Eric J. Walter

Large phonon band gap in SrTiO3 and the vibrational signatures of ferroelectricity in ATiO3 perovskites: First-principles lattice dynamics and inelastic neutron scattering

We report on 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 gives 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 to be 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.

Ab initio linear response and frozen phonons for the relaxor PbMg1/3Nb2/3O3

We report first principles density functional studies using plane wave basis sets and pseudopotentials and all electron linear augmented plane wave (LAPW) of the relative stability of various ferroelectric and antiferroelectric supercells of PMN for 1:2 chemical ordering along [111] and [001]. We used linear response with density functional perturbation theory (DFPT) as implemented in the code ABINIT to compute the Born effective charges, electronic dielectric tensors, long wavelength phonon frequencies and LO-TO splittings. The polar response is different for supercells ordered along [111] and [001]. Several polar phonon modes show significant coupling with the macroscopic electric field giving giant LO-TO splittings. For [111] ordering, a polar transverse optic (TO) mode with E symmetry is found to be unstable in the ferroelectric P3m1 structure and the ground state is found to be triclinic. Multiple phonon instabilities of polar modes and their mode couplings provide the pathway for polarization rotation. The Born effective charges in PMN are highly anisotropic and this anisotropy contributes to the observed huge electromechanical coupling in PMN solid solutions.

Phaseless auxiliary-field quantum Monte Carlo calculations with plane waves and pseudopotentials : Applications to atoms and molecules

The phaseless auxiliary-field quantum Monte Carlo (AF QMC) method [S. Zhang and H. Krakauer, Phys. Rev. Lett. 90, 136401 (2003)] is used to carry out a systematic study of the dissociation and ionization energies of second-row group 3A\char21{}7A atoms and dimers: Al, Si, P, S, and Cl. In addition, the

Anisotropic infrared response of vanadium dioxide microcrystals

{\mathrm{P}}_{2}

Optimized norm-conserving Hartree-Fock pseudopotentials for plane-wave calculations

dimer is compared to the third-row

Insulating phases of vanadium dioxide are Mott-Hubbard insulators

{\mathrm{As}}_{2}

Spin- and charge-density waves in the Hartree?Fock ground state of the two-dimensional Hubbard model

dimer, which is also triply bonded. This method projects the many-body ground state by means of importance-sampled random walks in the space of Slater determinants. The Monte Carlo phase problem, due to the electron-electron Coulomb interaction, is controlled via the phaseless approximation, with a trial wave function

Magnetic order in the Hubbard model in three dimensions and the crossover to two dimensions

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High sensitivity of O17 NMR to p-d hybridization in transition metal perovskites: First principles calculations of large anisotropic chemical shielding

. As in previous calculations, a mean-field single Slater determinant is used as

Phonon density of states of model ferroelectrics

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