Showing papers by "Alex Zunger published in 2014"

Hidden spin polarization in inversion-symmetric bulk crystals

Abstract: Spin polarization due to spin–orbit coupling requires broken inversion symmetry. Now, calculations show that the effect arises from local site-asymmetry rather than global space-group asymmetry, and that a hitherto overlooked form of spin polarization should also exist in centrosymmetric structures.

Electric Field Induced Topological Phase Transition in Two-Dimensional Few-layer Black Phosphorus

Abstract: Phosphorene is a novel two-dimensional material that can be isolated through mechanical exfoliation from layered black phosphorus, but unlike graphene and silicene, monolayer phosphorene has a large band gap. It was thus unsuspected to exhibit band inversion and the ensuing topological insulator behavior. It has recently attracted interest because of its proposed application as field effect transistors. Using first-principles calculations with applied perpendicular electric field F we predict a continuous transition from the normal insulator to a topological insulator and eventually to a metal as a function of F. The continuous tuning of topological behavior with electric field would lead to spin-separated, gapless edge states, i.e., quantum spins Hall effect. This finding opens the possibility of converting normal insulating materials into topological ones via electric field, and making a multi-functional field effect topological transistor that could manipulate simultaneously both spins and charge carrier.

A polarity-induced defect mechanism for conductivity and magnetism at polar–nonpolar oxide interfaces

Abstract: The discovery of conductivity and magnetism at the polar–nonpolar interfaces of insulating nonmagnetic oxides such as LaAlO3 and SrTiO3 has raised prospects for attaining interfacial functionalities absent in the component materials. Yet, the microscopic origin of such emergent phenomena remains unclear, posing obstacles to design of improved functionalities. Here we present first principles calculations of electronic and defect properties of LaAlO3/SrTiO3 interfaces and reveal a unifying mechanism for the origins of both conductivity and magnetism. We demonstrate that the polar discontinuity across the interface triggers thermodynamically the spontaneous formation of certain defects that in turn cancel the polar field induced by the polar discontinuity. The ionization of the spontaneously formed surface oxygen vacancy defects leads to interface conductivity, whereas the unionized Ti-on-Al antisite defects lead to interface magnetism. The proposed mechanism suggests practical design principles for inducing and controlling both conductivity and magnetism at general polar–nonpolar interfaces. The interface between LaAlO3 and SrTiO3shows unusual phenomena such as a two-dimensional electron gas as well as magnetism. Here, Yu and Zunger show how the formation of defects contributes to the properties of this system.

Assessing capability of semiconductors to split water using ionization potentials and electron affinities only

Abstract: We show in this article that the position of semiconductor band edges relative to the water reduction and oxidation levels can be reliably predicted from the ionization potentials (IP) and electron affinities (AE) only. Using a set of 17 materials, including transition metal compounds, we show that accurate surface dependent IPs and EAs of semiconductors can be computed by combining density functional theory and many-body GW calculations. From the extensive comparison of calculated IPs and EAs with available experimental data, both from photoemission and electrochemical measurements, we show that it is possible to sort candidate materials solely from IPs and EAs thereby eliminating explicit treatment of semiconductor/water interfaces. We find that at pH values corresponding to the point of zero charge there is on average a 0.5 eV shift of IPs and EAs closer to the vacuum due to the dipoles formed at material/water interfaces.

Hidden spin polarization in inversion-symmetric bulk crystals

Abstract: Spin polarization due to spin–orbit coupling requires broken inversion symmetry. Now, calculations show that the effect arises from local site-asymmetry rather than global space-group asymmetry, and that a hitherto overlooked form of spin polarization should also exist in centrosymmetric structures.

Self-Doping and Electrical Conductivity in Spinel Oxides: Experimental Validation of Doping Rules

Abstract: Self-doping of cations on the tetrahedral and octahedral sites in spinel oxides creates “anti-site” defects, which results in functional optical, electronic, magnetic, and other materials properties. Previously, we divded the III–II spinel family into four doping types (DTs) based on first-principle calculations in order to understand their electrical behavior. Here, we present experimental evidence on two prototype spinels for each major doping type (DT1 and DT4) that test the first principles calculations. For the DT-1 Ga2ZnO4 spinel, we show that the anti-site defects in a stoichiometric film are equal in concentration and compenstate each other, whereas, for nonstoichiometric Cr2MnO4, a representative DT-4 spinel, excess Mn on the tetrahedral sites becomes electrically inactive as the Mn species switch from (III) to (II). The agreement between experiment and theory validates the Doping Rules distilled from the theoretical framework and significantly enhances our understanding of the defect chemistry o...

Structurally unstable A III BiO 3 perovskites are predicted to be topological insulators but their stable structural forms are trivial band insulators

Abstract: Structurally unstable compounds can be predicted to be topological insulators (TIs) but their stable structural forms are trivial band insulators. The authors illustrate the danger in such false positive predictions for the A${}^{I\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}I}$BiO${}_{3}$ bismuth oxides that have been recently proposed as TIs in a hypothetical ``perovskite'' crystal structure-type. They show that the inversion of anti-bonding with bonding electron levels may destabilize the material and argue that to predict new candidate TIs by quantum atomistic modeling, one should simultaneously test structural stability and topological character of the candidate materials.

Design of TaIrGe: a ternary half-Heusler transparent hole conductor

Abstract: Ternary equiatomic ABX compounds constitute a fascinating group of materials, and manifesting extraordinary functionalities. Surprisingly, many of these compounds are still 'missing' materials, i.e. no record of them in the existing databases. There have been prior attempts to predict the properties of ABX compounds in assumed crystal structures. However, the stability of the assumed crystal structure was generally not examined. The fact that newly discovered materials that have escaped synthesis tend to have interesting properties, makes the proposition of prediction and realization of 'Missing Materials' attractive. Here, we use first-principles thermodynamics and laboratory realization to show that TaIrGe is a stable new compound with the 'Filled Tetrahedral Structure' having interband transitions (1.74, 2.64 and 3.1 eV) and manifest intrinsic p-type behavior and high hole mobility (2730 cm2/Vs), thus being one of the rare occurrences of a 'transparent hole conductor'. This work suggests that theory-driven design can accelerate discovery of new materials.

Confinement-Induced vs. Correlation-Induced Electron Localization in Model Semiconductor Nano Circuits

Abstract: Single-particle plus many-particle calculations of the electronic states of semiconductor nano dumbbells illustrate how geometrical features (e.g. the width of the dumbbell wire) determine, through quantum confinement and electron-electron correlation effects, the localization of the wave functions. Remarkably, we find that many-body effects can alter carrier localization, thus affecting the transport properties of nano circuits that include quantum dots and quantum wires. This is important, as most of the current transport calculations (using Landauer formula) neglect many-particle effects. We further show how the degree of entanglement and the exciton binding energies depend on the nano circuit geometry.