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

Relative energetics and structural properties of zirconia using a self-consistent tight-binding model

We describe an empirical, self-consistent, orthogonal tight-binding model for zirconia, which allows for the polarizability of the anions at dipole and quadrupole levels and for crystal field splitting of the cation d orbitals. This is achieved by mixing the orbitals of different symmetry on a site with coupling coefficients driven by the Coulomb potentials up to octapole level. The additional forces on atoms due to the self-consistency and polarizabilities are exactly obtained by straightforward electrostatics, by analogy with the Hellmann-Feynman theorem as applied in first-principles calculations. The model correctly orders the zero temperature energies of all zirconia polymorphs. The Zr-O matrix elements of the Hamiltonian, which measure covalency, make a greater contribution than the polarizability to the energy differences between phases. Results for elastic constants of the cubic and tetragonal phases and phonon frequencies of the cubic phase are also presented and compared with some experimental data and first-principles calculations. We suggest that the model will be useful for studying finite temperature effects by means of molecular dynamics.Comment: to be published in Physical Review B (1 march 2000

Колебательная, релаксационная и трансляционная подвижность ионов Li+ в монокристалле LiNbGeO5.

Dielectric properties of single crystals of LiNbGeO5, crystallizing in the andalusite (β-Al2SiO5) structure, have been studied. Dielectric and conducting properties along the major crystallographic axes were investigated at frequencies from 1 to 1000 kHz and in the range of temperatures between 100 and 600 K. Determined potential barriers between the equilibrium positions of Li+ ions are classified as low energy (0.23-0.79 eV, along the a axis) and high energy (0.9-1.90 eV, along the a, b, and c axes). Additional interstitial sites with the distances between them 0.3-0.4 Å have been found along the a axis, while no such sites observed along the b and c axes. Activation energy (Ea) in the direction a is 0.47 eV and ?a(570 K) = 2.10-5 S/cm, which correspond to the super-ionic conductivity.Исследованы диэлектрические свойства монокристаллов LiNbGeO5 со структурой минерала андалузита (β-Al2SiO5). Диэлектрические и проводящие свойства изучены вдоль главных кристаллографических осей a, b, c при частоте измерительного поля 1-1000 кГц в интервале температур 100-600 К. Найденные значения потенциальных барьеров между положениями равновесия ионов Li+ подразделяются на низкоэнергетические (0.23-0.79 эВ, a-ось) и высокоэнергетические (0.9-1.80 эВ, a, b и c-оси). В направлении оси a обнаружены дополнительные междуузельные позиции с расстояниями между ними 0.3-0.4 A, в то время как для осей b и c такие позиции отсутствуют. В направлении a энергия активации Еa = 0.47 эВ и значение ?a(570 К) = 2.10-5 См/см, что соответствует суперионной проводимости

Observation of Mott Transition in VO_2 Based Transistors

An abrupt Mott metal-insulator transition (MIT) rather than the continuous Hubbard MIT near a critical on-site Coulomb energy U/U_c=1 is observed for the first time in VO_2, a strongly correlated material, by inducing holes of about 0.018% into the conduction band. As a result, a discontinuous jump of the density of states on the Fermi surface is observed and inhomogeneity inevitably occurs. The gate effect in fabricated transistors is clear evidence that the abrupt MIT is induced by the excitation of holes.Comment: 4 pages, 4 figure