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

Anonymous — Wed, 24 Jun 2015 06:00:00 +0000

Design and discovery of a novel half-Heusler transparent hole conductor made of all-metallic heavy elements Anonymous (not verified) Wed, 06/24/2015 - 00:00

Transparent conductors combine two generally contradictory physical properties, but there are numerous applications where both functionalities are crucial. Previous searches focused on doping wide-gap metal oxides. Focusing instead on the family of 18 valence electron ternary ABX compounds that consist of elements A, B and X in 1:1:1 stoichiometry, we search theoretically for electronic structures that simultaneously lead to optical transparency while accommodating intrinsic defect structures that produce uncompensated free holes. This leads to the prediction of a stable, never before synthesized TaIrGe compound made of all-metal heavy atom compound. Laboratory synthesis then found it to be stable in the predicted crystal structure and p-type transparent conductor with a strong optical absorption peak at 3.36 eV and remarkably high hole mobility of 2,730 cm2 V−1 s−1 at room temperature. This methodology opens the way to future searches of transparent conductors in unexpected chemical groups.

F. Yan, X. Zhang, Yonggang Yu, L.Yu, A. Nagaraja, T.O. Mason, and Alex Zunger “ Design and discovery of a novel Half‐Heusler transparent hole conductor made of all metallic heavy elements” Nature Communication 6, 7308 (2015). (PDF)

Nature Communication 6, 7308 (2015)

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

Anonymous — Tue, 24 Mar 2015 06:00:00 +0000

Prediction and accelerated laboratory discovery of previously unknown 18-electron ABX compounds Anonymous (not verified) Tue, 03/24/2015 - 00:00

Chemists and material scientists have often focused on the properties of previously reported compounds, but neglect numerous unreported but chemically plausible compounds that could have interesting properties. For example, the 18-valence electron ABX family of compounds features examples of topological insulators, thermoelectrics and piezoelectrics, but only 83 out of 483 of these possible compounds have been made. Using first-principles thermodynamics we examined the theoretical stability of the 400 unreported members and predict that 54 should be stable. Of those previously unreported ‘missing’ materials now predicted to be stable, 15 were grown in this study; X-ray studies agreed with the predicted crystal structure in all 15 cases. Among the predicted and 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.

R. Gautier, X. Zhang, L. Hu, L. Yu, Y. Lin, T. O. L. Sunde, D. Chon, K. R. Poeppelmeier, A. Zunger,"Prediction and accelerated laboratory discovery of previously unknown 18-electron ABX compounds", Nature Chemistry 7, 308-316 (2015).(PDF)

Nature Chemistry 7, 308-316 (2015)

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Intrinsic Circular Polarization in Centrosymmetric Stacks of Transition-Metal Dichalcogenide Compounds

Anonymous — Fri, 27 Feb 2015 07:00:00 +0000

Intrinsic Circular Polarization in Centrosymmetric Stacks of Transition-Metal Dichalcogenide Compounds Anonymous (not verified) Fri, 02/27/2015 - 00:00

The circular polarization (CP) that the photoluminescence inherits from the excitation source in nmonolayers 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 symmetry, charge carriers in different band valleys could be selectively excited by different circular polarized light. Although several experiments observed CP in centrosymmetric MX2 systems, e.g., for bilayer 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.

Q. Liu, X. Zhang, A. Zunger,"Intrinsic Circular Polarization in Centrosymmetric Stacks of Transition- Metal Dichalcogenide Compounds," Physical Review Letters 114, 087402 (2015).(PDF)

Physical Review Letters 114, 087402 (2015)

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Switching a Normal Insulator into a Topological Insulator via Electric Field with Application to Phosphorene

Anonymous — Wed, 21 Jan 2015 07:00:00 +0000

Switching a Normal Insulator into a Topological Insulator via Electric Field with Application to Phosphorene Anonymous (not verified) Wed, 01/21/2015 - 00:00

The study of topological insulators has generally involved search of materials that have this property as an innate quality, distinct from normal insulators. Here we focus on the possibility of converting a normal insulator into a topological one by application of an external electric field that shifts different bands by different energies and induces a specific band inversion, which leads to a topological state. Phosphorene is a two-dimensional (2D) material that can be isolated through mechanical exfoliation from layered black phosphorus, but unlike graphene and silicene, single-layer phosphorene has a large band gap (1.5–2.2 eV). Thus, it was unsuspected to exhibit band inversion and the ensuing topological insulator behavior. Using first-principles calculations with applied perpendicular electric field F⊥ on few-layer phosphorene we predict a continuous transition from the normal insulator to a topological insulator and eventually to a metal as a function of F⊥. The tuning of topological behavior with electric field would lead to spin-separated, gapless edge states, that is, quantum spin Hall effect. This finding opens the possibility of converting normal insulating materials into topological ones via electric field and making a multifunctional “field effect topological transistor” that could manipulate simultaneously both spin and charge carrier. We use our results to formulate some design principles for looking for other 2D materials that could have such an electrical-induced topological transition.

Q. Liu, X. Zhang, L. B. Abdalla, A. Fazzio, A. Zunger,"Switching a normal insulator into a topological insulator via electric field with application to phosphorene," Nanoletters 15, 1222-1228 (2015).(PDF)

Nano Letters 15, 1222 (2015)

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Evolution of Electronic Structure as a Function of Layer Thickness in Group-VIB Transition Metal Dichalcogenides: Emergence of Localization Prototypes

Anonymous — Tue, 06 Jan 2015 07:00:00 +0000

Evolution of Electronic Structure as a Function of Layer Thickness in Group-VIB Transition Metal Dichalcogenides: Emergence of Localization Prototypes Anonymous (not verified) Tue, 01/06/2015 - 00:00

Layered group-VIB transition metal dichalcogenides (with the formula of MX2) are known to show a transition from an indirect band gap in the thick n-monolayer stack (MX2)n to a direct band gap at the n = 1 monolayer limit, thus converting the system into an optically active material suitable for a variety of optoelectronic applications. The origin of this transition has been attributed predominantly to quantum confinement effect at reduced n. Our analysis of the evolution of band-edge energies and wave functions as a function of n using ab initio density functional calculations including the long-range dispersion interaction reveals (i) the indirect-to-direct band gap transformation is triggered not only by (kinetic-energy controlled) quantum confinement but also by (potential-energy controlled) band repulsion and localization. On its own, neither of the two effects can explain by itself the energy evolution of the band-edge states relevant to the transformation; (ii) when n decreased, there emerge distinct regimes with characteristic localization prototypes of band-edge states deciding the optical response of the system. They are distinguished by the real-space direct/indirect in combination with momentum-space direct/indirect nature of electron and hole states and give rise to distinct types of charge distribution of the photoexcited carriers that control excitonic behaviors; (iii) the various regimes associated with different localization prototypes are predicted to change with modification of cations and anions in the complete MX2 (M = Cr, Mo, W and X = S, Se, Te) series. These results offer new insight into understanding the excitonic properties (e.g., binding energy, lifetime etc.) of multiple layered MX2 and their heterostructures.

L. Zhang, A. Zunger,"Evolution of electronic structure as a function of layer thickness in group-VIB transition metal dichalcogenides: emergence of localization prototypes," Nanoletters 15, 949-957(2015).(PDF)

Nano Letters 15, 949 (2015)

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Reinterpretation of the Expected Electronic Density of States of Semiconductor Nanowires

Anonymous — Sun, 30 Nov 2014 07:00:00 +0000

Reinterpretation of the Expected Electronic Density of States of Semiconductor Nanowires Anonymous (not verified) Sun, 11/30/2014 - 00:00

One-dimensional semiconductor nanowires hold the promise for various optoelectronic applications since they combine the advantages of quantized in-plane energy levels (as in zero-dimensional quantum dots) with a continuous energy spectrum along the growth direction (as in three-dimensional bulk materials). This dual characteristic is reflected in the density of states (DOS), which is thus the key quantity describing the electronic structures of nanowires, central to the analysis of electronic transport and spectroscopy. By comparing the DOS derived from the widely used “standard model”, the effective mass approximation (EMA) in single parabolic band mode, with that from direct atomistic pseudopotential theory calculations for GaAs and InAs nanowires, we uncover significant qualitative and quantitative shortcomings of the standard description. In the EMA description the nanowire DOS is rendered as a series of sharply rising peaks having slowly decaying tails, with characteristic peak height and spacing, all being classifiable in the language of atomic orbital momenta 1S, 1P, 1D, etc. Herein we find in the thinner nanowires that the picture changes significantly in that not only does the profile of each DOS peak lose its pronounced asymmetry, with significant changes in peak width, height, and spacing, but also the origin of the high-energy peaks changes fundamentally: below some critical diameter, the region of atomic orbital momentum classified states is occupied by a new set of DOS peaks folded-in from other non-Γ-valleys. We describe explicitly how distinct physical effects beyond the conventional EMA model contribute to these realistic DOS features. These results represent a significant step toward understanding the intriguing electronic structure of nanowires reflecting the coexistence of discrete and continuum states. Experimental examinations of the predicted novel DOS features are called for.

J. Wang, J.W. Luo, L. Zhang, A. Zunger,"Reinterpretation of the Expected Electronic Density of States of Semiconductor Nanowires," Nanoletters 15, 88-95 (2015).(PDF)

Nano Letters 15, 88 (2015)

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

Anonymous — Mon, 13 Oct 2014 06:00:00 +0000

A polarity-induced defect mechanism for conductivity and magnetism at polar–nonpolar oxide interfaces Anonymous (not verified) Mon, 10/13/2014 - 00:00

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.

L. Yu, A. Zunger,"A polarity-induced defect mechanism for conductivity and magnetism at polar-nonpolar oxide interfaces," Nature Communications 5, 5118 (2014).(PDF)

Nature Communications 5, 5118 (2014)

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Hidden spin polarization in centrosymmetric materials

Anonymous — Sun, 13 Apr 2014 06:00:00 +0000

Hidden spin polarization in centrosymmetric materials Anonymous (not verified) Sun, 04/13/2014 - 00:00

Spin–orbit coupling can induce spin polarization in nonmagnetic 3D crystals when the inversion symmetry is broken, as manifested by the bulk Rashba and Dresselhaus effects. We establish that these spin-polarization effects originate fundamentally from specific atomic site asymmetries, rather than, as generally accepted, from the 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 crystals. 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 particular centrosymmetric crystals by first-principles calculations. This new understanding could considerably broaden the range of currently useful spintronic materials and enable the control of spin polarization by means of operations on the atomic scale.

X. Zhang, Q. Liu, J.W. Luo, A. J. Freeman, A. Zunger,"Hidden spin polarization in inversion-symmetric bulk crystals," Nature Physics 10, 387-393 (2014).(PDF)

Nature Physics 10, 387 (2014)

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Genetic design of enhanced valley splitting towards a spin qubit in silicon

Anonymous — Mon, 09 Sep 2013 06:00:00 +0000

Genetic design of enhanced valley splitting towards a spin qubit in silicon Anonymous (not verified) Mon, 09/09/2013 - 00:00

The long spin coherence time and microelectronics compatibility of Si makes it an attractive material for realizing solid-state qubits. Unfortunately, the orbital (valley) degeneracy of the conduction band of bulk Si makes it difficult to isolate individual two-level spin-1/2 states, limiting their development. This degeneracy is lifted within Si quantum wells clad between Ge-Si alloy barrier layers, but the magnitude of the valley splittings achieved so far is small—of the order of 1 meV or less—degrading the fidelity of information stored within such a qubit. Here we combine an atomistic pseudopotential theory with a genetic search algorithm to optimize the structure of layered-Ge/Si-clad Si quantum wells to improve this splitting. We identify an optimal sequence of multiple Ge/Si barrier layers that more effectively isolates the electron ground state of a Si quantum well and increases the valley splitting by an order of magnitude, to ~9meV.

L. Zhang, J.W. Luo, A. Saraiva, B. Koiller, A. Zunger,"Genetic design of enhanced valley splitting towards a spin qubit in silicon," Nature Communications 4, 2396 (2013).(PDF)

Nature Communications 4, 2396 (2013)

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Mapping the orbital wavefunction of the surface states in three-dimensional topological insulators

Anonymous — Sun, 21 Jul 2013 06:00:00 +0000

Mapping the orbital wavefunction of the surface states in three-dimensional topological insulators Anonymous (not verified) Sun, 07/21/2013 - 00:00

Understanding the structure of the wavefunction is essential for depicting the surface states of a topological insulator. Owing to the inherent strong spin–orbit coupling, the conventional hand-waving picture of the Dirac surface state with a single chiral spin texture is incomplete, as this ignores the orbital components of the Dirac wavefunction and their coupling to the spin textures. Here, by combining orbital-selective angle-resolved photoemission experiments and first-principles calculations, we deconvolve the in-plane and out-of-plane p-orbital components of the Dirac wavefunction. The in-plane orbital wavefunction is asymmetric relative to the Dirac point. It is predominantly tangential (radial) to the k-space constant energy surfaces above (below) the Dirac point. This orbital texture switch occurs exactly at the Dirac point, and therefore should be intrinsic to the topological physics. Our results imply that the Dirac wavefunction has a spin–orbital texture—a superposition of orbital wavefunctions coupled with the corresponding spin textures.

Yue Cao, J. A.Waugh, X-W. Zhang, J-W. Luo, Q.Wang, T. J. Reber, S. K. Mo, Z. Xu,A. Yang, J. Schneeloch, G. D. Gu, M. Brahlek, N. Bansal, S. Oh, A. Zunger and D. S. Dessau" Mapping the orbital wavefunction of the surface states in three-dimensional topological insulators" Nature Physics 9 , 499 (2013). (PDF)

Nature Physics 9 , 499 (2013)

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