arXiv subjects

Alex Zunger

Publications and source records attributed to Alex Zunger.

At least 73 records · Page 4Linked to original sources

arXiv2016

Polytypism in LaOBiS2-type compounds based on different three-dimensional stacking sequences of two-dimensional BiS2 layers

LaOBiS2-type materials have drawn much attention recently because of various interesting physical properties, such as low-temperature superconductivity, hidden spin polarization, and electrically tunable Dirac cones. However, it was generally assumed that each LaOBiS2-type compound has a unique and specific crystallographic structure (with a space group P4/nmm) separated from other phases. Using first-principles total energy and stability calculations we confirm that the previous assignment of the centrosymetric P4/nmm structure to LaOBiS2 is incorrect as a phonon instability renders this structure impossible. Furthermore, we find that the unstable structure is replaced by a family of energetically closely spaced modifications (polytypes) differing by the layer sequences and orientations. We find that the local Bi-S distortion leads to three polytypes of LaOBiS2 with different stacking patterns of the distorted BiS2 layers. The energy difference between the polytypes of LaOBiS2 is merely ~1 meV/u.c., indicating the possible coexistence of all polytypes in the real sample and that the particular distribution of polytypes may be growth-induced. The in-plane distortion can be suppressed by pressure, leading to a phase transition from polytypes to the high-symmetry P4/nmm structure with a pressure larger than 2.5 GPa. In addition, different choices of the intermediate atoms (replacing La) or active atoms (BiS2) could also manifest different ground state structures. One can thus tune the distortion and the ground state by pressure or substituting covalence atoms in LaOBiS2-family.

cond-mat.mtrl-sci↗

arXiv2016

Transforming common III-V and II-VI semiconductor compounds into topological heterostructures: The case of CdTe/InSb superlattices

Currently known topological insulators (TIs) are limited to narrow gap compounds incorporating heavy elements, thus severely limiting the material pool available for such applications. We show via first-principle calculations how a heterovalent superlattice made of common semiconductor building blocks can transform its non-TI components into a topological nanostructure, illustrated by III-V/II-VI superlattice InSb/CdTe. The heterovalent nature of such interfaces sets up, in the absence of interfacial atomic exchange, a natural internal electric field that along with the quantum confinement leads to band inversion, transforming these semiconductors into a topological phase while also forming a giant Rashba spin splitting. We reveal the relationship between the interfacial stability and the topological transition, finding a window of opportunity where both conditions can be optimized. Once a critical InSb layer thickness above ~ 1.5 nm is reached, both [111] and [100] superlattices have a relative energy of 5-14 meV/A2 higher than that of the atomically exchanged interface and an excitation gap up to ~150 meV, affording room-temperature quantum spin Hall effect in semiconductor superlattices. The understanding gained from this study could significantly broaden the current, rather restricted repertoire of functionalities available from individual compounds by creating next-generation super-structured functional materials.

cond-mat.mtrl-sci↗

arXiv2015

Intrinsic transparent conductors without doping

Transparent conductors (TC's) combine the usually contraindicated properties of electrical conductivity with optical transparency and are generally made by starting with a transparent insulator and making it conductive via heavy doping, an approach that generally faces severe 'doping bottlenecks'. We propose a different idea for TC design-starting with a metallic conductor and designing transparency by control of intrinsic interband transitions and intraband plasmonic frequency. We identify the specific design principles for three such prototypical intrinsic TC classes and then search computationally for materials that satisfy them. Remarkably, one of the intrinsic TC, Ag3Al22O34, is predicted also to be a prototype 3D compounds that manifest natural 2D electron gas (2DEG) regions with very high electron density and conductivity.

cond-mat.mtrl-sci↗

arXiv2015

Search and design of nonmagnetic centrosymmetric layered crystals with large local spin polarization

Until recently, spin-polarization in nonmagnetic materials was the exclusive territory of non- centrosymmetric structures. It was recently shown that a form of hidden spin polarization (named the Rashba-2 or R-2 effect) could exist in globally centrosymmetric crystals provided the individual layers belong to polar point group symmetries. This realization could considerably broaden the range of materials that might be considered for spin-polarization spintronic applications to include the hitherto forbidden spintronic compound that belong to centrosymetric symmetries. Here we take the necessary steps to transition from such general, material-agnostic condensed matter theory arguments to material-specific design principles that could aid future laboratory search of R-2 materials. Specifically, we (i) classify different prototype layered structures that have been broadly studied in the literature in terms of their expected R-2 behavior, including the Bi2Se3-structure type (a prototype topological insulator), MoS2-structure type (a prototype valleytronic compound) and LaBiOS2-structure type (a host of superconductivity upon doping); (ii) formulate the properties that ideal R-2 compounds should have in terms of combination of their global unit cell symmetries with specific point group symmetries of their constituent sectors; (iii) use first-principles band theory to search for compounds from the prototype family of LaOBiS2-type structures that satisfy these R-2 design metrics. We initially consider both stable and hypothetical compounds to establish an understanding of trends of R-2 with composition, and then indicate the predictions that are expected to be stable and synthesizable. We predict large spin splittings (up to ~ 200 meV for holes in LaOBiTe2) as well as surface Rashba states. Experimental testing of such predictions is called for.

cond-mat.mtrl-sci↗

arXiv2015

Intrinsic circular polarization in centrosymmetric stacks of transition-metal dichalcogenides

The circular polarization (CP) that the photoluminescence inherits from the excitation source in n monolayers of transition-metal dichalcogenides (MX2)n has been previously explained as a special feature of odd values of n, where the inversion symmetry is absent. This valley polarization effect results from the fact that in the absence of inversion, charge carriers in different band valleys could be selectively excited by different circular polarized light. Such restriction to non-centrosymmetric systems poses a limitation on the material selection for achieving CP. Although several experiments observed CP in centrosymmetric MX2 systems e.g., for bilayer in MX2, they were dismissed as being due to some extrinsic sample irregularities. Here we show that also for n = even where inversion symmetry is present and valley polarization physics is strictly absent, such intrinsic selectivity in CP is to be expected on the basis of fundamental spin-orbit physics. First-principles calculations of CP predict significant polarization for n = 2 bilayers: from 69% in MoS2 to 93% in WS2. This realization could broaden the range of materials to be considered as CP sources.

cond-mat.mtrl-sci↗

arXiv2014

Prediction and accelerated laboratory discovery of previously unknown 18-electron ABX compounds

Chemists and material scientists have often focused on the properties of previously reported compounds, leaving out numerous unreported but chemically plausible compounds that could have interesting properties. For example, the 18-valence electron ABX family of compounds includes the half-Heusler subgroup, and features examples of topological insulators, thermoelectrics and superconductors, but only 83 out of 483 of these possible compounds have been made. Using first-principles thermodynamics we have examined the theoretical stability of 400 unreported members and predict that 54 should be stable. 15 previously missing materials, now predicted stable, were grown in this study; X-ray studies agreed with the predicted crystal structure in all 15 cases. Among the characterized properties of the missing compounds are potential transparent conductors, thermoelectric materials and topological semimetals. This integrated process-prediction of functionality in unreported compounds followed by laboratory synthesis and characterization-could be a route to the systematic discovery of hitherto missing, realizable functional materials.

cond-mat.mtrl-sci↗

arXiv2014

Design and discovery of a novel Half-Heusler transparent hole conductor made of all-metallic heavy elements

Metallic conductors that are optically transparent represent a rare breed of generally contraindicated physical properties that are nevertheless critically needed for application where both functionalities are crucial. Such rare materials have traditionally been searched in the general chemical neighborhood of compounds containing metal oxides, expected to be wide gap insulators that might be doped to induce conductivity.Focusing on the family of 18 valence electron ABX compounds we have searched theoretically for the ability of the compound's electronic structure to simultaneously lead to optical transparency, in parallel with the ability of its intrinsic defect structures to produce uncompensated free holes.This led to the prediction of a stable, never before synthesized TaIrGe compound made of all-metal heavy atom compound as the "best of class" from the V-IX-IV group. Laboratory synthesis then found it to be stable in the predicted crystal structure and p-type transparent conductor with measured strong direct absorption of 3.36 eV and remarkably high (albeit not predicted) hole mobility of 2730 cm2/Vs at room temperature. This methodology opens the way to future searches of transparent conductors in unexpected chemical groups.

cond-mat.mtrl-sci↗

arXiv2014

Supercoupling between heavy-hole and light-hole states in self-assembled quantum dots

Spintronics, quantum computing and quantum communication science utilizing cubic semiconductors rely largely on the properties of the hole states, composed of light and heavy hole wavefunction components. The admixture of light-hole (LH) into ground hole state predominately by the heavy hole (HH) would induce unique features of LH in optical transitions, spin relaxation, and spin polarization. We point to an unexpected source of HH-LH mixing in quantum dots, arguing that in contrast with current models the mixing does not reflect the strain between the dot and its matrix and does not scale inversely with the energy splitting between the bulk HH and LH states. Instead, we show via atomistic pseudopotential calculations on a range of strained and unstrained dots of different symmetries that the HH-LH mixing is enabled by the presence in the QD of a dense ladder of intermediate states between the HH and LH states which amplifies and propagates this interaction and leads to "supercoupling" (analogous to super-exchange in magnetism). This explains a number of outstanding puzzles regarding the surprising large coupling seen in unstrained QD (GaAs/AlAs) of ideal shapes and the surprising fact that in strained QD (InAs/GaAs) the coupling is very strong despite the fact that the 12-fold increase in bulk HH-LH splitting overrides the ~4 fold enhancement of the coupling matrix element by strain in comparison with unstrained GaAs QDs.

cond-mat.mes-hall↗

arXiv2014

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

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.

cond-mat.mtrl-sci↗

arXiv2014

Split Dirac cones in HgTe/CdTe quantum wells due to symmetry-enforced level anticrossing at interfaces

We describe the fine structure of Dirac states in HgTe/CdHgTe quantum wells of critical and close-to-critical thickness and demonstrate the formation of an anticrossing gap between the tips of the Dirac cones driven by interface inversion asymmetry. By combining symmetry analysis, atomistic calculations, and k-p theory with interface terms, we obtain a quantitative description of the energy spectrum and extract the interface mixing coefficient. The zero-magnetic-field splitting of Dirac cones can be experimentally revealed in studying magnetotransport phenomena, cyclotron resonance, Raman scattering, or THz radiation absorption.

cond-mat.mes-hall↗

arXiv2014

Cation ordering induced polarization enhancement for $PbTiO_3$-$SrTiO_3$ ferroelectric-dielectric superlattices

In this paper, an efficient computational material design approach (cluster expansion) is employed for the ferroelectric $PbTiO_3$/$SrTiO_3$ system. Via exploring a conguration space including over $3 \times 10^6$ candidates, two special cation ordered configurations: either perfect or mixed 1/1 (101) superlattice, are identified with the mostly enhanced ferroelectric polarization by up to about 100% in comparison with the (001) superlattice. Analyzing these two special configurations reveals the exotic couplings between the antiferrodistortive distortion (AFD) and ferroelectric polarization (FE) modes, e.g., tilting along x or y axis and polarization in z direction, as the origin for the best polarization property in this system. The identified cation ordering motifs and the exotic AFD-FE couplings could provide fresh ideas to design multifunctional perovskite heterostructures.

cond-mat.mtrl-sci↗

arXiv2014

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

The discovery of conducting two-dimensional electron gas (2DEG) and magnetism at the interface between insulating nonmagnetic oxides, as exemplified by the polar LaAlO3 and nonpolar 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. Using first principles defect calculations, we reveal a unifying polarity-induced defect mechanism for both conductivity and magnetism at polar-nonpolar interfaces of nonmagnetic insulating oxides. 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. It turns out that the 2DEG originates from those spontaneously formed surface donor defects (oxygen vacancy), but the density of 2DEG is controlled by the interfacial anti-site acceptor defects (Al-on-Ti). The interface magnetism is found to originate from the un-ionized deep Ti-on-Al anti-site donor defects within the LaAlO3 side near the interface. Our results suggest practical design guidelines for inducing and controlling both 2DEG and magnetism at polar-nonpolar oxide interfaces.

cond-mat.mtrl-sci↗

arXiv2014

Hidden spin polarization in inversion-symmetric bulk crystals

Spin-orbit coupling (SOC) can induce spin polarization in nonmagnetic 3D crystals when the inversion symmetry is broken, as manifested by the bulk Rashba (R-1) and Dresselhaus (D-1) effects. We determine that these spin polarization effects originate fundamentally from specific atomic site asymmetries, rather than from the generally accepted asymmetry of the crystal space-group. This understanding leads to the recognition that a previously overlooked hidden form of spin polarization should exist in centrosymmetric materials. Although all energy bands must be doubly degenerate in centrosymmetric materials, we find that the two components of such doubly degenerate bands could have opposite polarizations each spatially localized on one of the two separate sectors forming the inversion partners. We demonstrate such hidden spin polarizations in centrosymmetric crystals (denoted as R-2 and D-2) by first-principles calculations. This new understanding could considerably broaden the range of currently useful spintronic materials and enable control of spin polarization via operations on atomic scale.

cond-mat.mtrl-sci↗

arXiv2013

Statistical Properties of Exciton Fine Structure Splittings and Polarization Angles in Quantum Dot Ensembles

We propose an effective model to describe the statistical properties of exciton fine structure splitting (FSS) and polarization angle of quantum dot ensembles (QDEs). We derive the distributions of FSS and polarization angle for QDEs and show that their statistical features can be fully characterized using at most three independent measurable parameters. The effective model is confirmed using atomistic pseudopotential calculations as well as experimental measurements for several rather different QDEs. The model naturally addresses three fundamental questions that are frequently encountered in theories and experiments: (I) Why the probability of finding QDs with vanishing FSS is generally very small? (II) Why FSS and polarization angle differ dramatically from QD to QD? and (III) Is there any direct connection between FSS, optical polarization and the morphology of QDs? The answers to these fundamental questions yield a completely new physical picture for understanding optical properties of QDEs.

cond-mat.mes-hall↗

arXiv2013

Genetic Design of Enhanced Valley Splitting towards a Spin Qubit in Silicon

Electronic spins in Silicon (Si) are rising contenders for qubits -- the logical unit of quantum computation-- owing to its outstanding spin coherence properties and compatibility to standard electronics. A remarkable limitation for spin quantum computing in Si hosts is the orbital degeneracy of this material's conduction band, preventing the spin-1/2 states from being an isolated two-level system. So far available samples of Si quantum wells cladded by Ge-Si alloy barriers provide relatively small valley splitting (VS), with the order of 1 meV or less, degrading the fidelity of qubits encoded in spin "up" and "down" states in Si. Here, based on an atomically resolved pseudopotential theory, we demonstrate that ordered Ge-Si layered barriers confining a Si slab can be harnessed to enhance the VS in the active Si region by up to one order of magnitude compared to the random alloy barriers adopted so far. A biologically inspired genetic-algorithm search is employed to identify magic Ge/Si layer sequences of the superlattice barriers that isolate the electron ground state in a single valley composition with VS as large as ~9 meV. The enhanced VS is preserved with the reasonable inter-layer mixing between different species, and is interestingly "protected" even if some larger mixing occurs. Implementation of the optimized layer sequences of barriers, within reach of modern superlattice growth techniques, overcomes in a practical systematic way the main current limitations related to the orbital degeneracy, thus providing a roadmap for reliable spin-only quantum computing in Si.

cond-mat.mes-hall↗

arXiv2011

False-positive and False-negative assignments of Topological Insulators in Density-Functional Theory and Hybrids

Density-functional Theory (DFT) approaches have recently been used to judge the topological order of various materials despite its well-known band gap underestimation. Use of the more accurate quasi-particle GW approach reveals here few cases where DFT identifications are false-positive, possibly misguiding experimental searches of materials that are topological insulators (TI) in DFT but not expected to be TI in reality. We also present the case of false-positive due to the incorrect choice of crystal structures and adress the relevancy of such choice of crystal structure with respect to the ground state one and thermodynamical instability with respect to binary competing phases. We conclude that it is then necessary to consider both the correct ground state crystal structure and the correct Hamiltonian in order to predict new TI.

cond-mat.mtrl-sci↗

arXiv2010

Manybody GW calculation of the oxygen vacancy in ZnO

Density functional theory (DFT) calculations of defect levels in semiconductors based on approximate functionals are subject to considerable uncertainties, in particular due to inaccurate band gap energies. Testing previous correction methods by many-body GW calculations for the O vacancy in ZnO, we find that: (i) The GW quasi-particle shifts of the VO defect states increase the spitting between occupied and unoccupied states due to self-interaction correction, and do not reflect the conduction versus valence band character. (ii) The GW quasi-particle energies of charged defect states require important corrections for supercell finite size effects. (iii) The GW results are robust with respect to the choice of the underlying DFT or hybrid-DFT functional, and the (2+/0) donor transition lies below mid-gap, close to our previous prediction employing rigid band edge shifts.

cond-mat.mtrl-sci↗

arXiv2009

Prediction of large linear-in-k spin splitting for holes in the 2D GaAs/AlAs system

The spin-orbit interaction generally leads to spin splitting (SS) of electron and hole energy states in solids, a splitting that is characterized by a scaling with the wavevector $\bf k$. Whereas for {\it 3D bulk zincblende} solids the electron (heavy hole) SS exhibits a cubic (linear) scaling with $k$, in {\it 2D quantum-wells} the electron (heavy hole) SS is currently believed to have a mostly linear (cubic) scaling. Such expectations are based on using a small 3D envelope function basis set to describe 2D physics. By treating instead the 2D system explicitly in a multi-band many-body approach we discover a large linear scaling of hole states in 2D. This scaling emerges from hole bands coupling that would be unsuspected by the standard model that judges coupling by energy proximity. This discovery of a linear Dresselhaus k-scaling for holes in 2D implies a different understanding of hole-physics in low-dimensions.

cond-mat.mtrl-sci↗