Showing papers by "Alex Zunger published in 2016"

Cu-In Halide Perovskite solar absorbers

Abstract: The long-term chemical instability and the presence of toxic Pb in otherwise stellar solar absorber APbX$_{3}$ have hindered their large-scale commercialization. Previously explored ways to achieve Pb-free halide perovskites involved replacing Pb$^{2+}$ with other similar M$^{2+}$ cations in ns$^2$ electron configuration, e.g., Sn$^{2+}$ or by Bi$^{3+}$ (plus Ag$^+$), but unfortunately this showed either poor stability (M = Sn) or weakly absorbing oversized indirect gaps (M = Bi), prompting concerns that perhaps stability and good optoelectronic properties might be contraindicated. Herein, we exploit the electronic structure underpinning of classic Cu[In,Ga]Se$_{2}$ (CIGS) chalcopyrite solar absorbers to design Pb-free halide perovskites by transmuting 2Pb to the pair [B$^{IB}$ + C$^{III}$]. The resulting group of double perovskites with formula A$_2$BCX$_6$ (A = K, Rb, Cs; B = Cu, Ag; C = Ga, In; X = Cl, Br, I) benefits from the ionic, yet narrow-gap character of halide perovskites, and at the same time borrows the advantage of the strong and rapidly rising Cu(d)/Se(p) $\rightarrow$ Ga/In(s/p) valence-to-conduction-band absorption spectra known from CIGS. This constitutes a new group of CuIn-based Halide Perovskite (CIHP). Our first-principles calculations guided by such design principles indicate that the CIHPs class has members with clear thermodynamic stability, showing rather strong direct-gap optical transitions, and manifesting a wide-range of tunable gap values (from zero to about 2.5 eV) and combination of light electron and heavy-light hole effective masses. Materials screening of candidate CHIPs then identifies the best-of-class Rb$_2$[CuIn]Cl$_6$, Rb$_2$[AgIn]Br$_6$ and Cs$_2$[AgIn]Br$_6$, having direct band gaps of 1.36, 1.46 and 1.50 eV, and a theoretical spectroscopic limited maximal efficiency comparable to chalcopyrites and CH$_3$NH$_3$PbI$_3$.

Quasi-Direct Optical Transitions in Silicon Nanocrystals with Intensity Exceeding the Bulk

Abstract: Comparison of the measured absolute absorption cross section on a per Si atom basis of plasma-synthesized Si nanocrystals (NCs) with the absorption of bulk crystalline Si shows that while near the band edge the NC absorption is weaker than the bulk, yet above ∼2.2 eV the NC absorbs up to 5 times more than the bulk. Using atomistic screened pseudopotential calculations we show that this enhancement arises from interface-induced scattering that enhances the quasi-direct, zero-phonon transitions by mixing direct Γ-like wave function character into the indirect X-like conduction band states, as well as from space confinement that broadens the distribution of wave functions in k-space. The absorption enhancement factor increases exponentially with decreasing NC size and is correlated with the exponentially increasing direct Γ-like wave function character mixed into the NC conduction states. This observation and its theoretical understanding could lead to engineering of Si and other indirect band gap NC materia...

Single-dot absorption spectroscopy and theory of silicon nanocrystals

Abstract: Photoluminescence excitation measurements have been performed on single, unstrained oxide-embedded Si nanocrystals. Having overcome the challenge of detecting weak emission, we observe four broad p ...

Polytypism in LaOBi S 2 -type compounds based on different three-dimensional stacking sequences of two-dimensional Bi S 2 layers

Abstract: $\mathrm{LaOBi}{\mathrm{S}}_{2}$-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 $\mathrm{LaOBi}{\mathrm{S}}_{2}$-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 $P4$/nmm structure to $\mathrm{LaOBi}{\mathrm{S}}_{2}$ is incorrect. 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 $\mathrm{LaOBi}{\mathrm{S}}_{2}$ with different stacking patterns of the distorted $\mathrm{Bi}{\mathrm{S}}_{2}$ layers. The energy difference between the polytypes of $\mathrm{LaOBi}{\mathrm{S}}_{2}$ is merely \ensuremath{\sim}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 ($\mathrm{Bi}{\mathrm{S}}_{2}$) could also manifest different ground-state structures. One can thus tune the distortion and the ground state by pressure or by substituting covalence atoms in the $\mathrm{LaOBi}{\mathrm{S}}_{2}$ family.

The enabling electronic motif for topological insulation in ABO3 perovskites and its structural stability

Abstract: Stable oxide topological insulators (TIs) that could bring together the traditional oxide functionalities with the dissipationless surface states of TIs have been sought for years but none was found. Yet, heavier chalcogenides (selenides, tellurides) were readily found to be TIs. We clarify here the basic contradiction between TI-ness and stability which is maximal for oxides, and trace the basic design principles necessary to identify the window of opportunity of stable TIs. We first identify the electronic motif that can achieve topological band inversion ("topological gene") in ABO3 as being a lone-pair electron-rich B atom (e.g. Te, I, Bi) at the octahedral site. We then illustrate that poorly screened oxide systems with large inversion energies can undergo energy-lowering atomic distortions that remove the band inversion. We identify the coexistence windows of TI functionality and structure stability for different pressures and find that the common cubic ABO3 structures have inversion energies lying outside this coexistence window at zero pressure but could be moved into the coexistence window at moderate pressures. Our study demonstrates the interplay between topological band inversion and structural stability and traces the basic principles needed to design stable oxide topological insulators at ambient pressures.

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

Abstract: 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.

Transforming Common III–V and II–VI Semiconductor Compounds into Topological Heterostructures: The Case of CdTe/InSb Superlattices

Abstract: Currently, known topological insulators (TIs) are limited to narrow gap compounds incorporating heavy elements, thus severely limiting the material pool available for such applications. It is shown via first-principle calculations that 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. The relationship between the interfacial stability and the topological transition is revealed, 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 1.7–9.5 meV A–2, 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 broaden the current, rather restricted repertoire of functionalities available from individual compounds by creating next-generation superstructured functional materials.

Strong Absorption Enhancement in Si Nanorods.

Abstract: We report two orders of magnitude stronger absorption in silicon nanorods relative to bulk in a wide energy range. The local field enhancement and dipole matrix element contributions were disentangled experimentally by single-dot absorption measurements on differently shaped particles as a function of excitation polarization and photon energy. Both factors substantially contribute to the observed effect as supported by simulations of the light-matter interaction and atomistic calculations of the transition matrix elements. The results indicate strong shape dependence of the quasidirect transitions in silicon nanocrystals, suggesting nanostructure shape engineering as an efficient tool for overcoming limitations of indirect band gap materials in optoelectronic applications, such as solar cells.

Predicted Realization of Cubic Dirac Fermion in Quasi-One-Dimensional Transition-Metal MonoChalcogenides

Abstract: We show that the previously predicted Fermion particle that has no analogue in the standard model of particle theory - the cubically dispersed Dirac semimetal (CDSM) - is realized in a specific, stable solid state system that has been made years ago, but was not appreciated to host such a unique Fermion, composed of six Weyl Fermions, 3 with left-handed and 3 with right-handed chirality. We identified the crystal symmetry constraints and found the space group P63/m as one of the two that can support a CDSM, of which the characteristic band crossing has linear dispersion along the principle axis but cubic dispersion in the plane perpendicular to it. We then conducted a material search using density functional theory identifying a group of quasi-one-dimensional molybdenum mono-chalcogenide compounds A(MoX)3 (A = Na, K, Rb, In, Tl, X = S, Se, Te) as ideal CDSM candidates. Studying the stability of the A(MoX)3 family reveals a few candidates such as Rb(MoTe)3 and Tl(MoTe)3 that are predicted to be resilient to Peierls distortion, thus retaining the metallic character. The combination of one-dimensionality and metallic nature in this family provides a platform for unusual optical signature - polarization dependent metallic vs insulating response.

Orbital mapping of energy bands and the truncated spin polarization in three-dimensional Rashba semiconductors

Abstract: Associated with spin-orbit coupling (SOC) and inversion symmetry breaking, Rashba spin polarization opens an avenue for spintronic applications that was previously limited to ordinary magnets. However, spin-polarization effects in actual Rashba systems are far more complicated than what conventional single-orbital models would suggest. By studying via density functional theory and a multiorbital $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ model a three-dimensional bulk Rashba system (free of complications by surface effects), BiTeI, we find that the physical origin of the leading spin-polarization effects is SOC-induced hybridization between spin and multiple orbitals, especially those with nonzero orbital angular momenta. In this framework we establish a general understanding of the orbital mapping, common to the surface of topological insulators and the Rashba system. Consequently, the intrinsic mechanism of various spin-polarization effects---which pertain to all Rashba systems, even those with global inversion symmetry---is understood as a manifestation of the orbital textures. This finding suggests a route for designing high-spin-polarization materials by considering the atomic-orbital content.

Minimal ingredients for orbital-texture switches at Dirac points in strong spin–orbit coupled materials

Abstract: Recent angle-resolved photoemission spectroscopy measurements on strong spin–orbit coupled materials have shown an in-plane orbital-texture switch at their respective Dirac points, regardless of whether they are topological insulators or ‘trivial’ Rashba materials. This feature has also been demonstrated in a few materials (Bi2Se3, Bi2Te3 and BiTeI) though DFT calculations. Here we present a minimal orbital-derived tight binding model to calculate the electron wavefunction in a two-dimensional crystal lattice. We show that the orbital components of the wavefunction demonstrate an orbital-texture switch in addition to the usual spin switch seen in spin polarised bands. This orbital-texture switch is determined by the existence of three main properties: local or global inversion symmetry breaking, strong spin–orbit coupling and non-local physics (the electrons are on a lattice). Using our model, we demonstrate that the orbital-texture switch is ubiquitous and to be expected in many real systems. The orbital hybridisation of the bands is the key aspect for understanding the unique wavefunction properties of these materials, and this minimal model helps to establish the quantum perturbations that drive these hybridisations. For materials with inherent strong spin-orbit coupling (SOC), the conventional spin picture does not provide a complete description of the Dirac states as the orbital components are usually overlooked. Orbital-selective photoemission observations and theoretical calculations have recently confirmed the presence of orbital texture switch in certain materials. Here Waugh, Zunger, and Dessau from University of Colorado Boulder together with their co-workers develop a model to calculate the electronic structure and understand the underlying physics. This model not only shows the orbital texture switch, but predicts that this feature is ubiquitous and present in many systems with strong SOC and broken inversion symmetry. The orbital hybridization holds the key to understanding the unique wavefunction properties, and this model serves to establish the quantum perturbations that drive these hybridizations.

Cubic Dirac fermion in quasi-one-dimensional transition-metal mono-chalcogenide

Abstract: We show that a previously predicted fermion particle that has no analogue in the standard model of particle theory, the cubically dispersed Dirac semimetal (CDSM), will exist in a specific, stable solid state system that has been made years ago, but was not appreciated to host such a unique fermion. Our prediction was derived by combining crystal symmetry with topological invariants and identified the space group P63/m as one of the two that can have a CDSM. We then conduct a material search using density functional theory identifying a group of quasi-one-dimensional molybdenum mono-chalcogenide compounds A(MoX)3 (A = Na, K, Rb, In, Tl; X = S, Se, Te) as ideal CDSM candidates. Studying the stability of the A(MoX)3 family reveals a few candidates such as Rb(MoTe)3 and Tl(MoTe)3 that are resilient to Peierls distortion, thus retaining the metallic character. The importance of this theoretical discovery is not only in identifying a unique, never before realized fermion type in actual materials, but also in the possibilities it opens for new material properties associated with Luttinger liquid and topological superconductivity.