Large Bulk Band Gap Research Articles

Surface Electronic Structure of Cr Doped Bi2Se3 Single Crystals

Here, by using angle-resolved photoemission spectroscopy, we showed that Bi2−xCrxSe3 single crystals have a distinctly well-defined band structure with a large bulk band gap and undistorted topological surface states. These spectral features are unlike their thin film forms in which a large nonmagnetic gap with a distorted band structure was reported. We further provide laser-based high resolution photoemission data which reveal a Dirac point gap even in the pristine sample. The gap becomes more pronounced with Cr doping into the bulk of Bi2Se3. These observations show that the Dirac point can be modified by the magnetic impurities as well as the light source.

Large-Gap and Topological Crystalline Insulating Phase in RbZnBi and CsZnBi.

Topological insulators (TIs) are a new class of materials with gapless boundary states inside the bulk insulating gap. This metallic boundary state hosts intriguing phenomena such as helical spin textures and Dirac crossing points. Here, we theoretically propose RbZnBi and CsZnBi as a new family of TIs exhibiting large bulk band gaps and unique gapless surface states. Our first-principles density functional calculations show that two materials can be stabilized in two different structures depending on the stacking order of hexagonal ZnBi layers. While both materials in the AA-stacked structure become TI, the AB-stacked RbZnBi and CsZnBi are topological crystalline insulators with hourglass-shaped Fermion surface states protected by nonsymmorphic glide symmetry. The calculated bulk gap is about 1.5-1.8 times larger than that of Bi2Se3, which makes RbZnBi and CsZnBi promising candidates for future applications.

Epitaxial growth of BP-like Bi(1 1 0) with moiré pattern and potential topological edge state

Black-phosphorus-like (BP-like) Bi(110) thin film, a potential large band gap two-dimensional (2D) topological insulator (TI) candidate that hosts a non-dissipative topological edge state, has attracted a lot of attention in recent years. However, the Bi(110) thin film with a large bulk band gap and topological edge states has not been definitely confirmed experimentally. Here, high-quality BP-like Bi(110) thin film is epitaxially grown on a semiconductor SnSe substrate. Employing scanning tunneling microscopy/spectroscopy (STM/S), the BP-like structure was determined, and the corresponding electronic structure was also investigated. In addition, a distinct quasi-squared moiré pattern was observed, and the electronic states far away from Fermi energy in the 1BL Bi(110) film are significantly modulated by the moiré pattern spatially; instead, the electronic states near the Fermi energy are less influenced by the moiré pattern. Importantly, we also observed the enhanced states at the edge of 1BL and 2BL Bi(110) films, especially for 2BL film, with a clear sign of edge state peak extending to 2 nm depth, suggesting that 2BL BP-like Bi(110) on SnSe is a potential 2D TI. As a result, our research provides a platform for the further topological investigation of the BP-like Bi(110) thin film.

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.

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.

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.

Two-dimensional obstructed atomic insulators with fractional corner charge in the MA2Z4 family

According to topological quantum chemistry (TQC), a class of electronic materials has been called obstructed atomic insulators (OAIs), in which a portion of valence electrons necessarily have their centers located on some empty Wyckoff positions (WPs) without atom occupation in the lattice. The obstruction of centering these electrons coinciding with their host atoms is nontrivial and results in metallic boundary states when the boundary is properly cut. Here, on the basis of first-principles calculations in combination with TQC analysis, we propose two-dimensional ${MA}_{2}{Z}_{4}$ $(M=\mathrm{Cr}, \mathrm{Mo}, and \mathrm{W}; A=\mathrm{Si} \mathrm{and} \mathrm{Ge}; Z=\mathrm{N}, \mathrm{P}, \mathrm{and} \mathrm{As})$ monolayers are all OAIs. A typical case is the recently synthesized ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ (${\ensuremath{\alpha}}_{1}\text{\ensuremath{-}}{\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$). Although both ${\ensuremath{\alpha}}_{1}$- and ${\ensuremath{\alpha}}_{2}\text{\ensuremath{-}}{\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ monolayers are topological trivial insulators, it has valence electrons centering empty WPs and the in-gap corner states at the three vertices of triangle-shaped nanodisk samples respecting ${C}_{3}$ rotation symmetry, which can be explained by the filling anomaly of electrons. In addition, the ${\ensuremath{\alpha}}_{2}\text{\ensuremath{-}}{\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ monolayer exhibits unique OAI-induced metallic edge states along the $(1\overline{1})$ edge. The readily synthesized ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ is quite stable and has a large bulk bandgap of 1.94 eV, which makes the identification of these edge and corner states most possible for experimental clarification.

Computational analysis of the enhancement of photoelectrolysis using transition metal dichalcogenide heterostructures

Finding a material with all the desired properties for a photocatalytic water splitter is a challenge yet to be overcome, requiring both a surface with ideal energetics for all steps in the hydrogen and oxygen evolution reactions (HER and OER) and a bulk band gap large enough to mediate said steps. We have instead examined separating these challenges by investigating the energetic properties of two-dimensional transition metal dichalcogenides (TMDCs) that could be used as a surface coating to a material with a large enough bulk band gap. First we investigated the energetics of monolayer MoS2 and PdSe2 using density functional theory and then investigated how these energetics changed when they were combined into a heterostructure. Our results show that the surface properties were practically (0.2 eV) unchanged when combined and the MoS2 layer aligns well with the OER and HER. This work highlights the potential of TMDC monolayers as surface coatings for bulk materials that have sufficient band gaps for photocatalytic applications.

Ta4SiTe4 : A possible one-dimensional topological insulator

Using the first-principles calculations, we present a prediction of the nontrivial topological phase in one-dimensional (1D) ${\mathrm{Ta}}_{4}{\mathrm{SiTe}}_{4}$ with high stability further verified by the molecular dynamics simulation and phonon spectrum. In this material, the band inversion intrinsically occurs between $\mathrm{Ta}\text{\ensuremath{-}}d$ and $\mathrm{Te}\text{\ensuremath{-}}p$ orbitals without the spin-orbit coupling effect. As a novel topological insulator, protected by time-reversal symmetry Tˆ and inversion symmetry Pˆ, we explicitly demonstrate the existence of nontrivial topological invariants and the protected edge states in it with a large bulk band gap of $\ensuremath{\sim}163\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$, which could facilitate the experimental verification at the room temperature. In addition, we find its topological states are robust against the external pressure. Our results uncover a potential 1D topological-insulating ${\mathrm{Ta}}_{4}{\mathrm{SiTe}}_{4}$ and promote it as a concrete material platform for exploring the intriguing physics of low-dimensional topological phases.

Topological electronic states and thermoelectric transport at phase boundaries in single-layer WSe2: an effective Hamiltonian theory

Monolayer transition metal dichalcogenides in the distorted octahedral 1T′ phase exhibit a large bulk bandgap and gapless boundary states, which is an asset in the ongoing quest for topological electronics. In single-layer tungsten diselenide (WSe2), the boundary states have been observed at well ordered interfaces between 1T′ and semiconducting (1H) phases. This paper proposes an effective 4-band theory for the boundary states in single-layer WSe2, describing a Kramers pair of in-gap states as well as the behaviour at the spectrum termination points on the conduction and valence bands of the 1T′ phase. The spectrum termination points determine the temperature and chemical potential dependences of the ballistic conductance and thermopower at the phase boundary. Notably, the thermopower shows an ambipolar behaviour, changing the sign in the bandgap of the 1T′–WSe2 and reflecting its particle-hole asymmetry. The theory establishes a link between the bulk band structure and ballistic boundary transport in single-layer WSe2 and is applicable to a range of related topological materials.

Atomically Thin Quantum Spin Hall Insulators.

Atomically thin topological materials are attracting growing attention for their potential to radically transform classical and quantum electronic device concepts. Among them is the quantum spin Hall (QSH) insulator-a 2D state of matter that arises from interplay of topological band inversion and strong spin-orbit coupling, with large tunable bulk bandgaps up to 800 meV and gapless, 1D edge states. Reviewing recent advances in materials science and engineering alongside theoretical description, the QSH materials library is surveyed with focus on the prospects for QSH-based device applications. In particular, theoretical predictions of nontrivial superconducting pairing in the QSH state toward Majorana-based topological quantum computing are discussed, which are the next frontier in QSH materials research.

Nonmagnetic doping induced quantum anomalous Hall effect in topological insulators

Quantum anomalous Hall effect (QAHE) has been experimentally observed in magnetically doped topological insulators. However, ultra-low temperature (usually below 300 mK), which is mainly attributed to inhomogeneous magnetic doping, becomes a daunting challenge for potential applications. Here, a \textit{nonmagnetic}-doping strategy is proposed to produce ferromagnetism and realize QAHE in topological insulators. We numerically demonstrated that magnetic moments can be induced by nitrogen or carbon substitution in Bi$_2$Se$_3$, Bi$_2$Te$_3$, and Sb$_2$Te$_3$, but only nitrogen-doped Sb$_2$Te$_3$ exhibits long-range ferromagnetism and preserve large bulk band gap. We further show that its corresponding thin-film can harbor QAHE at temperatures of 17-29 Kelvin, which is two orders of magnitude higher than the typical temperatures in similar systems. Our proposed \textit{nonmagnetic} doping scheme may shed new light in experimental realization of high-temperature QAHE in topological insulators.

First-principles investigation of two-dimensional topological insulators BiX (X=H, F, O)

In order to exploit the nontrivial topological phase of matter for realizing room temperature transport applications, two-dimensional (2D) topological insulators (TIs) with large bulk band gaps are necessary. Based on first-principles calculations, the electronic and topological properties of 2D large gap TIs BiX (X = H, F, O) have been thoroughly investigated. We confirmed their nontrivial topological nature by calculating topological invariant (using Fu-Kane method) and observed odd number pairs of topologically protected gapless edge states from band structures of their zigzag nanoribbons. The strong spin–orbit coupling effect arising from heavy bismuth atoms not only affects the electronic structures, but also obviously influences the structural parameters such as lattice constant, bond length and buckling height. The key role of orbital filtering effect of X atoms in the electronic and topological properties has been illustrated by employing atomic-orbital-resolved band structures. Furthermore, we proposed a large band gap substrate, hexagonal boron nitride (h-BN), in a BiF/h-BN heterostructure, in which the h-BN substrate preserves the nontrivial topological behavior of BiF. To the best of our knowledge, BiF/h-BN with the gap of 1.041 eV has the largest band gap among all previously studied TI/substrate heterostructures. These findings make this structure a promising platform for high temperature low-dissipation spintronic nanodevices.

Edge mode bifurcations of two-dimensional topological lasers.

Topological lasers are of growing interest as a way to achieve disorder-robust single-mode lasing using arrays of coupled resonators. We study lasing in a two-dimensional coupled resonator lattice exhibiting transitions between trivial and topological phases, which allows us to systematically characterize the lasing modes throughout a topological phase. We show that, unlike conventional topological robustness that requires a sufficiently large bulk band gap, bifurcations in topological edge mode lasing can occur even when the band gap is maximized. We show that linear mode bifurcations from single-mode to multi-mode lasing can occur deep within the topological phase, sensitive to both the pump shape and lattice geometry. We suggest ways to suppress these bifurcations and preserve single-edge mode lasing.

Topological edge states at single layer WSe2 1T′–1H lateral heterojunctions

Transition metal dichalcogenides can be epitaxially grown at the single layer limit, which also adopt a variety of structural polymorphs with significantly different electronic properties. Lateral heterostructures of different polymorphs can be further synthesized for emergent functionality. Here, we selectively grow semiconducting 1H and metastable 1T′ phases of WSe2 on epitaxial graphene/SiC(0001) by molecular beam epitaxy and further show that the 1T′ phase is a two-dimensional topological insulator. Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, we determine that 1T′–WSe2 exhibits a large bulk bandgap of 120 meV, and edge states at the 1T′–1H lateral heterojunction extend within 1.6 nm from the heterointerface at the 1T′ side. These edge states are robust and persist regardless of the fact that if the edge is (1 × 1) zigzag or (3 × 1) reconstructed, confirming their topological nature. This further facilitates the epitaxial growth of 1T′–1H lateral junction superlattices with multiple helical edge channels, underpinning ultrahigh-density 2D topological nano quantum devices.

Two-dimensional ligand-functionalized plumbene: A promising candidate for ferroelectric and topological order with a large bulk band gap

Abstract Multifunctional two-dimensional (2D) materials has been attracted attentions to explore due to their versatile applications in electronic devices. Ferroelectric and topological materials have separately attracted investigated recently. However, materials with two properties coexisting are little known. 2D materials with coexisting topological and ferroelectric ordering have been the focus of 2D condensed matter physics in recent years, and their interaction in a single system may make important untapped phenomena and applications possible. Here, based on first-principles calculations, we demonstrate a class of ferroelectric topological insulators (FETIs) in CH2OCH3-functionalized plumbene (Pb–CH2OCH3). We find that the large nontrivial band gap of Pb–CH2OCH3 reaching 0.80 eV, which is introduced by the orbital filtering effect in the presence of spin-orbit coupling (SOC). Noticeably, the in-plane spontaneous polarization can be engineered by rotating the CH2OCH3 molecule at the surface of Plumbene. These results on the combination of topological and ferroelectric properties may provide a new platform for the development of microelectronic devices and the application of spintronics at room temperature.

Higher-Order Topological States in Surface-Wave Photonic Crystals.

Photonic topological states have revolutionized the understanding of the propagation and scattering of light. The recent discovery of higher‐order photonic topological insulators opens an emergent horizon for 0D topological corner states. However, the previous realizations of higher‐order topological insulators in electromagnetic‐wave systems suffer from either a limited operational frequency range due to the lumped components involved or a bulky structure with a large footprint, which are unfavorable for achieving compact photonic devices. To overcome these limitations, a planar surface‐wave photonic crystal realization of 2D higher‐order topological insulators is hereby demonstrated experimentally. The surface‐wave photonic crystals exhibit a very large bulk bandgap (a bandwidth of 28%) due to multiple Bragg scatterings and host 1D gapped edge states described by massive Dirac equations. The topology of those higher‐dimensional photonic bands leads to the emergence of in‐gap 0D corner states, which provide a route toward robust cavity modes for scalable compact photonic devices.

Current-Induced Gap Opening in Interacting Topological Insulator Surfaces.

Two-dimensional topological insulators (TIs) host gapless helical edge states that are predicted to support a quantized two-terminal conductance. Quantization is protected by time-reversal symmetry, which forbids elastic backscattering. Paradoxically, the current-carrying state itself breaks the time-reversal symmetry that protects it. Here we show that the combination of electron-electron interactions and momentum-dependent spin polarization in helical edge states gives rise to feedback through which an applied current opens a gap in the edge state dispersion, thereby breaking the protection against elastic backscattering. Current-induced gap opening is manifested via a nonlinear contribution to the system's I-V characteristic, which persists down to zero temperature. We discuss prospects for realizations in recently discovered large bulk band gap TIs, and an analogous current-induced gap opening mechanism for the surface states of three-dimensional TIs.

Epitaxial growth and electronic properties of few-layer stanene on InSb (1 1 1)

Stanene has been theoretically predicted to be a 2D topological insulator with a large band gap, potentially hosting quantum spin Hall effect at room temperature. Here, few-layer stanene films have been epitaxially grown on Sb-terminated InSb (1 1 1) surface and their structural and electrical properties are characterized. Scanning tunneling spectrum results reveal a large bulk bandgap in single-layer stanene (over 0.2 eV). Moreover, spectroscopy evidence for a filled edge state near the steps was observed. The gap decreases dramatically with increasing number of layers, and multilayer stanene should become a Dirac semimetal in the bulk limit. The changeover may involve nontrivial topological phase transitions. Clear and reproducible Shubnikov–de Haas oscillations were observed on the single-layer stanene films that were exposed to atmospheric conditions for an extended period of time, showing the possibility for device experiments using nanofabrication and magneto-transport. Our results demonstrate that the single-layer stanene is a promising topological material for exploring fundamental physics and quantum applications.

Designing Flexible Quantum Spin Hall Insulators through 2D Ordered Hybrid Transition-Metal Carbides

Quantum spin Hall (QSH) insulators have attracted much attention due to their potential applications ranging from electronic devices to quantum computing. In general, a large band gap is regarded as a critical descriptor in the design of QSH insulators; however, it faces challenges when additional factors such as strain and surface oxidation are involved in practical applications. In this work, taking M″2M′C2O2 (M′ = Ti, Zr, Hf; M″ = Mo, W) as a representative, results reveal that 2D ordered transition-metal carbides (MXenes) are promising candidates for flexible spintronic devices, which is ascribed to the mechanical flexibility and robust QSH states under strain. Although a large bulk band gap is shown in M″2HfC2O2, a strain-induced topological phase transition may limit its flexible application. On the contrary, M″2TiC2O2 has a smaller gap, and its topological nontrivial state survives under strain. When n changes from 0 to 4 in M″2TinCn+1O2, a topologically nontrivial–trivial phase transition is obser...