Strong Topological Insulator Research Articles

Half-quantum mirror Hall effect

We predict a half-quantized mirror Hall effect induced by mirror symmetry in strong topological insulator films. These films are known to host a pair of gapless Dirac cones in the first Brillouin zone associated with surface electrons. Our findings reveal that mirror symmetry assigns a unique mirror parity to each Dirac cone, resulting in a half-quantized Hall conductance of ±e22h\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm \\! \\frac{{e}^{2}}{2h}$$\\end{document} for each cone. Despite the total electrical Hall conductance being null due to time-reversal invariance, the difference in the Hall conductance between the two cones yields a quantized Hall conductance of e2h\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{{e}^{2}}{h}$$\\end{document} for the difference in mirror currents. The effect of helical edge mirror current - a crucial feature of this quantum effect - may, in principle, be determined by means of electrical measurements. The half-quantum mirror Hall effect reveals a type of mirror-symmetry induced quantum anomaly in a time-reversal invariant lattice system, giving rise to a topological metallic state of matter with time-reversal invariance.

Electron Transport along Screw Dislocations in a Strong Topological Insulator

Electron Transport along Screw Dislocations in a Strong Topological Insulator

Signal oscillations in helium scattering by bismuth atoms in the low energy range

Low Energy Ion Scattering (LEIS) analysis of bismuth selenide (Bi2Se3), a strong 3D topological insulator, revealed oscillations of the detected signal in dependence on primary ion beam energy. Bismuth is a part of the group of elements where oscillatory behaviour was already noticed, such as gallium, indium, or lead. Generally, it is ascribed to quasi-resonant charge exchange processes between primary ion and target atom. Moreover, clear differences exist between data for target atoms in elemental form and for atoms in compound form. A study of Bi signal yields in different material forms was performed for primary He+ projectiles within the energy range from 0.50 to 6.00 keV. A foil of pure Bi, Bi2Se3, Bi2O3 and thin Bi films deposited on several substrates (SiO2, CuOx) by Molecular Beam Epitaxy were analysed. The energy shift in the position of oscillations was observed when the oxygen atoms were present at the surface and bonded to Bi atoms. A description of the Bi ion yield variation is provided to facilitate the quantification process in LEIS.

Exploring the effects of a one-dimensional periodic potential on a three-dimensional topological insulator

High mobility two-dimensional systems with superposed 1D lateral periodic potentials exhibit characteristic commensurability (Weiss) oscillations that reflect the interplay of the cyclotron radius at the Fermi level and the superlattice period. Here, we impose a one-dimensional periodic potential on strained HgTe, which is a strong 3D topological insulator. By tuning the Fermi level with top gates, the effects of the artificial potential can be studied in the bulk gap, where only Dirac surface states exist, in the conduction band, and in the valence band, where Dirac electrons and holes coexist. On the electron side, we observe clear commensurability oscillations whose period is governed by the carrier density of the top-surface Dirac electrons. Unexpectedly, weak commensurability oscillations are also observed in the valence band with a period that depends on both electron and hole density. Published by the American Physical Society 2024

COF‐Topological Quantum Material Nano‐heterostructure for CO2 to Syngas Production under Visible Light

AbstractEfficient solar‐driven syngas production (CO+H2 mixture) from CO2 and H2O with a suitable photocatalyst and fundamental understanding of the reaction mechanism are the desired approach towards the carbon recycling process. Herein, we report the design and development of an unique COF‐topological quantum material nano‐heterostructure, COF@TI with a newly synthesized donor‐acceptor based COF and two dimensional (2D) nanosheets of strong topological insulator (TI), PbBi2Te4. The intrinsic robust metallic surfaces of the TI act as electron reservoir, minimising the fast electron‐hole recombination process, and the presence of 6s2 lone pairs in Pb2+ and Bi3+ in the TI helps for efficient CO2 binding, which are responsible for boosting overall catalytic activity. In variable ratio of acetonitrile‐water (MeCN : H2O) solvent mixture COF@TI produces syngas with different ratios of CO and H2. COF@TI nano‐heterostructure enables to produce higher amount of syngas with more controllable ratios of CO and H2 compared to pristine COF. The electron transfer route from COF to TI was realized from Kelvin probe force microscopy (KPFM) analysis, charge density difference calculation, excited state lifetime and photoelectrochemical measurements. Finally, a probable mechanistic pathway has been established after identifying the catalytic sites and reaction intermediates by in situ DRIFTS study and DFT calculation.

Polarization and Weak Topology in Chern Insulators.

Chern insulators, and more broadly, topological insulators, present an obstruction to the construction of exponentially localized electronic Wannier functions. This implies a fundamental difficulty in determining whether such insulators exhibit electric polarization. Here, we show that these insulators can indeed exhibit bound charges and adiabatic currents consistent with changes in bulk polarization over space and time, respectively. We also show that the change in polarization across crystalline domains within these strong topological insulators is quantized in the presence of crystalline symmetries.

COF-Topological Quantum Material Nano-heterostructure for CO2 to Syngas Production under Visible Light.

Efficient solar-driven syngas production (CO+H2 mixture) from CO2 and H2O with a suitable photocatalyst and fundamental understanding of the reaction mechanism are the desired approach towards the carbon recycling process. Herein, we report the design and development of an unique COF-topological quantum material nano-heterostructure, COF@TI with a newly synthesized donor-acceptor based COF and two dimensional (2D) nanosheets of strong topological insulator (TI), PbBi2Te4. The intrinsic robust metallic surfaces of the TI act as electron reservoir, minimising the fast electron-hole recombination process, and the presence of 6s2 lone pairs in Pb2+ and Bi3+ in the TI helps for efficient CO2 binding, which are responsible for boosting overall catalytic activity. In variable ratio of acetonitrile-water (MeCN : H2O) solvent mixture COF@TI produces syngas with different ratios of CO and H2. COF@TI nano-heterostructure enables to produce higher amount of syngas with more controllable ratios of CO and H2 compared to pristine COF. The electron transfer route from COF to TI was realized from Kelvin probe force microscopy (KPFM) analysis, charge density difference calculation, excited state lifetime and photoelectrochemical measurements. Finally, a probable mechanistic pathway has been established after identifying the catalytic sites and reaction intermediates by in situ DRIFTS study and DFT calculation.

Nontrivial Topological Phases in Ternary Borides M2XB2 (M=W, Mo; X=Co, Ni)

The nontrivial band topologies protected by certain symmetries have attracted significant interest in condensed matter physics. The discoveries of nontrivial topological phases in real materials provide a series of archetype materials to further explore the topological physics. Ternary borides M2XB2 (M = W, Mo; X = Co, Ni) have been widely investigated as the wear-resistant and high-hardness materials. Based on first-principles calculations, we find the nontrivial topological properties in these materials. Taking W2NiB2 as an example, this material shows the nodal line semimetal state in the absence of spin-orbit coupling. Two types of nodal lines appear near the Fermi level simultaneously. One is protected by the combined space-inversion and time-reversal symmetry, and the other is by the mirror symmetry. Part of these two-type nodal lines form nodal chains. When spin-orbit coupling is included, these nodal lines are fully gapped and the system becomes a strong topological insulator with nontrivial Z 2 index (1;000). Our calculations demonstrate that a nontrivial spin-momentum locked surface Dirac cone appears on the (1¯10) surface. We also find that other isostructural ternary borides Mo2NiB2, Mo2CoB2, and W2CoB2 possess similar topological band structures. Therefore, our work not only enriches the understanding of band topology for ternary borides, but also lays the foundation for the further study of topological phases manipulation and potential spintronic applications in realistic materials.

Tailoring weak and metallic phases in a strong topological insulator by strain and disorder: Conductance fluctuations signatures

Tailoring weak and metallic phases in a strong topological insulator by strain and disorder: Conductance fluctuations signatures

Controllable strain-driven topological phase transition and dominant surface-state transport in HfTe5

The fine-tuning of topologically protected states in quantum materials holds great promise for novel electronic devices. However, there are limited methods that allow for the controlled and efficient modulation of the crystal lattice while simultaneously monitoring the changes in the electronic structure within a single sample. Here, we apply significant and controllable strain to high-quality HfTe5 samples and perform electrical transport measurements to reveal the topological phase transition from a weak topological insulator phase to a strong topological insulator phase. After applying high strain to HfTe5 and converting it into a strong topological insulator, we found that the resistivity of the sample increased by 190,500% and that the electronic transport was dominated by the topological surface states at cryogenic temperatures. Our results demonstrate the suitability of HfTe5 as a material for engineering topological properties, with the potential to generalize this approach to study topological phase transitions in van der Waals materials and heterostructures.

A combined first-principles calculations and X-ray photoelectron spectroscopy of Ce2T3X9 (T = Rh, Ru, Ir; X = Al, Ga): Possible strong topological insulator state in Ce2Ir3Al9

We calculated electronic structure of Ce2Rh3Ga9, Ce2Ru3Ga9, La2Ir3Al9 and Ce2Ir3Al9 compounds and we found that Ce2Ir3Al9 may exhibit strong topological insulator state according to Z2 index estimation based on parity algorithm and shape of band structure near Fermi level. The same algorithm shows strong topological insulator state for La2Ir3Al9 and Ce2Rh3Ga9 compounds, but their band structure near Fermi level is more like semimetallic one. Ce2Ru3Ga9 is a normal metal. Our calculations show no magnetic ordering for studied compounds. We also measured the XPS (X-ray Photoelectron Spectroscopy) spectra for these materials, and compared the valence band with simulated spectra, where good agreement was found. Core level states were also investigated and compared with Mulliken population analysis results. According to our measurements and calculations, the highest Ce valence is observed for Ce2Ir3Al9 whereas for Ce2Rh3Ga9 and Ce2Ru3Ga9 the Ce valence is smaller. Hybridization between conduction electrons and 4f states is very small, and for Ce2Ru3Ga9 essentially negligible.

ZrOsSi: a Z2 topological metal with a superconducting ground state

The silicide superconductors (Ta, Nb, Zr)OsSi are among the best candidate materials for investigating the interplay of topological order and superconductivity. Here, we investigate in detail the normal-state topological properties of (Ta, Nb, Zr)OsSi, focusing on ZrOsSi, by employing a combination of 29Si nuclear magnetic resonance (NMR) measurements and first-principles band-structure calculations. We show that, while (Ta, Nb)OsSi behave as almost ideal metals, characterized by weak electronic correlations and a relatively low density of states, the replacement of Ta (or Nb) with Zr expands the crystal lattice and shifts ZrOsSi towards an insulator. Our ab initio calculations indicate that ZrOsSi is a Z2 topological metal with clear surface Dirac cones and properties similar to a doped strong topological insulator.

Structural and topological phase transitions in Se doping-controlled intrinsic magnetic topological material FeBi2Te4

Defects profoundly affect the magnetic, electronic structure and topological behaviour of intrinsic magnetic topological insulators. Here, we investigate the magnetic, structural stability and topological properties of different Te atomic sites in the intrinsic magnetic topological insulator FeBi2Te4 with Se substitution of the FM-z magnetic order. When Se replaces the outermost site of the septuple layer (the Te atomic layer connected by van der Waals bonds), the Se–Bi bond length formed with its nearest neighbour Bi is drastically shortened and the lattice constant and volume contract accordingly. We speculate that this defect-induced chemical bond enhancement is related to the strong electronegativity of Se over Te. In contrast, the system has lower formation energy, a more stable structure, and enhanced magnetic moment when Se replaces the Te atomic layer inside the septuple layer. Also, we reveal a variety of topological phase transitions due to substitution defects. FeBi2Te4 is considered to belong to the topological classification of higher-order topological insulators with z 4 = 2. The system after Se substitution can be transformed into Weyl semimetals, strong 3D topological insulators, and unknown topological materials without symmetry-base indicators, respectively.

Band topology and phase stability of half-Heusler semiconductors with d0 alkaline earth atoms

In the previous studies, the topological insulators in the half-Heusler ternary structure are assumed to be in the γ-phase having the transition metal atom at the unique middle site. In contrast, the minimum-energy half-Heusler phase can well be the β-phase with the highly electronegative anion positioned in between the electropositive metallic ions. For this subtle reason in the following a systematic study of the phase stability and the band topology of XAuZ compounds containing d0 alkaline earth atoms (X = Ca, Sr, Ba; Z = As, Sb) in their respective minimum-energy half-Heusler phases is presented. It is thus found that all the above compounds are mechanically (or dynamically) stable and have formation energies that are within a reasonable distance from the thermodynamic hull. The band structures of all compounds obtained from the more accurate TB-mBJ exchange potential with the SOC included indicate a strong band inversion such that the s-like Γ6 band falls below the p-like (Γ7+Γ8) bands. Furthermore, due to the negative spin–orbit splitting in the β-phase compounds SrAuAs and BaAuAs, the Γ8 falls below the Γ7, which opens a significant bulk insulating gap at the Fermi level. The outputs of WannierTools, such as the Z2 topological quantum numbers, surface states spectra and surface Fermi loops, indicate that the β-phase compounds SrAuAs and BaAuAs are indeed strong topological insulators with a polar character due to the lack of inversion symmetry, while non-uniform strains are needed to turn the γ-phase compounds into Weyl semimetals (ca>1) and topological insulators (ca<1).

Topological insulator path toward efficient hydrogen evolution catalysts in the Li2Pt family

Topological materials, such as topological semimetals and topological insulators, with robust topological surface states have bright application prospects in electrochemical catalysis. Here, the first-principles calculations indicate the strong topological insulator ${\mathrm{Li}}_{2}\mathrm{Pt}$ family promotes efficient catalytic response to the hydrogen evolution reaction. For ${\mathrm{Li}}_{2}\mathrm{Pt}$ and ${\mathrm{Li}}_{2}\mathrm{Pd}$, the calculated Gibbs free energy $\mathrm{\ensuremath{\Delta}}{G}_{{\mathrm{H}}^{*}}$ of the bridge site is 0.054 eV and 0.041 eV, while that for the top site is 0.187 eV and 0.641 eV, respectively. The better hydrogen evolution reaction performance of the bridge site can ascribe to H hybridizes with the ${d}_{xy}+{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital, which donates the nontrivial topological surface states, while H hybridizes with the ${d}_{{z}^{2}}$ orbital that withholds contribution to topological surface states for the top site. Noticeably, the $\mathrm{\ensuremath{\Delta}}{G}_{{\mathrm{H}}^{*}}$ of the bridge site for ${\mathrm{Li}}_{2}\mathrm{Pt}$ (0.054 eV) and ${\mathrm{Li}}_{2}\mathrm{Pd}$ (0.041 eV) is nearly half of the value of Pt (0.09 eV), indicating an excellent hydrogen evolution reaction activity. This work uncovers the hybridization between adsorbate and topological surface states plays a vital role in enhancing the hydrogen evolution reaction performance and provides a promising route to design topological quantum catalysts.

Evidence of charge transfer in three-dimensional topological insulator/antiferromagnetic Mott insulator Bi1Sb1Te1.5Se1.5/α−RuCl3 heterostructures

We report magnetotransport measurements on ${\mathrm{Bi}}_{1}{\mathrm{Sb}}_{1}{\mathrm{Te}}_{1.5}{\mathrm{Se}}_{1.5}/{\mathrm{RuCl}}_{3}$ heterostructure nanodevices. ${\mathrm{Bi}}_{1}{\mathrm{Sb}}_{1}{\mathrm{Te}}_{1.5}{\mathrm{Se}}_{1.5}$ (BSTS) is a strong three-dimensional topological insulator (3D-TI) that hosts conducting topological surface states (TSS) enclosing an insulating bulk. $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ (namely, ${\mathrm{RuCl}}_{3}$) is an antiferromagnet that is predicted to behave as a Kitaev-like quantum spin liquid carrying chargeless Majorana excitations. Temperature ($T$)-dependent resistivity measurements on BSTS show the interplay between parallel bulk and surface transport channels. For $T&lt;150$ K, surface state transport dominates over bulk transport. Multichannel weak antilocalization (WAL) is observed as a sharp cusp in the low-field magnetoconductivity, indicating strong spin-orbit coupling. The presence of top and bottom topological surface states (TSS) including a pair of electrically coupled Rashba surface states (RSS) is indicated. The nonlinear Hall effect, explained by a two-band model, further supports this interpretation. We observed that ${\mathrm{RuCl}}_{3}$ has an electrostatic effect on the TSS of BSTS, that is, it changes bandbending at the interface as a result of charge transfer. In this regard, the change in low-$T$ logarithmic resistivity upturn in the presence of out-of-plane magnetic field is analyzed employing the Lu-Shen model, supporting the presence of gapless surface states (TSS and RSS) with a $\ensuremath{\pi}$ Berry phase.

Prediction of wide-gap topological insulating phase in metastable BiTeI

BiTeI crystals possess many unique pressure-induced electronic properties; however, high-pressure conditions limit their practical applications to some extent. We conducted crystal structure prediction from 3000 structures to determine several structures with high thermodynamic stability. By studying the topological invariants, we found one strong topological insulator with inversion symmetry and one weak topological insulator. Spin splitting was found in two structures, and their spin texture was analyzed by effective SOC Hamiltonian and symmetry. Our study showed that some structures are not only topologically nontrivial but can also simultaneously possess Rashba-type spin splitting with a band gap of up to 860 meV.

Multiple surface states, nontrivial band topology, and antiferromagnetism in GdAuAl4Ge2

Magnetic topological states of matter provide a fertile playground for emerging topological physics and phenomena. The current main focus is on materials whose magnetism stems from 3d magnetic transition elements, e.g., MnBi2Te4, Fe3Sn2, and Co3Sn2S2. In contrast, topological materials with the magnetism from rare earth elements remain largely unexplored. Here we report rare earth antiferromagnet GdAuAl4Ge2 as a candidate magnetic topological metal. Angle resolved photoemission spectroscopy (ARPES) and first-principles calculations have revealed multiple bulk bands crossing the Fermi level and pairs of low energy surface states. According to the parity and Wannier charge center analyses, these bulk bands possess nontrivial Z 2 topology, establishing a strong topological insulator state in the nonmagnetic phase. Furthermore, the surface band pairs exhibit strong termination dependence which provides insight into their origin. Our results suggest GdAuAl4Ge2 as a rare earth platform to explore the interplay between band topology, magnetism and f electron correlation, calling for further study targeting on its magnetic structure, magnetic topology state, transport behavior, and microscopic properties.

Topology Hierarchy of Transition Metal Dichalcogenides Built from Quantum Spin Hall Layers.

The evolution of the physical properties of two-dimensional material from monolayer limit to the bulk reveals unique consequences from dimension confinement and provides a distinct tuning knob for applications. Monolayer 1T'-phase transition metal dichalcogenides (1T'-TMDs) with ubiquitous quantum spin Hall (QSH) states are ideal two-dimensional building blocks of various three-dimensional topological phases. However, the stacking geometry was previously limited to the bulk 1T'-WTe2 type. Here, we introduce the novel 2M-TMDs consisting of translationally stacked 1T'-monolayers as promising material platforms with tunable inverted bandgaps and interlayer coupling. By performing advanced polarization-dependent angle-resolved photoemission spectroscopy as well as first-principles calculations on the electronic structure of 2M-TMDs, we revealed a topology hierarchy: 2M-WSe2 , MoS2, and MoSe2 are weak topological insulators (WTIs), whereas 2M-WS2 is a strong topological insulator (STI). Further demonstration of topological phase transitions by tunning interlayer distance indicates that band inversion amplitude and interlayer coupling jointly determine different topological states in 2M-TMDs. We propose that 2M-TMDs are parent compounds of various exotic phases including topological superconductors and promise great application potentials in quantum electronics due to their flexibility in patterning with two-dimensional materials. This article is protected by copyright. All rights reserved.

Quantum oscillations and three-dimensional quantum Hall effect in ZrTe5

Recent experiments have reported a lot of spectacular transport properties in topological materials, such as quantum oscillations and three-dimensional (3D) quantum Hall effect (QHE) in ${\mathrm{ZrTe}}_{5}$. In this paper, by using a strong topological insulator model to describe ${\mathrm{ZrTe}}_{5}$, we study the magnetotransport property of the 3D system. With fixed carrier density, we find that there exists a deferring effect in the chemical potential, which favors distinguishing the saddle points of the inverted LLs. On the other hand, with fixed chemical potential, the features of 3D QHE are demonstrated and we attribute the underlying mechanisms to the interplay between Dirac fermions, magnetic field, and impurity scatterings.