Uncovering spin-orbit coupling-independent hidden spin polarization of energy bands in antiferromagnets.

Many textbook physical effects in crystals are enabled by some specific symmetries. In contrast to such 'apparent effects', 'hidden effect X' refers to the general condition where the nominal global system symmetry would disallow the effect X, whereas the symmetry of local sectors within the crystal would enable effect X. Known examples include the hidden Rashba and/or hidden Dresselhaus spin polarization that require spin-orbit coupling, but unlike their apparent counterparts are demonstrated to exist in non-magnetic systems even in inversion-symmetric crystals.

Degeneracy Removal of Spin Bands in Collinear Antiferromagnets with Non-Interconvertible Spin-Structure Motif Pair.

Energy bands in antiferromagnets are supposed to be spin degenerate in the absence of spin-orbit coupling (SOC). Recent studies have identified formal symmetry conditions for antiferromagnetic crystals in which this degeneracy can be lifted, spin splitting,even in the vanishing SOC (i.e.

Intrinsic doping limitations in inorganic lead halide perovskites.

Inorganic halide perovskites (HP's) of the CsPbX (X = I, Br, Cl) type have reached prominence in photovoltaic solar cell efficiencies, leading to the expectation that they are a new class of semiconductors relative to the traditional ones. Peculiarly, they have shown an asymmetry in their ability to be doped by holes electrons. Indeed, both structural defect-induced doping as well as extrinsic impurity-induced doping strangely often result in HP's in a unipolar doping (dominantly p-type) with low free carriers' concentration.

Understanding Doping of Quantum Materials.

Doping mobile carriers into ordinary semiconductors such as Si, GaAs, and ZnO was the enabling step in the electronic and optoelectronic revolutions. The recent emergence of a class of "quantum materials", where uniquely quantum interactions between the components produce specific behaviors such as topological insulation, unusual magnetism, superconductivity, spin-orbit-induced and magnetically induced spin splitting, polaron formation, and transparency of electrical conductors, pointed attention to a range of doping-related phenomena associated with chemical classes that differ from the traditional semiconductors. These include wide-gap oxides, compounds containing open-shell d electrons, and compounds made of heavy elements yet having significant band gaps.

Origin of band gaps in 3d perovskite oxides.

With their broad range of properties, ABO transition metal perovskite oxides have long served as a platform for device applications and as a testing bed for different condensed matter theories. Their insulating character and structural distortions are often ascribed to dynamical electronic correlations within a universal, symmetry-conserving paradigm. This view restricts predictive theory to complex computational schemes, going beyond density functional theory (DFT).

Antidoping in Insulators and Semiconductors Having Intermediate Bands with Trapped Carriers.

Ordinary doping by electrons (holes) generally means that the Fermi level shifts towards the conduction band (valence band) and that the conductivity of free carriers increases. Recently, however, some peculiar doping characteristics were sporadically recorded in different materials without noting the mechanism: electron doping was observed to cause a portion of the lowest unoccupied band to merge into the valance band, leading to a decrease in conductivity. This behavior, that we dub as "antidoping," was seen in rare-earth nickel oxides SmNiO_{3}, cobalt oxides SrCoO_{2.

Uncovering and tailoring hidden Rashba spin-orbit splitting in centrosymmetric crystals.

Hidden Rashba and Dresselhaus spin splittings in centrosymmetric crystals with subunits/sectors having non-centrosymmetric symmetries (the R-2 and D-2 effects) have been predicted theoretically and then observed experimentally, but the microscopic mechanism remains unclear. Here we demonstrate that the spin splitting in the R-2 effect is enforced by specific symmetries, such as non-symmorphic symmetry in the present example, which ensures that the pertinent spin wavefunctions segregate spatially on just one of the two inversion-partner sectors and thus avoid compensation. We further show that the effective Hamiltonian for the conventional Rashba (R-1) effect is also applicable for the R-2 effect, but applying a symmetry-breaking electric field to a R-2 compound produces a different spin-splitting pattern than applying a field to a trivial, non-R-2, centrosymmetric compound.

Electron Doping of Proposed Kagome Quantum Spin Liquid Produces Localized States in the Band Gap.

Carrier doping of quantum spin liquids is a long-proposed route to the emergence of high-temperature superconductivity. Electrochemical intercalation in kagome hydroxyl halide materials shows that samples remain insulating across a wide range of electron counts. Here we demonstrate through first-principles density-functional calculations, corrected for self-interaction, the mechanism by which electrons remain localized in various Zn-Cu hydroxyl halides, independent of the chemical identity of the dopant-the formation of polaronic states with attendant lattice displacements and a dramatic narrowing of bandwidth upon electron addition.

Topological Insulators versus Topological Dirac Semimetals in Honeycomb Compounds.

Intriguing physical properties of materials stem from their chemical constituents, whereas the connection between them is often not clear. Here, we uncover a general chemical classification for the two quantum phases in the honeycomb ABX structure-topological insulator (TI) and topological Dirac semimetal (TDSM). First, we find among the 816 (existing as well as hypothetical) calculated compounds, 160 TIs (none were noted before), 96 TDSMs, 282 normal insulators (NIs), and 278 metals.

Rapid Transition of the Hole Rashba Effect from Strong Field Dependence to Saturation in Semiconductor Nanowires.

The electric field manipulation of the Rashba spin-orbit coupling effects provides a route to electrically control spins, constituting the foundation of the field of semiconductor spintronics. In general, the strength of the Rashba effects depends linearly on the applied electric field and is significant only for heavy-atom materials with large intrinsic spin-orbit interaction under high electric fields. Here, we illustrate in 1D semiconductor nanowires an anomalous field dependence of the hole (but not electron) Rashba effect (HRE).

Cu-In Halide Perovskite Solar Absorbers.

The long-term chemical instability and the presence of toxic Pb in otherwise stellar solar absorber APbX made of organic molecules on the A site and halogens for X have hindered their large-scale commercialization. Previously explored ways to achieve Pb-free halide perovskites involved replacing Pb with other similar M cations in ns electron configuration, e.g.

Strong Absorption Enhancement in Si Nanorods.

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.

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

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.

Intrinsic Transparent Conductors without Doping.

Transparent conductors (TCs) 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.

Prediction and Synthesis of Strain Tolerant RbCuTe Crystals Based on Rotation of One-Dimensional Nano Ribbons within a Three-Dimensional Inorganic Network.

A unique possibility for a simple strain tolerant inorganic solid is envisioned whereby a set of isolated, one-dimensional (1D) nano objects are embedded in an elastically soft three-dimensional (3D) atomic matrix thus forming an interdimensional hybrid structure (IDHS). We predict theoretically that the concerted rotation of 1D nano objects could allow such IDHSs to tolerate large strain values with impunity. Searching theoretically among the 1:1:1 ABX compounds of I-I-VI composition, we identified, via first-principles thermodynamic theory, RbCuTe, which is a previously unreported but now predicted-to-be-stable compound in the MgSrSi-type structure, in space group Pnma.

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.

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, 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.

Intrinsic circular polarization in centrosymmetric stacks of transition-metal dichalcogenide compounds.

The circular polarization (CP) that the photoluminescence inherits from the excitation source in n monolayers of transition-metal dichalcogenides (MX_{2})_{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 MX_{2} systems, e.

Switching a normal insulator into a topological insulator via electric field with application to phosphorene.

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.

Evolution of electronic structure as a function of layer thickness in group-VIB transition metal dichalcogenides: emergence of localization prototypes.

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.

Reinterpretation of the expected electronic density of states of semiconductor nanowires.

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.