In-gap States Research Articles

Non-Hermitian CNH=2 Chern insulator protected by generalized rotational symmetry

We propose a non-Hermitian topological system protected by the generalized rotational symmetry which invokes rotation in space and Hermitian conjugation. The system, described by the tight-binding model with reciprocal imaginary next-nearest-neighbor hopping, is found to host two pairs of in-gap edge modes in the gapped topological phase, and is characterized by the non-Hermitian (NH) Chern number ${C}_{\mathrm{NH}}=2$. The quantization of the NH Chern number is shown to be protected by the generalized rotational symmetry ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{H}}^{+}(\mathrm{g}\stackrel{P\vec}{k})={\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{U}}_{g}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{H}(\stackrel{P\vec}{k}){\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{U}}_{g}^{+}$ of the system. Our finding paves the way towards non-Hermitian topological systems characterized by large values of topological invariants and hosting multiple in-gap edge states, which can be used for topologically resilient multiplexing.

Fermionic Chern insulator from twisted light with linear polarization

The breaking of time-reversal symmetry is a crucial ingredient to topological bands. It can occur intrisically in materials with magnetic order, or be induced by external fields, such as magnetic fields in quantum Hall systems, or circularly polarized light fields in Floquet Chern insulators. Apart from polarization, photons can carry another degree of freedom, orbital angular momentum, through which time-reversal symmetry can be broken. In this Letter, we pose the question whether this property allows for inducing topological bands via a linearly polarized but twisted light beam. To this end, we study a graphene-like model of electrons on a honeycomb lattice interacting with a twisted light field. To identify topological behavior of the electrons, we calculate their local markers of Chern number, and monitor the presence of in-gap edge states. Our results are shown to be fully analogous to the behavior found in paradigmatic models for static and driven Chern insulators, and realizing the state is experimentally straightforward. With this, our work establishes a new mechanism for generating Fermionic topological phases of matter that can harness the central phase singularity of an optical vortex beam.

Synthetic Route of Layered Titanium Nitride Chloride TiNCl Using Sodium Amide.

A synthetic route in a closed system for layered titanium nitride chloride TiNCl has been developed using sodium amide NaNH2 as a nitrogen source. A highly crystalline sample is obtained by an appropriate thermal decomposition of aminated titanium chloride. The obtained TiNCl was also characterized using electronic resistivity measurement and photoemission spectroscopy. TiNCl showed hopping conduction compatible with an in-gap state revealed by photoelectron spectroscopy. However, it appeared highly electron-doped, albeit without showing superconductivity. Comparison with the spectrum of superconducting sodium-doped samples suggests the presence of the microstructure required to exhibit superconductivity.

Reversible Manipulation of Photoconductivity Caused by Surface Oxygen Vacancies in Perovskite Stannates with Ultraviolet Light (Adv. Mater. 5/2022)

2D Photoconductivity Light–matter interaction under different oxygen pressures can reversibly manipulate photoconductivity from surface oxygen vacancies of perovskite stannate semiconductors. In article number 2107650, utilizing a surface-selective photochemical reaction, Junwoo Son and co-workers report that ultraviolet-excited electron–hole pairs of a perovskite compound under low oxygen pressure accelerate the unprecedented formation of spatially confined and optically programmable in-gap states induced by oxygen vacancies at room temperature.

Bending as a control knob for the electronic and optical properties of phosphorene nanoribbons

We have assessed mechanical bending as a powerful controlling tool for the electronic structure and optical properties of phosphorene nanoribbons. We use state-of-the-art density functional approximations in our work. The overall performance of the recently developed meta-generalized gradient approximation (meta-GGA) mTASK [Phys. Rev. Materials 5, 063803 (2021)] functional establishes the method as a useful alternative to the screened hybrid HSE06 for better band gaps of phosphorene nanoribbons. We present a detailed analysis to interpret the optical absorption of bent phosphorene nanoribbons using the GW-Bethe-Salpeter approximation. We demonstrate the important role of the unoccupied in-gap state and conclude that this in-gap state in armchair nanoribbons introduced by bending can significantly affect the properties of low-energy excitons and add some useful opportunities for applications in optoelectronic devices.

Dual Passivation of Point Defects at Perovskite Grain Boundaries with Ammonium Salts Greatly Inhibits Nonradiative Charge Recombination.

Experiments demonstrate that grain boundaries (GBs) exhibit detrimental effect on carrier lifetimes in MAPbI3 (MA= CH3NH3+). On the basis of the nonadiabatic (NA) molecular dynamics simulations, we demonstrated that NH4Cl can simultaneously passivate the common point defects that introduce recombination centers at GBs and accelerate electron-hole recombination but shows small effects in the bulk. The MA interstitial (MAi) and the substitutional MA to Pb (MAPb) in pristine MAPbI3 leave the band gap and charge recombination rates largely unchanged but create deep electron traps at GBs by separately either distorting inorganic octahedra or creating an I-dimer. Cl- and NH4+ remove the in-gap states by either restoring the distorted octahedra or destroying the I-dimer. Thus, the band gap recovers to the pristine system, NA coupling decreases, and decoherence accelerates, extending the carrier lifetime even twice longer than MAPbI3. This study shows that the negative role of GBs can be removed by dually passivating with NH4Cl.

Local breaking of the spin degeneracy in the vortex states of Ising superconductors: Induced antiphase ferromagnetic order

Ising spin-orbital coupling is usually easy to identify in the Ising superconductors via an in-plane critical field enhancement, but we show that the Ising spin-orbital coupling also manifests in the vortex physics for perpendicular magnetic fields. By self-consistently solving the Bogoliubov--de Gennes equations of a model Hamiltonian built on the honeycomb lattice with the Ising spin-orbital coupling pertinent to the transition metal dichalcogenides, we numerically investigate the local breaking of the spin and sublattice degeneracies in the presence of a perpendicular magnetic field. It is revealed that the ferromagnetic orders are induced inside the vortex core region by the Ising spin-orbital coupling. The induced magnetic orders are antiphase in terms of their opposite polarizations inside the two nearest-neighbor vortices with one of the two polarizations coming predominantly from one sublattice site, implying the local breaking of the spin and sublattice degeneracies. The finite-energy peaks of the local density of states for spin-up and spin-down in-gap states are split and shifted oppositely by the Ising spin-orbital coupling, and the relative shifts of them on sublattices $A$ and $B$ are also of opposite algebraic sign. The calculated results and the proposed scenario may not only serve as experimental signatures for identifying the Ising spin-orbital coupling in the Ising superconductors, but also be prospective in the manipulation of electron spins in motion through the orbital effect in the superconducting vortex states.

Epitaxial growth and optical band gap variation of ultrathin ZnTe films

Zinc telluride (ZnTe) has attracted interests for its semiconducting, optoelectronic, and electrical switching properties. However, the growth mechanism of ultrathin epitaxial films is not well established. Here we present a systematic study of the growth ultrathin ZnTe films on GaAs (001) by molecular-beam epitaxy. In situ reflection high-energy electron diffraction and synchrotron based high-resolution X-ray diffraction showed that both surface atomic ordering and single crystalline phase aligned to the substrate orientation with small variation of c-axis lattice in the ultrathin films. While the deviation of chemical compositions depended on the growth conditions, information on the variation of the band gap and in-gap states was obtained through spectroscopic ellipsometry analysis. Our study showed that single crystal ZnTe films can serve as a model system in the development of Ovonic threshold switching devices for cross-point device applications.

The enhanced photocatalytic performance and first-principles computational insights of Ba doping-dependent TiO2 quantum dots.

Degradation in the presence of visible light is essential for successfully removing dyes from industrial wastewater, which is pivotal for environmental and ecological safety. In recent years, photocatalysis has emerged as a prominent technology for wastewater treatment. This study aimed to improve the photocatalytic efficiency of synthesized TiO2 quantum dots (QDs) under visible light by barium (Ba) doping. For this, different weight ratios (2% and 4%) of Ba-doped TiO2 QDs were synthesized under ambient conditions via a simple and modified chemical co-precipitation approach. The QD crystal structure, functional groups, optical features, charge-carrier recombination, morphological properties, interlayer spacing, and presence of dopants were analyzed. The results showed that for 4% Ba-doped TiO2, the effective photocatalytic activity in the degradation process of methylene blue (MB) dye was 99.5% in an alkaline medium. Density functional theory analysis further corroborated that the band gap energy was reduced when Ba was doped into the TiO2 lattice, implying a considerable redshift of the absorption edge due to in-gap states near the valence band.

Non-trivial impurity and field effects in topological Kondo insulator SmB6

Topological Kondo Insulator SmB6 is a strongly correlated material where a spin–orbit interaction between the localized odd-parity f-electron and even parity d-electron levels lead to a band inversion and opening of an insulating gap. The non-trivial topology (Z2 = -1) leads to the formation of a topologically protected conducting surface state due to the presence of strong correlation physics. Although the bulk material is known to form a topological Kondo gap, recent magnetic quantum oscillation experiments on SmB6 find an unconventional Fermi surface whose origin remains a matter of intense debate, and the possible role of topological surfacestate and impurity induced in-gap states have been proposed. By utilizing a realistic multi-orbital tight-binding Hamiltonian, this work aims to develop a microscopic model to study the effects of perturbations like external magnetic fields, and impurities on the low energy bulk and topological surface state properties of SmB6. We further examine the role of an external electric field in tuning the non-trivial topological surface state in SmB6 and provide experimentally observable signatures in the local density of state (LDOS) near non-magnetic and magnetic impurities. Additionally, we study the surface Ferromagnetism that arises in SmB6 as observed by magnetoresistance experiments.

In-Plane Magnetization-Induced Corner States in Bismuthene

We theoretically demonstrate that the electronic second-order topological insulator with robust corner states, having a buckled honeycomb lattice, can be realized in bismuthene by inducing in-plane magnetization. Based on the sp 3 Slater–Koster tight-binding model with parameters extracted from first-principles results, we show that spin-helical edge states along zigzag boundaries are gapped out by the in-plane magnetization whereas four robust in-gap electronic corner states at the intersection between two zigzag boundaries arise. By regulating the orientation of in-plane magnetization, we show different position distribution of four corner states with different energies. Nevertheless, it respects some spatial symmetries and thus can protect the higher-order topological phase. Combined with the Kane–Mele model, we discuss the influence of the magnetization orientation on the position distribution of corner states.

Defect-Stabilized Substoichiometric Polymorphs of Hafnium Oxide with Semiconducting Properties.

Hafnium oxide plays an important role as a dielectric material in various thin-film electronic devices such as transistors and resistive or ferroelectric memory. The crystallographic and electronic structure of the hafnia layer often depends critically on its composition and defect structure. Here, we report two novel defect-stabilized polymorphs of substoichiometric HfO2-x with semiconducting properties that are of particular interest for resistive switching digital or analog memory devices. The thin-film samples are synthesized by molecular beam epitaxy with oxygen engineering that allows us to cover the whole range of metallic Hf with oxygen interstitials to HfO2. The crystal and defect structures, in particular of a cubic low-temperature phase c-HfO1.7 and a hexagonal phase hcp-HfO0.7 are identified by X-ray diffraction, in vacuo electron spectroscopic, and transmission electron microscopic methods. With the help of UV/Vis transmission data, we propose a consistent band structure model for the whole oxidation range involving oxygen vacancy-induced in-gap defect states. Our comprehensive study of engineered hafnia thin films has an impact on the design of resistive memory devices and can be transferred to chemically similar suboxide systems.

Higher-order topological insulators in two-dimensional Dirac materials

As a novel topological state, a higher-order topological insulator has attracted enormous interest, which in d spatial dimensions has gapless boundary states at (d−n) dimensions (integer n is larger than 1). Until now, merely few two-dimensional (2D) materials have been identified as higher-order topological insulators and their experimental confirmations are still absent. Here we propose a universal strategy of antidot engineering to realize second-order topological insulators (SOTIs) in 2D Dirac materials. Based on symmetry analysis, tight-binding model, and first-principles calculations, we demonstrate SOTIs in antidot-decorated Xene (X=C, Si,and Ge) by displaying its finite bulk quadrupole moment, weak topological edge states, and in-gap topological corner states. An inherent connection is established for the existing various mechanisms of the SOTIs, including quadrupole polarization, filling anomaly, and generalized Su-Schrieffer-Heeger model on a Kekulé lattice. The robustness of topological corner states of the SOTIs against edge perturbations and bulk disorders is explicitly demonstrated, rendering our strategy appealing to experimental realization of topological corner states.Received 5 September 2021Accepted 30 November 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.L042044Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEdge statesFirst-principles calculationsTopological materialsPhysical SystemsGermaneneGrapheneHoneycomb latticeTechniquesDensity functional theorySymmetriesTight-binding modelWannier function methodsCondensed Matter, Materials & Applied Physics

In-gap band in the one-dimensional two-orbital Kanamori-Hubbard model with interorbital Coulomb interaction

We study the electronic spectral properties at zero temperature of the one-dimensional (1D) version of the degenerate two-orbital Kanamori Hubbard model (KHM), one of the well established frameworks to study transition metal compounds, using state-of-the-art numerical techniques based on the Density Matrix Renormalization Group. While the system is Mott insulating for the half-filled case, as expected for an interacting 1D system, we find interesting and rich structures in the single-particle density of states (DOS) for the hole-doped system. In particular, we find the existence of in-gap states which are pulled down to lower energies from the upper Hubbard band (UHB) with increasing the inter-orbital Coulomb interaction $V$. We analyze the composition of the DOS by projecting it onto different local excitations and we observe that for large dopings these in-gap excitations are formed mainly by inter-orbital holon-doublon (HD) states and their energies follow approximately the HD states in the atomic limit. We observe that the Hund interaction $J$ increases the width of the in-gap band, as expected from the two-particle fluctuations in the Hamiltonian. The observation of a finite density of states within the gap between the Hubbard bands for this extended 1D model indicates that these systems present a rich excitation spectra which could help us understand the microscopic physics behind multi-orbital compounds.

Disclosing the response of the surface electronic structure in SrTiO3 (001) to strain

Combining angle-resolved photoemission spectroscopy and density functional theory calculations, we addressed the surface electronic structure of bent SrTiO3 (STO) (001) wafers. Using a custom-made device, we observe that the low-dimensional states that emerge at the STO (001) surface are robust to an external tensile strain of about 0.1%. Our results show that this value of strain is too small to sensibly alter the surface conduction band of STO, but, surprisingly, it is enough to shift the energy of the in-gap states. In order to access higher strain values of around 2%, standard for STO-based heterostructures, we performed density functional theory calculations of STO slabs under different strain configurations. The simulations predict that such levels of both compressive and tensile strain significantly alter the orbital splitting of the surface conduction band. Our study indicates that the strain generated in STO can tailor the electronic properties of its bare surface and of STO-based interfaces.

Transport features of topological corner states in honeycomb lattice with multihollow structure

Higher-order topological phase in 2-dimensional (2D) systems is characterized by in-gap corner states, which are hard to detect and utilize. We numerically investigate transport properties of topological corner states in 2D honeycomb lattice, where the second-order topological phase is induced by an in-plane Zeeman field in the conventional Kane-Mele model. Through engineering multihollow structures with appropriate boundaries in honeycomb lattice, multiple corner states emerge, which greatly increases the probability to observe them. A typical two-probe setup is built to study the transport features of a diamond-shaped system with multihollow structures. Numerical results reveal the existence of global resonant states in bulk insulator, which corresponds to the resonant tunneling of multiple corner states and occupies the entire scattering region. Furthermore, based on the well separated energy levels of multiple corner states, a single-electron source is constructed.

Reversible Manipulation of Photoconductivity Caused by Surface Oxygen Vacancies in Perovskite Stannates with Ultraviolet Light.

Programmable optoelectronic devices call for the reversible control of the photocarrier recombination process by in-gap states in oxide semiconductors. However, previous approaches to produce oxygen vacancies as a source of in-gap states in oxide semiconductors have hampered the reversible formation of oxygen vacancies and their related phenomena. Here, a new strategy to manipulate the 2D photoconductivity from perovskite stannates is demonstrated by exploiting spatiallyselective photochemical reaction under ultraviolet illumination at room temperature. Remarkably, the ideal trap-free photocurrent of air-illuminated BaSnO3 (≈200pA) is reversibly switched into three orders of magnitude higher photocurrent of vacuum-illuminated BaSnO3 (≈335nA) with persistent photoconductivity depending on ambient oxygen pressure under illumination. Multiple characterizations elucidate that ultraviolet illumination of BaSnO3 under low oxygen pressure induces surface oxygen vacancies as a result of surface photolysis combined with the low oxygen-diffusion coefficient of BaSnO3 ; the concentrated oxygen vacancies are likely to induce a two-step transition of photocurrent response by changing the characteristics of in-gap states from the shallow level to the deep level. These results suggest a novel strategy that uses light-matter interaction in a reversible and spatiallyconfined way to manipulate functionalities related to surface defect states, for the emerging applications using newly discovered oxide semiconductors.

Development of In-Gap State and Related Collapse of Two-Step Energy Gap Structure by Nonmagnetic Ion Substitution in Kondo Insulator YbB12

YbB12, which has attracted attention as an unconventional strongly correlated insulator, has a two-step energy gap structure (\(\Delta E_{1},\Delta E_{2}\)) with an in-gap state. These characterist...

Charge-Compensated Doping Extends Carrier Lifetimes in SrTiO3 by Passivating Oxygen Vacancy Defects

Experiments reported that oxygen vacancies shorten the charge carrier lifetime of SrTiO3 but that it is greatly improved upon Al and Na doping. Using nonadiabatic (NA) molecular dynamics, we demonstrate that the in-gap hole trap state created by an oxygen vacancy can be eliminated by charge-compensated doping when two Ti4+ ions or two Sr2+ ions are equally replaced by Al3+ or Na+ ions. Nevertheless, Al3+ and Na+ reduce the strength of NA coupling to a different extent, resulting in increased charge carrier lifetimes of 4.6 and 1.3 ns. The lifetimes are several times longer than that of the pristine system and 3 orders of magnitude longer than that of defective SrTiO3, which is within 50 ps due to strong NA coupling. The weakly correlated electron and hole wave functions in doped systems accelerate decoherence, further delaying charge recombination. Our study rationalizes the complex charge-phonon dynamics in SrTiO3 and proposes charge-compensated doping for the design of advanced visible-light photocatalysts.

Calculations of in-gap states of ferromagnetic spin chains on s -wave wide-band superconductors

Magnetic impurities create in-gap states on superconductors. Recent experiments explore the topological properties of one-dimensional arrays of magnetic impurities on superconductors, because in certain regimes p-wave pairing can be locally induced leading to new topological phases. A by-product of the new accessible phases is the appearance of zero-energy edge states that have non-Abelian exchange properties and can be used for topological quantum computation. Despite the large amount of theory devoted to these systems, most treatments use approximations that render their applicability limited when comparing with usual experiments of 1-D impurity arrays on wide-band superconductors. These approximations either involve tight-binding-like approximations where the impurity energy scales match the minute energy scale of the superconducting gap and are many times unrealistic, or they assume strongly-bound in-gap states. Here, we present a theory for s-wave superconductors based on a wide-band normal metal, with any possible energy scale for the magnetic impurities. The theory is based on free-electron Green's functions. We include Rashba coupling and compare with recent experimental results, permitting us to analyze the topological phases and the experimental edge states. The infinite-chain properties can be analytically obtained, giving us a way to compare with finite-chain calculations. We show that it is possible to converge to the infinite limit by doing finite numerical calculation, paving the way for numerical calculations not based on analytical Green's functions.