Fermi level. Their
magnetic moment and binding energy in the ground state display interesting
variation with the number of electrons of the TM atom. We also show that
specific chains of transition metal atoms adsorbed on a SWNT can lead to
semiconducting properties for the minority spin-bands, but semimetallic for the
majority spin-bands. Spin-polarization is maintained even when the underlying
SWNT is subjected to high radial strain. Spin-dependent electronic structure
becomes discretized when TM atoms are adsorbed on finite segments of SWNTs.
Once coupled with non-magnetic metal electrodes, these magnetic needles or
nanomagnets can perform as spin-dependent resonant tunnelling devices. The
electronic and magnetic properties of these nanomagnets can be engineered
depending on the type and decoration of adsorbed TM atom as well as the size
and symmetry of the tube. Our study is performed by using first-principles
pseudopotential plane wave method within spin-polarized Density Functional
Method.Comment: 8 pages, 6 figures, without proof readin
Nonlinear optics of III-V semiconductors in the terahertz regime: an ab-initio study
We compute from first principles the infrared dispersion of the nonlinear
susceptibility in zincblende semiconductors. At terahertz
frequencies the nonlinear susceptibility depends not only on the purely
electronic response , but also on three other parameters
, and describing the contributions from ionic motion. They
relate to the TO Raman polarizability, the second-order displacement-induced
dielectric polarization, and the third-order lattice potential. Contrary to
previous theory, we find that mechanical anharmonicity () dominates over
electrical anharmonicity (), which is consistent with recent experiments
on GaAs. We predict that the sharp minimum in the intensity of second-harmonic
generation recently observed for GaAs between and
does not occur for several other III-V compounds.Comment: 9 pages, 3 figures; updated bibliograph
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