arXiv subjects

Andrew M. Rappe

Publications and source records attributed to Andrew M. Rappe.

At least 91 records · Page 5Linked to original sources

arXiv2015

Polarization Dependence of Water Adsorption to CH$_3$NH$_3$PbI$_3$ (001) Surfaces

The instability of organometal halide perovskites when in contact with water is a serious challenge to their feasibility as solar cell materials. Although studies of moisture exposure have been conducted, an atomistic understanding of the degradation mechanism is required. Toward this goal, we study the interaction of water with the (001) surfaces of CH$_3$NH$_3$PbI$_3$ under both low and high water concentrations using density functional theory. We find that water adsorption is heavily influenced by the orientation of the methylammonium cations close to the surface. It is demonstrated that, depending on methylammonium orientation, the water molecule can infiltrate into the hollow site of the surface and get trapped. Controlling dipole orientation via poling or interfacial engineering could thus enhance its moisture stability. We do not see a direct reaction between the water and methylammonium molecules. Furthermore, calculations with an implicit solvation model indicate that a higher water concentration may facilitate degradation.

cond-mat.mtrl-sci↗

arXiv2015

Low Operation Voltage Ferroelectric Field-Effect Transistor Based on Polarization Rotation Effect

The effect of polarization rotation on the performance of metal oxide semiconductor field-effect transistors was investigated with a Landau-Ginzburg-Devonshire theory based model. In this analytical model, depolarization field, polarization rotations and the electrostatic properties of the doped silicon substrate are considered to illustrate the size effect of ferroelectric oxides and the stability of polarization in each direction. Based on this model, we demonstrate that MOSFET operation could be achieved by polarization reorientation with a low operating voltage, if the thickness of ferroelectric oxide is properly selected. Polarization reorientation can boost the surface potential of the silicon substrate, leading to a subthreshold swing S lower than 60 mV/decade. We believe that this model could provide guidance in designing electronic logic devices with low operating voltages and low active energy consumption.

cond-mat.mtrl-sci↗

arXiv2015

Rashba Spin-Orbit Coupling Enhanced Carrier Lifetime in Organometal Halide Perovskites

Organometal halide perovskites are promising solar-cell materials for next-generation photovoltaic applications. The long carrier lifetime and diffusion length of these materials make them very attractive for use in light absorbers and carrier transporters. While these aspects of organometal halide perovskites have attracted the most attention, the consequences of the Rashba effect, driven by strong spin-orbit coupling, on the photovoltaic properties of these materials are largely unexplored. In this work, taking the electronic structure of methylammonium lead iodide as an example, we propose an intrinsic mechanism for enhanced carrier lifetime in 3D Rashba materials. Based on first-principles calculations and a Rashba spin-orbit model, we demonstrate that the recombination rate is reduced due to the spin-forbidden transition. These results are important for understanding the fundamental physics of organometal halide perovskites and for optimizing and designing the materials with better performance. The proposed mechanism including spin degrees of freedom offers a new paradigm of using 3D Rashba materials for photovoltaic applications.

cond-mat.mtrl-sci↗

arXiv2015

Dirac Line Nodes in Inversion Symmetric Crystals

We propose and characterize a new $\mathbb{Z}_2$ class of topological semimetals with a vanishing spin--orbit interaction. The proposed topological semimetals are characterized by the presence of bulk one-dimensional (1D) Dirac Line Nodes (DLNs) and two-dimensional (2D) nearly-flat surface states, protected by inversion and time--reversal symmetries. We develop the $\mathbb{Z}_2$ invariants dictating the presence of DLNs based on parity eigenvalues at the parity--invariant points in reciprocal space. Moreover, using first-principles calculations, we predict DLNs to occur in Cu$_3$N near the Fermi energy by doping non-magnetic transition metal atoms, such as Zn and Pd, with the 2D surface states emerging in the projected interior of the DLNs. This paper includes a brief discussion of the effects of spin--orbit interactions and symmetry-breaking as well as comments on experimental implications.

cond-mat.mtrl-sci↗

arXiv2015

Structural and Magnetic Reconstruction of Thermodynamically Stable CaMnO$_{3}$ (001) Surfaces

The relative thermodynamic stability of surface reconstructions including vacancies, adatoms and additional layers on both CaO and MnO$_{2}$ terminations is calculated to predict the surface phase diagram of ($\sqrt{2}\times\sqrt{2}$)$\it{R}$45$^{\circ}$ CaMnO$_{3}$ (001) using $\it{ab}$ $\it{initio}$ thermodynamics. Stoichiometric and nonstoichiometric reconstructions are considered. A set of boundary conditions driven by the binary and ternary sub-phases from CaMnO$_{3}$ defines its bulk stability region, enclosing the stable surface reconstructions that can be in equilibrium with the bulk. Most of the surfaces take the magnetic ordering of the bulk ground state, G-type antiferromagnetic (AFM); however, some of the MnO$_{2}$-terminated surfaces are more stable when the surface layer spins flip. At 573 K, the stoichiometric CaO- and MnO$_{2}$-terminated surfaces are predicted to be thermodynamically stable, as well as a CaO-terminated surface reconstruction where half the Ca are replaced by Mn. The MnO$_{2}$-terminated surface reconstructions dominate the phase diagrams at high temperatures, including phases with MnO and MnO$_{2}$ adatoms per surface unit cell.

cond-mat.mtrl-sci↗

arXiv2015

Layered Topological Crystalline Insulators

Topological crystalline insulators (TCIs) are insulating materials whose topological property relies on generic crystalline symmetries. Based on first-principles calculations, we study a three-dimensional (3D) crystal constructed by stacking two-dimensional TCI layers. Depending on the inter-layer interaction, the layered crystal can realize diverse 3D topological phases characterized by two mirror Chern numbers (MCNs) ($μ_1,μ_2$) defined on inequivalent mirror-invariant planes in the Brillouin zone. As an example, we demonstrate that new TCI phases can be realized in layered materials such as a PbSe (001) monolayer/h-BN heterostructure and can be tuned by mechanical strain. Our results shed light on the role of the MCNs on inequivalent mirror-symmetric planes in reciprocal space and open new possibilities for finding new topological materials.

cond-mat.mtrl-sci↗

arXiv2015

Bulk photovoltaic effect enhancement via electrostatic control in layered ferroelectrics

The correlation between the shift current mechanism for the bulk photovoltaic effect (BPVE) and the structural and electronic properties of ferroelectric perovskite oxides is not well understood. Here, we study and engineer the shift current photovoltaic effect using a visible-light-absorbing ferroelectric Pb(Ni$_{x}$Ti$_{1-x}$)O$_{3-x}$ solid solution from first principles. We show that the covalent orbital character dicates the direction, magnitude, and onset energy of shift current in a predictable fashion. In particular, we find that the shift current response can be enhanced via electrostatic control in layered ferroelectrics, as bound charges face a stronger impetus to screen the electric field in a thicker material, delocalizing electron densities. This heterogeneous layered structure with alternative photocurrent generating and insulating layers is ideal for BPVE applications.

cond-mat.mtrl-sci↗

arXiv2015

First-Principles Calculation of the Bulk Photovoltaic Effect in KNbO$_{3}$ and (K,Ba)(Ni,Nb)O$_{3-δ}$

The connection between noncentrosymmetric materials' structure, electronic structure, and bulk photovoltaic performance remains not well understood. In particular, it is still unclear which photovoltaic (PV) mechanism(s) are relevant for the recently demonstrated visible-light ferroelectric photovoltaic (K,Ba)(Ni,Nb)O$_{3-δ}$. In this paper, we study the bulk photovoltaic effect (BPVE) of (K,Ba)(Ni,Nb)O$_{3-δ}$ and KNbO$_{3}$ by calculating the shift current from first principles. The effects of structural phase, lattice distortion, oxygen vacancies, cation arrangement, composition, and strain on BPVE are systematically studied. We find that (K,Ba)(Ni,Nb)O$_{3-δ}$ has a comparable BPVE with that of the broadly explored BiFeO$_{3}$, but for a much lower photon energy. In particular, the Glass coefficient of (K,Ba)(Ni,Nb)O$_{5}$ in a simply layered structure can be as large as 12 times that of BiFeO$_{3}$. Furthermore, the nature of the wavefunctions dictates the eventual shift current yield, which can be significantly affected and engineered by changing the O vacancy location, cation arrangement, and strain. This is not only helpful for understanding other PV mechanisms that relate to the motion of the photocurrent carriers, but also provides guidelines for the design and optimization of PV converters.

cond-mat.mtrl-sci↗

arXiv2014

First-Principles Calculation of the Bulk Photovoltaic Effect in the Polar Compounds LiAsS$_\text{2}$, LiAsSe$_\text{2}$, and NaAsSe$_\text{2}$

We calculate the shift current response, which has been identified as the dominant mechanism for the bulk photovoltaic effect, for the polar compounds LiAsS$_\text{2}$, LiAsSe$_\text{2}$, and NaAsSe$_\text{2}$. We find that the magnitudes of the photovoltaic responses in the visible range for these compounds exceed the maximum response obtained for BiFeO$_\text{3}$ by 10 - 20 times. We correlate the high shift current response with the existence of $p$ states at both the valence and conduction band edges, as well as the dispersion of these bands, while also showing that high polarization is not a requirement. With low experimental band gaps of less than 2 eV and high shift current response, these materials have potential for use as bulk photovoltaics.

cond-mat.mtrl-sci↗

arXiv2014

First-principles calculation of bulk photovoltaic effect in CH$_3$NH$_3$PbI$_3$ and CH$_3$NH$_3$PbI$_{3-x}$Cl$_{x}$

Hybrid halide perovskites exhibit nearly 20% power conversion efficiency, but the origin of their high efficiency is still unknown. Here, we compute the shift current, a dominant mechanism of bulk photovoltaic (PV) effect for ferroelectric photovoltaics, in CH$_3$NH$_3$PbI$_3$ and CH$_3$NH$_3$PbI$_{3-x}$Cl$_{x}$ from first principles. We find that these materials give approximately three times larger shift current PV response to near-IR and visible light than the prototypical ferroelectric photovoltaic BiFeO$_3$. The molecular orientations of CH$_3$NH$_3^{+}$ can strongly affect the corresponding PbI$_3$ inorganic frame so as to alter the magnitude of the shift current response. Specifically, configurations with dipole moments aligned in parallel distort the inorganic PbI$_3$ frame more significantly than configurations with near net zero dipole, yielding a larger shift current response. Furthermore, we explore the effect of Cl substitution on shift current, and find that Cl substitution at the equatorial site induces a larger response than does substitution at the apical site.

cond-mat.mtrl-sci↗

arXiv2014

The Structural Diversity of ABS3 Compounds with d0 Electronic Configuration for the B-cation

We use first-principles density functional theory (DFT) within the local density approx- imation (LDA) to ascertain the ground state structure of real and theoretical compounds with the formula ABS3 (A = K, Rb, Cs, Ca, Sr, Ba, Tl, Sn, Pb, and Bi; and B = Sc, Y, Ti, Zr, V, and Nb) under the constraint that B must have a d0 electronic configuration. Our findings indicate that none of these AB combinations prefer a perovskite ground state with corner-sharing BS6 octahedra, but that they prefer phases with either edge- or face-sharing motifs. Further, a simple two-dimensional structure field map created from A and B ionic radii provides a neat demarcation between combinations preferring face-sharing versus edge- sharing phases for most of these combinations. We then show that by modifying the common Goldschmidt tolerance factor with a multiplicative term based on the electronegativity dif- ference between A and S, the demarcation between predicted edge-sharing and face-sharing ground state phases is enhanced. We also demonstrate that, by calculating the free energy contribution of phonons, some of these compounds may assume multiple phases as synthesis temperatures are altered, or as ambient temperatures rise or fall.

cond-mat.mtrl-sci↗

arXiv2014

Density Functional Theory Study Of Hypothetical PbTiO3-Based Oxysulfides

Using density functional theory (DFT) within the local density approximation (LDA), we calculate the physical and electronic properties of PbTiO3 (PTO) and a series of hypothetical compounds PbTiO3-xSx x = 0.2, 0.25, 0.33, 0.5, 1, 2, and 3 arranged in the corner-sharing cubic perovskite structure. We determine that replacing the apical oxygen atom in the PTO tetragonal unit cell with a sulfur atom reduces the x = 0 LDA calculated band gap of 1.47 eV to 0.43 - 0.67 eV for x = 0.2 - 1 and increases the polarization. PBE0 and GW methods predict that the hypothetical compositions x = 0.2 to x = 2 will have band gaps in the visible range. For all values of x < 2, the oxysulfide perovskite retains the tetragonal phase of PbTiO3, and the a-lattice parameter remains within 2.5% of the oxide. Thermodynamic analysis indicates that chemical routes using high temperature gas, such as H2S and CS2, can be used to substitute O for S in PTO for the compositions x = 0.2 - 0.5.

cond-mat.mtrl-sci↗

arXiv2014

Coupling between octahedral rotations and local polar displacements in WO3/ReO3 superlattices

We model short-period superlattices of WO$_3$ and ReO$_3$ with first-principles calculations. In fully-relaxed superlattices, we observe that octahedral tilts about an axis in the planes of the superlattices do not propagate from one material, despite the presence of the corner-shared oxygen atoms. However, we find that octahedral rotation is enhanced within WO$_3$ layers in cases in which strain couples with native antiferroelectric displacements of tungsten within their octahedral cages. Resulting structures remain antiferroelectric with low net global polarization. Thermodynamic analysis reveals that superlattices with sufficiently thick ReO$_3$ layers, the absolute number being three or more layers and the Re fraction $\geq 50\%$, tend to be more stable than the separated material phases and also show enhanced octahedral rotations in the WO$_3$ layers.

cond-mat.mtrl-sci↗

arXiv2013

Bulk Dirac points in distorted spinels

We report on a Dirac-like Fermi surface in three-dimensional bulk materials in a distorted spinel structure on the basis of density functional theory (DFT) as well as tight-binding theory. The four examples we provide in this paper are BiZnSiO4, BiCaSiO4, BiMgSiO4, and BiAlInO4. A necessary characteristic of these structures is that they contain a Bi lattice which forms a hierarchy of chain-like substructures, with consequences for both fundamental understanding and materials design.

cond-mat.mtrl-sci↗

arXiv2013

Exploration of the momentum of ferroelectric domain walls via molecular dynamics simulations

The motion of ferroelectric domain walls (DW) is critical for various technological applications of ferroelectric materials. One important question that is of interest both scientifically and technologically is whether the ferroelectric DW has momentum. To address this problem, we performed canonical ensemble molecular dynamics simulations of 180$^\circ$ and 90$^\circ$ DWs under applied electric field. Examination of the evolution of the polarization and local structure of DWs reveals that they stop moving after the removal of electric field. Thus, our computational study shows that ferroelectric domain walls do not have momentum.

cond-mat.mtrl-sci↗

arXiv2013

Reinterpretation of bond-valence model with bond-order formalism: an improved bond-valence based interatomic potential for PbTiO$_3$

We present a modified bond-valence model of PbTiO$_3$ based on the principles of bond-valence and bond-valence vector conservation. The relationship between the bond-valence model and the bond-order potential is derived analytically in the framework of a tight-binding model. A new energy term, bond-valence vector energy, is introduced into the atomistic model and the potential parameters are re-optimized. The new model potential can be applied both to canonical ensemble ($NVT$) and isobaric-isothermal ensemble ($NPT$) molecular dynamics (MD) simulations. This model reproduces the experimental phase transition in $NVT$ MD simulations and also exhibits the experimental sequence of temperature-driven and pressure-driven phase transitions in $NPT$ simulations. We expect that this improved bond-valence model can be applied to a broad range of inorganic materials.

cond-mat.mtrl-sci↗

arXiv2012

Spin texture on the Fermi surface of tensile strained HgTe

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 zincblende 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 zincblende lattice in the (111) direction. We present ab initio calculations of ordered Hg_xZn_1-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.

cond-mat.mes-hall↗

arXiv2012

Anisotropic Local Correlations and Dynamics in a Relaxor Ferroelectric

Relaxor ferroelectrics have been a focus of intense attention due to their anomalous dielectric characteristics, diffuse phase transitions, and strong piezoelectricity. Understanding the structure and dynamics of relaxors has been one of the long-standing challenges in solid-state physics, with the current model of polar nanoregions in a non-polar matrix providing only a qualitative description of the relaxor phase transitions. In this paper, we investigate the local structure and dynamics in 75%PbMg$_{1/3}$Nb$_{2/3}$O$_3$-25%PbTiO$_3$ (PMN-PT) using molecular dynamics simulations and the dynamic pair distribution function technique. We show for the first time that relaxor transitions can be described by local order parameters. We find that structurally, the relaxor phase is characterized by the presence of highly anisotropic correlations between the local cation displacements. These correlations resemble the hydrogen bond network in water. Our findings contradict the current polar nanoregion model; instead, we suggest a new model of a homogeneous random network of anisotropically coupled dipoles.

cond-mat.mtrl-sci↗