arXiv subjects

Karin M. Rabe

Publications and source records attributed to Karin M. Rabe.

97 records · Page 6Linked to original sources

arXiv2001

Lattice instabilities of cubic NiTi from first principles

The phonon dispersion relation of NiTi in the simple cubic B2 structure is computed using first-principles density-functional perturbation theory with pseudopotentials and a plane-wave basis set. Lattice instabilities are observed to occur across nearly the entire Brillouin zone, excluding three interpenetrating tubes of stability along the (001) directions and small spheres of stability centered at R. The strongest instability is that of the doubly degenerate M5' mode. The atomic displacements of one of the eigenvectors of this mode generate a good approximation to the observed B19' ground-state structure.

cond-mat.mtrl-sci↗

arXiv1998

First Principles Investigation of Ferromagnetism and Ferroelectricity in Bismuth Manganite

We present results of local spin density approximation (LSDA) pseudopotential calculations for the perovskite structure oxide, bismuth manganite (BiMnO3). The origin of the differences between bismuth manganite and other perovskite manganites is determined by first calculating total energies and band structures of the high symmetry cubic phase, then sequentially lowering the magnetic and structural symmetry. Our results indicate that covalent bonding between bismuth cations and oxygen anions stabilizes different magnetic and structural phases compared with the rare earth manganites. This is consistent with recent experimental results showing enhancement of charge ordering in doped bismuth manganite.

cond-mat.mtrl-sci↗

arXiv1998

Configuration dependence of physical properties of a ferroelectric solid solution

In this article, we motivate the detailed comparison of the physical properties of individual configurations of a ferroelectric solid solution as a means toward developing first principles models for these systems. We compare energies, dielectric constants epsilon_infinity, mode effective charges of local polar distortions, and the zero temperature piezoelectric behavior of several ordered Pb_3GeTe_4 supercells. Cluster expansions of these properties show the importance of second-neighbor effects, which can be related to symmetry-preserving relaxation and its effect on the symmetry breaking polar instabilities.

cond-mat.mtrl-sci↗

arXiv1998

Temperature-dependent dielectric and piezoelectric response of ferroelectrics from first principles

A method for the calculation of the temperature dependence of dielectric and piezoelectric responses, based on the use of a first-principles effective Hamiltonian, is described. Results are presented for the ferroelectric perovskite PbTiO3. While the method includes only the soft-mode contributions to the responses, it is argued to give a good description of the divergences or near-divergences of the response functions near the cubic-tetragonal transition. The expression of the response functions in terms of correlation functions is used to provide a real-space interpretation of the responses which clearly distinguishes between PbTiO3 and the related materials BaTiO3 and KNbO3.

cond-mat.mtrl-sci↗

arXiv1997

Enhancement of piezoelectricity in a mixed ferroelectric

We use first-principles density-functional total energy and polarization calculations to calculate the piezoelectric tensor at zero temperature for both cubic and simple tetragonal ordered supercells of Pb_3GeTe_4. The largest piezoelectric coefficient for the tetragonal configuration is enhanced by a factor of about three with respect to that of the cubic configuration. This can be attributed to both the larger strain-induced motion of cations relative to anions and higher Born effective charges in the tetragonal case. A normal mode decomposition shows that both cation ordering and local relaxation weaken the ferroelectric instability, enhancing piezoelectricity.

cond-mat.mtrl-sci↗

arXiv1997

Ab Initio Study of the Structural Phase Transition in Cubic Pb_3GeTe_4

In the substitutionally disordered narrow-gap semiconductor Pb_{1-x}Ge_xTe, a finite-temperature cubic-rhombohedral transition appears above a critical concentration $x \approx 0.005$. As a first step towards a first-principles investigation of this transition in the disordered system, a (hypothetical) ordered cubic Pb_3GeTe_4 supercell is studied. First principles density-functional calculations of total energies and linear response functions are performed using the conjugate-gradients method with ab initio pseudopotentials and a plane-wave basis set. Unstable modes in Pb_3GeTe_4 are found, dominated by off-centering of the Ge ions coupled with displacements of their neighboring Te ions. A model Hamiltonian for this system is constructed using the lattice Wannier function formalism. The parameters for this Hamiltonian are determined from first principles. The equilibrium thermodynamics of the model system is studied via Metropolis Monte Carlo simulations. The calculated transition temperature, T_c, is approximately 620K for the cubic Pb_3GeTe_4 model, compared to the experimental value of T_c \approx 350K for disordered Pb_{0.75}Ge_{0.25}Te. Generalization of this analysis to the disordered Pb_{1-x}Ge_xTe system is discussed.

cond-mat.mtrl-sci↗

arXiv1996

Anomalous effective charges and far IR optical absorption of Al$_2$Ru from first principles

For the orthorhombic intermetallic semiconductor Al$_2$Ru, the bandstructure, valence charge density, zone center optical phonon frequencies, and Born effective charge and electronic dielectric tensors are calculated using variational density functional perturbation theory with {\em ab initio} pseudopotentials and a plane wave basis set. Good agreement is obtained with recent measurements on polycrystalline samples which showed anomalously strong far IR absorption by optical phonons, while analysis of the valence charge density shows that the static ionic charges of Al and Ru are negligible. Hybridization is proposed as the single origin both of the semiconducting gap and the anomalous Born effective charges. Analogous behavior is expected in related compounds such as NiSnZr, PbTe, skutterudites, and Al-transition-metal quasicrystals.

mtrl-th↗