Xiuwen Zhang

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

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.

Hidden spin polarization in inversion-symmetric bulk crystals

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.

False-positive and false-negative assignments of topological insulators in density functional theory and hybrids

Density-functional theory (DFT) approaches have been used recently to judge the topological order of various materials despite DFT’s well-known band-gap underestimation. Use of the more accurate quasi-particle GW approach reveals few cases where DFT identifications are false positive, which can possibly misguide experimental searches for materials that are topological insulators (TIs) in DFT but not expected to be TIs in reality. We also present the case of false positives due to the incorrect choice of crystal structures and address the relevance of 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 necessary to consider both the correct ground-state crystal structure and the correct Hamiltonian in order to predict new TIs.

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

Topological insulators are novel macroscopic quantum-mechanical phase of matter, which hold promise for realizing some of the most exotic particles in physics as well as application towards spintronics and quantum computation. In all the known topological insulators, strong spin-orbit coupling is critical for the generation of the protected massless surface states. Consequently, a complete description of the Dirac state should include both the spin and orbital (spatial) parts of the wavefunction. For the family of materials with a single Dirac cone, theories and experiments agree qualitatively, showing the topological state has a chiral spin texture that changes handedness across the Dirac point (DP), but they differ quantitatively on how the spin is polarized. Limited existing theoretical ideas predict chiral local orbital angular momentum on the two sides of the DP. However, there have been neither direct measurements nor calculations identifying the global symmetry of the spatial wavefunction. Here we present the first results from angle-resolved photoemission experiment and first-principles calculation that both show, counter to current predictions, the in-plane orbital wavefunctions for the surface states of Bi2Se3 are asymmetric relative to the DP, switching from being tangential to the k-space constant energy surfaces above DP, to being radial to them below the DP. Because the orbital texture switch occurs exactly at the DP this effect should be intrinsic to the topological physics, constituting an essential yet missing aspect in the description of the topological Dirac state. Our results also indicate that the spin texture may be more complex than previously reported, helping to reconcile earlier conflicting spin resolved measurements.

Theoretical Prediction and Experimental Realization of New Stable Inorganic Materials Using the Inverse Design Approach

Discovery of new materials is important for all fields of chemistry. Yet, existing compilations of all known ternary inorganic solids still miss many possible combinations. Here, we present an example of accelerated discovery of the missing materials using the inverse design approach, which couples predictive first-principles theoretical calculations with combinatorial and traditional experimental synthesis and characterization. The compounds in focus belong to the equiatomic (1:1:1) ABX family of ternary materials with 18 valence electrons per formula unit. Of the 45 possible V-IX-IV compounds, 29 are missing. Theoretical screening of their thermodynamic stability revealed eight new stable 1:1:1 compounds, including TaCoSn. Experimental synthesis of TaCoSn, the first ternary in the Ta-Co-Sn system, confirmed its predicted zincblende-derived crystal structure. These results demonstrate how discovery of new materials can be accelerated by the combination of high-throughput theoretical and experimental methods. Despite being made of three metallic elements, TaCoSn is predicted and explained to be a semiconductor. The band gap of this material is difficult to measure experimentally, probably due to a high concentration of interstitial cobalt defects.

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

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 cm(2) V(-1) s(-1) at room temperature. This methodology opens the way to future searches of transparent conductors in unexpected chemical groups.

Strong optical absorption in CuTaN2 nitride delafossite

We report on synthesis, stability, electronic structure and optical properties of CuTaN2 with the delafossite crystal structure and its potential use as an absorber for solar energy conversion applications. According to theoretical first-principles calculations, the formation enthalpy of CuTaN2 is negative (−0.66 eV per atom), but this material is metastable with respect to decomposition into Cu, Ta3N5 and N2. Nevertheless, the experimental thermal stability limit of single phase CuTaN2 powders synthesized using an ion exchange method is 250 °C in ambient atmosphere, according to combined temperature-dependent X-ray diffraction and thermo-gravimetric analyses. Electronic structure of the CuTaN2 is different compared to that of CuAlO2, in particular the band gap of this nitride delafossite (1.3 eV) calculated using HSE06+G0W0 is much smaller than the band gap of the oxide delafossite. The onset of optical absorption onset of CuTaN2 at 1.5 eV determined from experimental diffuse reflectance measurements is consistent with the theoretical 1.4 eV optical band gap and large calculated absorption coefficient (>105 cm−1 above 1.5 eV) determined from time-dependent HSE06 calculations corrected by a scissors operator. The significance of our findings is that optical properties of CuTaN2 are nearly optimal for photovoltaic energy conversion.

Structure prediction and targeted synthesis: a new NanN2 diazenide crystalline structure

Significant progress in theoretical and computational techniques for predicting stable crystal structures has recently begun to stimulate targeted synthesis of such predicted structures. Using a global space-group optimization (GSGO) approach that locates ground-state structures and stable stoichiometries from first-principles energy functionals by objectively starting from randomly selected lattice vectors and random atomic positions, we predict the first alkali diazenide compound Na(n)N(2), manifesting homopolar N-N bonds. The previously predicted Na(3)N structure manifests only heteropolar Na-N bonds and has positive formation enthalpy. It was calculated based on local Hartree-Fock relaxation of a fixed-structure type (Li(3)P-type) found by searching an electrostatic point-ion model. Synthesis attempts of this positive ΔH compound using activated nitrogen yielded another structure (anti-ReO(3)-type). The currently predicted (negative formation enthalpy) diazenide Na(2)N(2) completes the series of previously known BaN(2) and SrN(2) diazenides where the metal sublattice transfers charge into the empty N(2) Π(g) orbital. This points to a new class of alkali nitrides with fundamentally different bonding, i.e., homopolar rather than heteropolar bonds and, at the same time, illustrates some of the crucial subtleties and pitfalls involved in structure predictions versus planned synthesis. Attempts at synthesis of the stable Na(2)N(2) predicted here will be interesting.

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.

Polytypism inLaOBiS2-type compounds based on different three-dimensional stacking sequences of two-dimensionalBiS2layers

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 separated from other phases (with a space group P4/nmm). Using first-principles total energy and stability calculations we find that contrary to this view the three-dimensional structure of this important family of compounds represents instead a family of energetically closely spaced modifications differing by the layer sequences and orientations. We find that the local Bi-S distortion, mainly induced by the interaction between the medium LaO and the BiS2 layers and the electron hybridization in the BiS2 layer, leads to three polytypes of LaOBiS2 according to the different stacking pattern of distorted BiS2 layers. Although the energy difference between the polytypes of LaOBiS2 is relatively small (within several meV/f.u.), they have obvious impact on the presence of inversion symmetry, which is reflected by the band structure and spin polarization. In addition, different choices of the medium atoms (replacing La) or active atoms (BiS2) manifest different ground state polytypes. One can thus tune the distortion and the ground state by substituting covalence atoms in LaOBiS2-family.