## Rappe, A M

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Publication Dirac Semimetal in Three Dimensions(2012-04-06) Young, Steve M; Kane, Charles L; Zaheer, Saad; Mele, Eugene J.; Teo, Jeffrey C; Rappe, A MWe show that the pseudorelativistic physics of graphene near the Fermi level can be extended to three dimensional (3D) materials. Unlike in phase transitions from inversion symmetric topological to normal insulators, we show that particular space groups also allow 3D Dirac points as symmetry protected degeneracies. We provide criteria necessary to identify these groups and, as an example, present ab initio calculations of β-cristobalite BiO2 which exhibits three Dirac points at the Fermi level. We find that β-cristobalite BiO2 is metastable, so it can be physically realized as a 3D analog to graphene.Publication First Principles Calculation of the Shift Current Photovoltaic Effect in Ferroelectrics(2012-09-12) Young, Steve M; Rappe, Andrew MWe calculate the bulk photovoltaic response of the ferroelectrics BaTiO3 and PbTiO3 from first principles by applying the “shift current” theory to the electronic structure from density functional theory. The first principles results for BaTiO3 reproduce experimental photocurrent direction and magnitude as a function of light frequency, as well as the dependence of current on light polarization, demonstrating that shift current is the dominant mechanism of the bulk photovoltaic effect in BaTiO3. Additionally, we analyze the relationship between response and material properties in detail. Photocurrent does not depend simply or strongly on the magnitude of material polarization, as has been previously assumed; instead, electronic states with delocalized, covalent bonding that is highly asymmetric along the current direction are required for strong shift current enhancements. The complexity of the response dependence on both external and material parameters suggests applications not only in solar energy conversion, but in photocatalysis and sensor and switch type devices as well.Publication Anisotropic Local Correlations and Dynamics in a Relaxor Ferroelectric(2013-04-05) Takenaka, Hiroyuki; Grinberg, Ilya; Rappe, Andrew MRelaxor ferroelectrics have been a focus of intense attention due to their anomalous properties, and understanding the structure and dynamics of relaxors has been one of the long-standing challenges in solid-state physics. We investigate the local structure and dynamics in 75%PbMg1/3Nb2/3O3-25%PbTiO3 using molecular dynamics simulations and the dynamic pair distribution function technique. We show that relaxor transitions can be described by local order parameters. The relaxor phase is characterized by the presence of highly anisotropic correlations between the local cation displacements that resemble the hydrogen bond network in water. This contradicts the current model of polar nanoregion inside a nonpolar matrix. We therefore suggest a new model of a homogeneous random network of anisotropically coupled dipoles.Publication Evolution of the Structure and Thermodynamic Stability of the BaTiO3(001) Surface(2008-07-18) Kolpak, Alexie M; Li, Dongbo; Rappe, Andrew M; Shao, Rui; Bonnell, Dawn AWe report a series of new surface reconstructions on BaTiO3(001) as a function of environmental conditions, determined via scanning tunneling microscopy and low energy electron diffraction. Using density functional theory calculations and thermodynamic modeling, we construct a surface phase diagram and determine the atomic structures of the thermodynamically stable phases. Excellent agreement is found between the predicted phase diagram and experiment. The results enable prediction of surface structures and properties under the entire range of accessible environmental conditions.Publication Atomic and Electronic Structure of the BaTiO3(001) (√5×√5)R26.6° Surface Reconstruction(2012-12-20) Martirez, John Mark P; Morales, Erie H; Bonnell, Dawn A; Saidi, Wissam A; Rappe, A MThis contribution presents a study of the atomic and electronic structure of the (√5×√5)R26.6° surface reconstruction on BaTiO3 (001) formed by annealing in ultrahigh vacuum at 1300 K. Through density functional theory calculations in concert with thermodynamic analysis, we assess the stability of several BaTiO3 surface reconstructions and construct a phase diagram as a function of the chemical potential of the constituent elements. Using both experimental scanning tunneling microscopy (STM) and scanning tunneling spectroscopy measurements, we were able to further narrow down the candidate structures, and conclude that the surface is either TiO2-Ti3/5, TiO2-Ti4/5, or some combination, where Ti adatoms occupy hollow sites of the TiO2 surface. Density functional theory indicates that the defect states close to the valence band are from Ti adatom 3d orbitals (≈1.4 eV below the conduction band edge) in agreement with scanning tunneling spectroscopy measurements showing defect states 1.56±0.11 eV below the conduction band minimum (1.03±0.09 eV below the Fermi level). STM measurements show electronic contrast between empty and filled states’ images. The calculated local density of states at the surface shows that Ti 3d states below and above the Fermi level explain the difference in electronic contrast in the experimental STM images by the presence of electronically distinctive arrangements of Ti adatoms. This work provides an interesting contrast with the related oxide SrTiO3, for which the (001) surface (√5×√5)R26.6° reconstruction is reported to be the TiO2 surface with Sr adatoms.Publication First-Principles Calculation of the Bulk Photovoltaic Effect in Bismuth Ferrite(2012-12-04) Young, Steve M; Zheng, Fan; Rappe, A MWe compute the bulk photovoltaic effect (BPVE) in BiFeO3 using first-principles shift current theory, finding good agreement with experimental results. Furthermore, we reconcile apparently contradictory observations: by examining the contributions of all photovoltaic response tensor components and accounting for the geometry and ferroelectric domain structure of the experimental system, we explain the apparent lack of BPVE response in striped polydomain samples that is at odds with the significant response observed in monodomain samples. We reveal that the domain-wall-driven response in striped polydomain samples is partially mitigated by the BPVE, suggesting that enhanced efficiency could be obtained in materials with cooperative rather than antagonistic interaction between the two mechanisms.Publication Spin Texture on the Fermi Surface of Tensile-strained HgTe(2013-01-03) Zaheer, Saad; Young, Steve M.; Kane, Charles L.; Cellucci, Daniel; Mele, Eugene J.; Teo, Jeffrey C.Y.; Rappe, Andrew M.We present ab initio and k·p calculations of the spin texture on the Fermi surface of tensile-strained HgTe, which is obtained by stretching the zinc-blende lattice along the (111) axis. Tensile-strained HgTe is a semimetal with pointlike accidental degeneracies between a mirror symmetry protected twofold degenerate band and two nondegenerate bands near the Fermi level. The Fermi surface consists of two ellipsoids which contact at the point where the Fermi level crosses the twofold degenerate band along the (111) axis. However, the spin texture of occupied states indicates that neither ellipsoid carries a compensating Chern number. Consequently, the spin texture is locked in the plane perpendicular to the (111) axis, exhibits a nonzero winding number in that plane, and changes winding number from one end of the Fermi ellipsoids to the other. The change in the winding of the spin texture suggests the existence of singular points. An ordered alloy of HgTe with ZnTe has the same effect as stretching the zinc-blende lattice in the (111) direction. We present ab initio calculations of ordered HgxZn1−xTe that confirm the existence of a spin texture locked in a 2D plane on the Fermi surface with different winding numbers on either end.Publication Theoretical Investigation of the Evolution of the Topological Phase of Bi2Se3 under Mechanical Strain(2011-08-19) Young, Steve M.; Chowdhury, Sugata; Mele, Eugene J.; Kane, Charles L; Rappe, Andrew M; Walter, Eric J.The topological insulating phase results from inversion of the band gap due to spin-orbit coupling at an odd number of time-reversal symmetric points. In Bi2Se3, this inversion occurs at the Γ point. For bulk Bi2Se3, we have analyzed the effect of arbitrary strain on the Γ point band gap using density functional theory. By computing the band structure both with and without spin-orbit interactions, we consider the effects of strain on the gap via Coulombic interaction and spin-orbit interaction separately. While compressive strain acts to decrease the Coulombic gap, it also increases the strength of the spin-orbit interaction, increasing the inverted gap. Comparison with Bi2Te3 supports the conclusion that effects on both Coulombic and spin-orbit interactions are critical to understanding the behavior of topological insulators under strain, and we propose that the topological insulating phase can be effectively manipulated by inducing strain through chemical substitution.Publication Pb-free Semiconductor Ferroelectrics: A Theoretical Study of Pd-substituted Ba(Ti1-xCex)O3 Solid Solutions(2010-11-10) Grinberg, Ilya; Davies, Peter K.; Bennett, Joseph W.; Rappe, Andrew M.We use first-principles density-functional-theory calculations to investigate the ground state structures of Ba(Ti1−xCex)O3 solid solutions containing Pd. Previous studies have shown that the properties of BaTiO3, a Pb-free ferroelectric ABO3 perovskite, can be tailored via B-site substitution. In the present study, we substitute Ce for Ti to increase the overall volume of the perovskite, to then accommodate an O-vacancy-stabilized Pd substitution. Using the LDA+U method, we predict that these proposed materials will display a decreased band gap compared to BaTiO3 while maintaining polarization. These features, combined with their environmentally friendly characteristics make these materials promising candidates for use as semiconducting ferroelectrics in solar-energy conversion devices.Publication Relationship between Local Structure and Relaxor Behavior in Perovskite Oxides(2007-12-31) Grinberg, Ilya; Davies, Peter K; Juhás, Pavol; Rappe, Andrew MDespite intensive investigations over the past five decades, the microscopic origins of the fascinating dielectric properties of ABO3 relaxor ferroelectrics are currently poorly understood. Here, we show that the frequency dispersion that is the hallmark of relaxor behavior is quantitatively related to the crystal chemical characteristics of the solid solution. Density functional theory is used in conjunction with experimental determination of cation arrangement to identify the 0 K structural motifs. These are then used to parametrize a simple phenomenological Landau theory that predicts the universal dependence of frequency dispersion on the solid solution cation arrangement and off-center cation displacements.