Bulk Band Gap Research Articles

Edge states in coupled non-Hermitian resonators.

Small perturbations may dramatically influence the physical properties of a single non-Hermitian cavity. However, how these small perturbations interplay with bulk-edge properties is still to be demonstrated by experimentation. Here, we experimentally demonstrate edge states in coupled non-Hermitian resonators, based on a chain of all-dielectric coupled resonators where each resonator consists of two target particles. The evanescent coupling between the cavity and the target particles leads to tunable asymmetric backscattering, which plays a key role in the appearance of edge states in the bulk bandgap. We also demonstrate that these observed edge states are robust against weak disorders introduced to the system. Our study may inspire further explorations of the non-Hermitian bulk-edge properties.

Trivial Andreev Band Mimicking Topological Bulk Gap Reopening in the Nonlocal Conductance of Long Rashba Nanowires.

We consider a one-dimensional Rashba nanowire in which multiple Andreev bound states in the bulk of the nanowire form an Andreev band. We show that, under certain circumstances, this trivial Andreev band can produce an apparent closing and reopening signature of the bulk band gap in the nonlocal conductance of the nanowire. Furthermore, we show that the existence of the trivial bulk reopening signature in nonlocal conductance is essentially unaffected by the additional presence of trivial zero-bias peaks in the local conductance at either end of the nanowire. The simultaneous occurrence of a trivial bulk reopening signature and zero-bias peaks mimics the basic features required to pass the so-called "topological gap protocol." Our results therefore provide a topologically trivial minimal model by which the applicability of this protocol can be benchmarked.

High Chern number in strained thin films of dilute magnetic topological insulators

The quantum anomalous Hall effect was first observed experimentally by doping the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ materials family with chromium, where 5% doping induces an exchange field of around 0.1 eV. In ultrathin films, a topological phase transition from a normal insulator to a Chern insulator can be induced with an exchange field proportional to the hybridization gap. Subsequent transitions to states with higher Chern numbers require an exchange field larger than the (bulk) band gap, but are prohibited in practice by the detrimental effects of higher doping levels. Here, we show that threshold doping for these phase transitions in thin films is controllable by strain. As a consequence, higher Chern states can be reached with experimentally feasible doping, sufficiently dilute for the topological insulator to remain structurally stable. Such a facilitated realization of higher Chern insulators opens prospects for multichannel quantum computing, higher-capacity circuit interconnects, and energy-efficient electronic devices at elevated temperatures.

Computational and experimental studies on band alignment of ZnO/InxGa2-xO3/GaN heterojunctions.

The ZnO/GaN heterojunctions are extensively investigated now, owing to their good luminescent properties and devisable capability to form efficient hybrid structures. An electron-blocking layer inserted into heterojunctions can greatly change their properties. In this work, n-ZnO/β-InxGa2-xO3/p-GaN heterojunctions have been successfully formed using atomic layer deposition methods. We show that the doping of In can effectively tune the band edges of the heterojunctions. First-principle calculations reveal that the bandgap of bulk β-InxGa2-xO3 shrinks linearly with the increase in In contents, accompanied by an upward movement of the valence band maximum and a downward movement of the conduction band minimum. As the indium concentrations increase, the valence band offsets show an upward movement at both the InxGa2-xO3/GaN and ZnO/InxGa2-xO3 interfaces, while the conduction band offsets present different trends. A broad, reddish yellow-green emission appears after In doping, which verifies the effect of band alignment. What is more, we show that the amorphization of InxGa2-xO3 can play an important role in tuning the band edge. This work provides access to a series of band offsets tunable heterojunctions and can be used for the further design of direct white light-emitting diodes without any phosphors, based on this structure.

Light-induced switch based on edge modes in irradiated thin topological insulators

We investigate transport properties through the nano-ribbons of thin topological insulators irradiated by circularly polarized light in a high-frequency regime. It is demonstrated that pseudo-spin polarized edge modes appearing in the bulk band gap are responsible for the current flowing through this nano-junction whose localization on the top and bottom edges depend strongly on the polarization of light. Based on these edge modes, one can design a light-induced switch with a desirable on/off ratio of the current whose off-state is engineered by dividing the scattering region into two parts. Each part is irradiated by the light with opposite polarization in respect to the other one. This off-state arises from the quantum blocking of transition between two edge modes with opposite pseudo-spin polarization induced by irradiation. The local current on each bond shows how the current passes through the edges and jumps to the opposite edge. Furthermore, some other nano-junctions are proposed as a light-induced switch which are designed based on the gap opening induced by the perpendicular magnetization or structural inversion asymmetry.

Surface state mediated ferromagnetism in Mn0.14Bi1.86Te3 thin films

A spontaneous ferromagnetic moment can be induced in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films below a temperature $T\ensuremath{\approx}16$ K by the introduction of Mn dopants. We demonstrate that films grown via molecular beam epitaxy with the stoichiometry ${\mathrm{Mn}}_{0.14}{\mathrm{Bi}}_{1.86}{\mathrm{Te}}_{3}$ maintain the crystal structure of pure ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. The van der Waals nature of interlayer forces in the ${\mathrm{Mn}}_{0.14}{\mathrm{Bi}}_{1.86}{\mathrm{Te}}_{3}$ crystal causes lattice mismatch with the underlayer to have a limited effect on the resulting crystal structure, as we demonstrate by thin film growth on tetragonal ${\mathrm{MgF}}_{2}$ (110) and ${\mathrm{NiF}}_{2}$ (110). Electronic transport and magnetic moment measurements show that the ferromagnetic moment of the ${\mathrm{Mn}}_{0.14}{\mathrm{Bi}}_{1.86}{\mathrm{Te}}_{3}$ thin films is enhanced as the Fermi level moves from the bulk conduction band and toward the bulk band gap, suggesting that electronic surface states play an important role in mediating the ferromagnetic order. Ferromagnetic ${\mathrm{Mn}}_{0.14}{\mathrm{Bi}}_{1.86}{\mathrm{Te}}_{3}$/antiferromagnetic ${\mathrm{NiF}}_{2}$ bilayers show evidence that the ferromagnetic moment of the ${\mathrm{Mn}}_{0.14}{\mathrm{Bi}}_{1.86}{\mathrm{Te}}_{3}$ film is suppressed, suggesting the existence of an interface effect between the two magnetic layers.

Reduction and Diffusion of Cr-Oxide Layers into P25, BaLa4Ti4O15, and Al:SrTiO3 Particles upon High-Temperature Annealing.

Chromium oxide (Cr2O3) is a beneficial metal oxide used to prevent the backward reaction in photocatalytic water splitting. The present work investigates the stability, oxidation state, and the bulk and surface electronic structure of Cr-oxide photodeposited onto P25, BaLa4Ti4O15, and Al:SrTiO3 particles as a function of the annealing process. The oxidation state of the Cr-oxide layer as deposited is found to be Cr2O3 on the surface of P25 and Al:SrTiO3 particles and Cr(OH)3 on BaLa4Ti4O15. After annealing at 600 °C, for P25 (a mixture of rutile and anatase TiO2), the Cr2O3 layer diffuses into the anatase phase but remains at the surface of the rutile phase. For BaLa4Ti4O15, Cr(OH)3 converts to Cr2O3 upon annealing and diffuses slightly into the particles. However, for Al:SrTiO3, the Cr2O3 remains stable at the surface of the particles. The diffusion here is due to the strong metal-support interaction effect. In addition, some of the Cr2O3 on the P25, BaLa4Ti4O15, and Al:SrTiO3 particles is reduced to metallic Cr after annealing. The effect of Cr2O3 formation and diffusion into the bulk on the surface and bulk band gaps is investigated with electronic spectroscopy, electron diffraction, DRS, and high-resolution imaging. The implications of the stability and diffusion of Cr2O3 for photocatalytic water splitting are discussed.

The electronic structure of β-TeO2 as wide bandgap p-type oxide semiconductor

Wide bandgap oxide semiconductors have gained significant attention in the fields from flat panel displays to solar cells, but their uses have been limited by the lack of high mobility p-type oxide semiconductors. Recently, β-phase TeO2 has been identified as a promising p-type oxide semiconductor with exceptional device performance. In this Letter, we report on the electronic structure of β-TeO2 studied by a combination of high-resolution x-ray spectroscopy and hybrid density functional theory calculations. The bulk bandgap of β-TeO2 is determined to be 3.7 eV. Direct comparisons between experimental and computational results demonstrate that the top of a valence band (VB) of β-TeO2 is composed of the hybridized Te 5s, Te 5p, and O 2p states, whereas a conduction band (CB) is dominated by unoccupied Te 5p states. The hybridization between spatially dispersive Te 5s2 states and O 2p orbitals helps us to alleviate the strong localization in the VB, leading to small hole effective mass and high hole mobility in β-TeO2. The Te 5p states provide stabilizing effect to the hybridized Te 5s-O 2p states, which is enabled by structural distortions of a β-TeO2 lattice. The multiple advantages of large bandgap, high hole mobility, two-dimensional structure, and excellent stability make β-TeO2 a highly competitive material for next-generation opto-electronic devices.

High-sensitivity and high-stability near-ultraviolet self-powered photodetectors enabled by depolarization electric field and core-shell structured Cu@benzotriazole nanowire electrode

Currently semiconductor-based near ultraviolet and ultraviolet self-powered photodetectors usually display low detectivity and responsivity due to weak photogenerated electron and hole separation ability and not suitable electrode. BaTiO3 possesses a high depolarization electric field throughout the bulk and wide band gap (> 3 eV), and hence is a good candidate for fabricating near-ultraviolet and ultraviolet self-powered photodetectors. Cu nanowires (NWs) have been regarded as promising transparent electrode for photodetectors because they have not only equivalent optical transparency and electrical conductivity relative to Ag NWs currently in use, but also far lower cost. But, weak resistance to oxidation in the environment of light and electricity prevents large-scale use of Cu NWs in photodetectors. Herein, an organic corrosion inhibitor, benzotriazole (BTA), is used to passivate Cu NWs, and interestingly, the passivated NW networks exhibit significantly increased resistance to oxidation. At zero field, the BaTiO3 photodetector coated with Cu@BTA NWs exhibits stable photocurrent, high responsibility of 6.45 × 10−7A/W, large detectivity of 1.97 × 108 Jones, and fast response rate (rise and decay time being 28.8 ms and 37.2 ms, respectively) at the wavelength of 405 nm, which is far better than indium-tin-oxide electrode-coated BaTiO3 photodetector. This work provides a strong guideline for large-scale fabrication of low-cost and high-stability transparent electrodes applicable in near-ultraviolet and ultraviolet photoelectric devices.

Interlayer hybridization in a van der Waals quantum spin-Hall insulator/superconductor heterostructure

In this work, we present an angle-resolved photoemission spectroscopy study of a 1T′-WTe2 monolayer epitaxially grown on NbSe2 substrates, a prototypical quantum spin Hall insulator (QSHI)/superconductor heterojunction. Angle-resolved photoemission spectroscopy data indicate the formation of electronic states in the bulk bandgap of WTe2, which are absent in the nearly free-standing WTe2 grown on the highly oriented pyrolytic graphite substrate, where an energy gap of ∼100 meV is reported. The results are explained in terms of hybridization effects promoted by the QSHI–superconductor interaction at WTe2/NbSe2 interfaces, in line with recent scanning probe microscopy investigation and theoretical band structure calculations. Our findings highlight the important role of interlayer interaction on the electronic properties and ultimately on the engineering of topological properties of the QSHI/superconducting heterostructure.

Topological nature of large bulk band gap materials Sr3Bi2 and Ca3Bi2

Topological materials are an emerging class of materials attracting the attention of the scientific community due to their potential applications in the fields of spintronics and quantum computing. Using first-principles calculations, the structural, electronic, and topological properties of Sr3Bi2 and Ca3Bi2 compounds without and with spin–orbit coupling are investigated. In the absence of spin–orbit coupling, the projected bulk band structure revealed that the Sr3Bi2 compound host a type-I Dirac point along the F-Γ direction. Since the compound possesses time-reversal and space-inversion symmetries, this Dirac point is associated with the nodal line. The existence of a type-I nodal ring around the Γ-point in the kz = 0 planes, as well as a drumhead-like surface state within the nodal ring, suggested that Sr3Bi2 is a type-I nodal-line semimetal with no spin–orbit coupling. The inclusion of spin–orbit coupling introduced an energy gap of 0.36 eV between the valence band and conduction band at Dirac point. The topological surface states forming a Dirac cone between the bulk bandgap for (001) surface of Sr3Bi2 compound is calculated with spin–orbit coupling. The Z2 topological invariants (1;000), as calculated by using parity product criteria, suggested that Sr3Bi2 is a strong topological insulator. Ca3Bi2, another compound with a similar crystal structure, is also predicted to behave similarly to Sr3Bi2 compound without and with spin–orbit coupling. This research broadens the application of topological insulators and existing platforms for developing novel spintronic devices.

Emergence of metallic surface states and negative differential conductance in thin β-FeSi2 films on Si(001)

The electronic properties of the surface of β-FeSi2 have been debated for a long. We studied the surface states of β-FeSi2 films grown on Si(001) substrates using scanning tunnelling microscopy (STM) and spectroscopy (STS), with the aid of density functional theory calculations. STM simulations using the surface model proposed by Romanyuk et al (2014 Phys. Rev. B 90 155305) reproduce the detailed features of experimental STM images. The result of STS showed metallic surface states in accordance with theoretical predictions. The Fermi level was pinned by a surface state that appeared in the bulk band gap of the β-FeSi2 film, irrespective of the polarity of the substrate. We also observed negative differential conductance at ∼0.45 eV above the Fermi level in STS measurements performed at 4.5 K, reflecting the presence of an energy gap in the unoccupied surface states of β-FeSi2.

Tunneling-induced fractal transmission in valley Hall waveguides

The Valley Hall effect provides a popular route to engineer robust waveguides for bosonic excitations such a photons and phonons. The almost complete absence of backscattering in many experiments has its theoretical underpinning in a smooth-envelope approximation that neglects large momentum transfer and is accurate only for small bulk band gaps and/or smooth domain walls. For larger bulk band gaps and hard domain walls backscattering is expected to become significant. Here, we show that in this experimentally relevant regime, the reflection of a wave at a sharp corner becomes highly sensitive on the orientation of the outgoing waveguide relative to the underlying lattice. Enhanced backscattering can be understood as being triggered by resonant tunneling transitions in quasimomentum space. Tracking the resonant tunneling energies as a function of the waveguide orientation reveals a self-repeating fractal pattern that is also imprinted in the density of states and the backscattering rate at a sharp corner.

Energy spectrum and light absorption of arsenene quantum dots

As a close relative of phosphorene, puckered arsenene is stable and has been demonstrated experimentally as a promising two-dimensional semiconductor material. By means of a multi-orbital tight-binding model, we investigate theoretically the energy spectrum and magneto-optical properties of circular and rectangular arsenene quantum dots (AQDs). The tight-binding parameters are obtained by reproducing accurately the electron energy bands of puckered arsenene from the first-principle calculation. For both AQDs, the density of states (DOS), probability distribution and inverse participation ratio (IPR) of eigenstates, and optical absorption spectrum are calculated numerically. It is found that edge states of AQDs fill up the bulk band gap of puckered arsenene and are only localized near the armchair edges of AQDs. These features differ greatly from those in quantum dots made of graphene or phosphorene. The band edges of arsenene monolayer can be located roughly from the IPR and DOS spectrum. The light absorptivity of both AQDs under zero magnetic field depends strongly on the polarization direction of incident light. The position and intensity of absorption peaks can be tuned by the shape of quantum dots and the applied magnetic field. The anisotropy ratio of light absorption can be reduced by one order of magnitude under a magnetic field of tens of Tesla.

Large band gap quantum spin Hall insulators in plumbene monolayer decorated with amidogen, hydroxyl and thiol functional groups.

Two-dimensional Quantum Spin Hall (QSH) insulators featuring edge states that are topologically protected against back-scattering are arising as a novel state of quantum matter. One of the major obstacles to finding QSH insulators operable at room temperature is the insufficiency of suitable materials demonstrating the QSH effect with a large bulk band gap. Plumbene, the latest group-IV graphene analogous material, shows a large SOC-induced band gap opening but the coupling between topological states at different momentum points makes it a topologically trivial insulator. Pristine plumbene can be chemically functionalized to transform it from a conventional insulator to a topologically non-trivial insulator with a considerable bulk band gap. In this work, three new QSH phases in plumbene have been theoretically predicted through functionalization with amidogen (-NH2), hydroxyl (-OH) and thiol (-SH) groups. The derived electronic properties show non-trivial topological states in plumbene with very high bulk band gaps ranging from 1.0911 eV to as high as 1.1515 eV. External strain can be used to further enhance and tune these bulk gaps, as demonstrated in this work. We also propose a H-terminated SiC (0001) surface as a suitable substrate for the practical implementation of these monolayers to minimize lattice mismatch and maintain their topological order. The robustness of these QSH insulators against strain and substrate effects and the large bulk gaps provide a promising platform for potential applications of future low dissipation nanoelectronic devices and spintronic devices at room temperature.

Thermal transport of helical Majorana edge modes in a superconducting antiferromagnetic quantum spin Hall insulator

In the system composed of a two-dimensional antiferromagnetic quantum spin Hall insulator (AFQSHI) nanoribbon covered by s -wave superconductors, possessing helical Majorana edge mode (HMEM), and preserving effective time-reversal symmetry. Based on such the AFQSHI-SC hybrid system, we investigate the band structure and thermal transport properties. The two-terminal thermal conductance κ e l and electric conductance G are calculated by using the Landauer-Büttiker formula and the nonequilibrium Green’s function method. When a pair of intersecting edge states exists in the bulk band gap, a helical topological superconductor with Z 2 invariant ν = ± 1 is formed, which has a pair of Majorana edge states with opposite chirality, that is Majorana Kramer pairs, leading to a integer quantized thermal conductance plateau. We find that the quantized thermal conductance plateau is well preserved in a certain range of temperature. The presence and number of Majorana Kramer pairs in helical topological superconductor can be confirmed with the help of the quantized thermal conduction plateau, which is promising for the experimental detection of helical Majorana fermions. • In this manuscript, based on the Landauer-Buttiker formula and the nonequilibrium Green’s function method, we investigate the properties of helical Majorana fermions edge states thermal transport in a hybrid AFQSHI-SC structure. • When a pair of Majorana edge states with opposite chirality residing at the edge of the central topological superconductor region, a integer quantized thermal conductance plateau is discovered. • This quantized plateau can be used as strong evidence for the experimental detection of helical topological superconductors.

First-principles investigation of possible room-temperature topological insulators in monolayers.

A Quantum Spin Hall (QSH) insulator with a large bulk band gap and tunable topological properties is crucial for both fundamental research and practical application. Chemical function-alization has been proposed as an effective route to realize the QSH effect. Using the ABINIT package, we have investigated the properties of (1) TlP, the functionalized monolayers TlPX2 (X = F, Cl, Br, I); (2) TlAs, the functionalized monolayers TlAsX2 (X = F, Cl, Br, I), and (3) GaGeTe, InGeTe, and InSnTe systems. The topological nature is verified by the calculation of the Z2 topo-logical invariant. We discovered TlPF2, TlPCl2, TlPBr2, TlPI2, TlAs, TlAsF2, TlAsCl2, TlAsBr2, and TlAsI2 were promising 2D TIs with bulk band gaps as large as 0.21 eV. Each monolayer was suitable for room-temperature application, and show great potential for their future applications in quantum computers, nanoelectronics, and spintronics.

Controlled synthesis of photoresponsive bismuthinite (Bi2S3) nanostructures mediated through a new 1D bismuth-pyrimidylthiolate coordination polymer as a molecular precursor.

Bismuthinite (Bi2S3) nanostructures have garnered significant interest due to their appealing photoresponsivity which has positioned them as an attractive choice for energy conversion applications. However, to utilize their full potential, a simple and economically viable method of preparation is highly desirable. Herein, we present the synthesis and characterization including structural elucidation of a new air- and moisture-stable bismuth-pyrimidylthiolate complex. This complex serves as an efficient single-source molecular precursor for the facile preparation of phase-pure Bi2S3 nanostructures. Powder X-ray diffraction (PXRD), Raman spectroscopy, electron dispersive spectroscopy (EDS) and electron microscopy techniques were used to assess the crystal structure, phase purity, elemental composition and morphology of the as-prepared nanostructures. This study also revealed the profound effects of temperature and growth duration on the crystallinity, phase formation and morphology of nanostructures. The optical band gap of the nanostructures was tuned within the range of 1.9-2.3 eV, which is blue shifted with respect to the bulk bandgap and suitable for photovoltaic applications. Liquid junction photo-electrochemical cells fabricated from the as-prepared Bi2S3 nanostructure exhibit efficient photoresponsivity and good photo-stability, which project them as promising candidates for alternative low-cost photon absorber materials.

Controllable topological edge mode in an optically excited exciton-polariton lattice

We propose an all-optical scheme of topological lasing and switching based on the Aubry-Andr\'e-Harper (AAH) model of an exciton-polariton chain. We theoretically show that the phase parameter of the optical potential, with a tunable effective quasimomentum, allows the system to exhibit nontrivial topological properties which are attributed to higher dimensions. The topological modes emerging within the bulk band gaps are spatially localized at the edges of the polariton lattice, and their topological properties are characterized by the nonzero Chern numbers of the bulk bands. Polariton lasing in topological edge modes exhibits a higher efficiency and better robustness than in bulk modes, and can be switched between two opposite edges of the lattice by nonresonant excitation, which paves a way for topologically protected optical circuits.

Temperature-Driven Twin Structure Formation and Electronic Structure of Epitaxially Grown Mg3Sb2 Films on Mismatched Substrates.

Mg3Sb2-based compounds are one type of important room-temperature thermoelectric materials and the appropriate candidate of type-II nodal line semimetals. In Mg3Sb2-based films, compelling research topics such as dimensionality reduction and topological states rely on the controllable preparation of films with high crystallinity, which remains a big challenge. In this work, high quality Mg3Sb2 films are successfully grown on mismatched substrates of sapphire (000l), while the temperature-driven twin structure evolution and characteristics of the electronic structure are revealed in the as-grown Mg3Sb2 films by in situ and ex situ measurements. The transition of layer-to-island growth of Mg3Sb2 films is kinetically controlled by increasing the substrate temperature (Tsub), which is accompanied with the rational manipulation of twin structure and epitaxial strains. Twin-free structure could be acquired in the Mg3Sb2 film grown at a low Tsub of 573 K, while the formation of twin structure is significantly promoted by elevating the Tsub and annealing, in close relation to the processes of strain relaxation and enhanced mass transfer. Measurements of scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) elucidate the intrinsic p-type conduction of Mg3Sb2 films and a bulk band gap of ~0.89 eV, and a prominent Fermi level downshift of ~0.2 eV could be achieved by controlling the film growth parameters. As elucidated in this work, the effective manipulation of the epitaxial strains, twin structure and Fermi level is instructive and beneficial for the further exploration and optimization of thermoelectric and topological properties of Mg3Sb2-based films.