Weak Topological Insulator Research Articles

Supervised learning of an interacting two-dimensional hardcore boson model of a weak topological insulator using correlation functions

Supervised learning of an interacting two-dimensional hardcore boson model of a weak topological insulator using correlation functions

Signature of pressure-induced topological phase transition in ZrTe5

The layered van der Waals material ZrTe5 is known as a candidate topological insulator (TI), however its topological phase and the relation with other properties such as an apparent Dirac semimetallic state is still a subject of debate. We employ a semiclassical multicarrier transport (MCT) model to analyze the magnetotransport of ZrTe5 nanodevices at hydrostatic pressures up to 2 GPa. The temperature dependence of the MCT results between 10 and 300 K is assessed in the context of thermal activation, and we obtain the positions of conduction and valence band edges in the vicinity of the chemical potential. We find evidence of the closing and re-opening of the band gap with increasing pressure, which is consistent with a phase transition from weak to strong TI. This matches expectations from ab initio band structure calculations, as well as previous observations that CVT-grown ZrTe5 is a weak TI in ambient conditions.

Weak topological insulators, nodal-line semimetals, and Dirac semimetals in phononic crystal plates

Topological phases of matter help us to modulate elastic waves at the boundaries of mechanical systems with robustness against defects. Here, we propose to realize weak topological insulator phases, nodal-line semimetal phases, and Dirac semimetal phases in 2D mechanical systems. We implement this idea by theoretical analysis of tight-binding models and numerical simulations of phononic crystal plates based on the two-dimensional Su-Schrieffer-Heeger models. The topological nature of these topological phases is characterized by the topological invariants. The edge states, the visualization feature of the weak topological insulators, are observed unambiguously in the phononic crystal plates. Particularly, the robustness of these edge states against structural defects is verified by removing four single square plates from the center. Experimentally, we realize two types of weak topological insulators in 2D mechanical systems. This study fertilizes the topological phases in mechanical systems and opens excellent avenues for manipulating elastic wave propagation.

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.

Ideal weak topological insulator and protected helical saddle points

Ideal weak topological insulator and protected helical saddle points

Ab-initio simulations of coherent phonon-induced pumping of carriers in zirconium pentatelluride

Laser-driven coherent phonons can act as modulated strain fields and modify the adiabatic ground state topology of quantum materials. Here we use time-dependent first-principles and effective model calculations to simulate the effect of the coherent phonon induced by strong terahertz electric field on electronic carriers in the topological insulator ZrTe5. We show that a coherent A1g Raman mode modulation can effectively pump carriers across the band gap, even though the phonon energy is about an order of magnitude smaller than the equilibrium band gap. We reveal the microscopic mechanism of this effect which occurs via Landau–Zener-Stückelberg tunneling of Bloch electrons in a narrow region in the Brillouin zone center where the transient energy gap closes when the system switches from strong to weak topological insulator. The quantum dynamics simulation results are in excellent agreement with recent pump-probe experiments in ZrTe5 at low temperature.

Towards layer-selective quantum spin hall channels in weak topological insulator Bi4Br2I2

Weak topological insulators, constructed by stacking quantum spin Hall insulators with weak interlayer coupling, offer promising quantum electronic applications through topologically non-trivial edge channels. However, the currently available weak topological insulators are stacks of the same quantum spin Hall layer with translational symmetry in the out-of-plane direction—leading to the absence of the channel degree of freedom for edge states. Here, we study a candidate weak topological insulator, Bi4Br2I2, which is alternately stacked by three different quantum spin Hall insulators, each with tunable topologically non-trivial edge states. Our angle-resolved photoemission spectroscopy and first-principles calculations show that an energy gap opens at the crossing points of different Dirac cones correlated with different layers due to the interlayer interaction. This is essential to achieve the tunability of topological edge states as controlled by varying the chemical potential. Our work offers a perspective for the construction of tunable quantized conductance devices for future spintronic applications.

Anisotropic Topological Anderson Transitions in Chiral Symmetry Classes.

We study quantum phase transitions of three-dimensional disordered systems in the chiral classes (AIII and BDI) with and without weak topological indices. We show that the systems with a nontrivial weak topological index universally exhibit an emergent thermodynamic phase where wave functions are delocalized along one spatial direction but exponentially localized in the other two spatial directions, which we call the quasilocalized phase. Our extensive numerical study clarifies that the critical exponent of the Anderson transition between the metallic and quasilocalized phases, as well as that between the quasilocalized and localized phases, are different from that with no weak topological index, signaling the new universality classes induced by topology. The quasilocalized phase and concomitant topological Anderson transition manifest themselves in the anisotropic transport phenomena of disordered weak topological insulators and nodal-line semimetals, which exhibit the metallic behavior in one direction but the insulating behavior in the other directions.

Next-Generation Quantum Materials for Thermoelectric Energy Conversion

This review presents the recent advances in the search for thermoelectric (TE) materials, mostly among intermetallic compounds and in the enhancement of their TE performance. Herein, contemporary approaches towards improving the efficiency of heat–electricity conversion (e.g., energy harvesting and heat pumping) are discussed through the understanding of various emergent physical mechanisms. The strategies for decoupling the individual TE parameters, as well as the simultaneous enhancement of the TE power factor and the suppression of heat conduction, are described for nanoparticle-doped materials, high entropy alloys, and nanowires. The achievement of a superior TE performance due to emergent quantum phenomena is discussed for intermetallic chalcogenides and related systems (e.g., strong and weak topological insulators, Weyl and Dirac semimetals), and some of the most promising compounds within these classes are highlighted. It was concluded that high-entropy alloying provides a methodological breakthrough for employing band engineering methods along with various phonon scattering mechanisms towards significant TE efficiency improvement in conventional TE materials. Finally, topological semimetals and magnetic semimetals with several intriguing features, such as a violation of the Wiedemann–Franz law and outstanding perpendicular Nernst signals, are presented as strong candidates for becoming next-generation TE quantum materials.

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.

Bulk-surface coupling in dual topological insulator Bi1Te1 and Sb-doped Bi1Te1 single crystals via electron-phonon interaction

Recently, has been proved to be a dual topological insulator (TI), a new subclass of symmetry-protected topological phases, and predicted to be higher order topological insulator (HOTI). Being a dual TI (DTI), Bi1Te1 is said to host quasi-1D surface states (SSs) due to weak TI phase and topological crystalline insulating SSs at the same time. On the other hand, HOTI supports topologically protected hinge states. So, is a unique platform to study the electrical signature of topological SS (TSS) of fundamentally different origins. Though there is a report of magneto-transport measurements on large-scale Bi1Te1 thin films, the Bi1Te1 single crystal is not studied experimentally to date. Even the doping effect in a DTI Bi1Te1 is missing in the literature. In this regard, we performed the perpendicular and parallel field magneto-transport measurement on the exfoliated microflake of Bi1Te1 and Sb-doped Bi1Te1 single crystals, grown by the modified Bridgmann method. Our metallic sample shows the weak anti-localization behavior analyzed by the multi-channel Hikami-Larkin-Nagaoka equation. We observed the presence of a pair of decoupled TSS. Further, we extracted the dephasing index (β) from temperature (T)-dependence of phase coherence length (L), following the power law equation (L). The thickness-dependent value of β indicates the transition in the dephasing mechanism from electron-electron to electron-phonon interaction with the increase in thickness, indicating the enhancement in the strength of bulk-surface coupling. Sb-doped system shows weakened bulk-surface coupling, hinted by the reduced dephasing indices.

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.

Electronic Structure of the Weak Topological Insulator Candidate Zintl Ba3Cd2Sb4

One of the greatest triumph of condensed matter physics in the past ten years is the classification of materials by the principle of topology. The existence of topological protected dissipationless surface state makes topological insulators great potential for applications and hotly studied. However, compared with the prosperity of strong topological insulators, theoretical predicted candidate materials and experimental confirmation of weak topological insulators (WTIs) are both extremely rare. By combining systematic first-principles calculation and angle-resolved photoemission spectroscopy measurements, we have studied the electronic structure of the dark surface of the WTI candidate Zintl Ba3Cd2Sb4 and another related material Ba3Cd2As4. The existence of two Dirac surface states on specific side surfaces predicted by theoretical calculations and the observed two band inversions in the Brillouin zone give strong evidence to prove that the Ba3Cd2Sb4 is a WTI. The spectroscopic characterization of this Zintl Ba3Cd2N4 (N = As and Sb) family materials will facilitate applications of their novel topological properties.

High-quality thin film growth of the weak topological insulator BiSe on Si (111) substrates via pulsed laser deposition

In this work, we report the growth of high-quality BiSe thin films deposited on Si (111) substrates at different temperatures via pulsed laser deposition. We observe poor sample quality at a low substrate temperature (Tsub=175°C), and as the substrate temperature increases, the crystallinity of the samples increases. At a substrate temperature, Tsub=250°C, BiSe Raman modes (modes centered around 97.6 and 112.9 cm−1) start to emerge with less intensity and evolve with the increase in the substrate temperature and at Tsub=325°C closely match with that of single crystals. These modes correspond to the vibrations of Se-atoms from the Bi2Se3 quintuple layers and Bi-atoms from the Bi-bilayer. By carefully investigating the structural properties and the Raman modes of BiSe thin films at each substrate temperature, we provide an optimal condition to grow high-quality thin films of BiSe by pulsed laser deposition.

Temperature dependence of the energy band gap in ZrTe5 : Implications for the topological phase

Using Landau level spectroscopy, we determine the temperature dependence of the energy band gap in zirconium pentatelluride (ZrTe$_5$). We find that the band gap reaches $E_g=(5 \pm 1)$ meV at low temperatures and increases monotonously when the temperature is raised. This implies that ZrTe$_5$ is a weak topological insulator, with non-inverted ordering of electronic bands in the center of the Brillouin zone. Our magneto-transport experiments performed in parallel show that the resistivity anomaly in ZrTe$_5$ is not connected with the temperature dependence of the band gap.

Pressure-induced superconductivity in the weak topological insulator BiSe

Layered BiSe in the trigonal $P\overline{3}m1$ phase is a weak topological insulator and a candidate topological crystalline insulator. Here using structural, spectroscopic, resistance measurements at high pressure and density functional theory calculations, we report that BiSe exhibits a rich phase diagram with the emergence of superconductivity above 7 GPa. Structural transitions into SnSe-type energetically tangled orthorhombic structures and, subsequently, into a CsCl-type cubic structure having distinct superconducting properties are identified at 8 and 13 GPa, respectively. Superconductivity is preserved as the system transforms back to the trigonal phase upon release of pressure. Spin-orbit coupling plays a significant role in enhancement of ${T}_{c}$ in the trigonal and cubic phases. In the orthorhombic $Cmcm$ phase, ${T}_{c}$ decreases monotonically with increasing pressure, whereas unusual pressure-independent ${T}_{c}$ is observed in the cubic $Pm\overline{3}m$ phase. Theoretical analysis reveals topological surface states in the cubic phase. The emergence of superconductivity within the topological phases makes BiSe a candidate topological superconductor.

Evidence of pressure-induced multiple electronic topological transitions in BiSe

BiSe, an exciting member of homologous family (Bi2)m (Bi2Se3)n [where, m = 1; n = 2], has recently gained attention in thermoelectric research due to its intrinsically ultralow lattice thermal conductivity and promising n-type thermoelectric performance near room temperature. It is experimentally observed to be a weak topological insulator and possesses a fascinating crystal structure where Bi2 bilayer is sandwiched between two Bi2Se3 quintuple layers, thus the structure resembles natural van der Waals heterostructure (Bi2Se3–Bi2–Bi2Se3). Herein, we have studied the pressure induced electronic topological transition (ETT) in BiSe through high pressure (0–10 GPa) synchrotron X-ray diffraction, Raman spectroscopy and first-principles density functional theory (DFT). We have observed clear anomalies at ∼1 and ∼2.2 GPa in the pressure dependent lattice parameters (a, c, and c/a ratio), cell volume, Raman mode shift and line width data suggesting the presence of two ETTs which is further verified by electronic structures and Fermi surface calculations under pressure. The origin of these ETTs is associated with the two different vibrational modes arising from Bi2 bilayer and Bi2Se3 quintuple layers of BiSe, respectively.

Exploring strong and weak topological states on isostructural substitutions in TlBiSe_2

Topological Insulators (TIs) are unique materials where insulating bulk hosts linearly dispersing surface states protected by the Time-Reversal Symmetry. These states lead to dissipationless current flow, which makes this class of materials highly promising for spintronic applications. Here, we predict TIs by employing state-of-the-art first-principles based methodologies, viz., density functional theory and many-body perturbation theory (G_0W_0) combined with spin-orbit coupling effects. For this, we take a well-known 3D TI, TlBiSe_2 and perform complete substitution with suitable materials at different sites to check if the obtained isostructural materials exhibit topological properties. Subsequently, we scan these materials based on SOC-induced parity inversion at Time-Reversal Invariant Momenta. Later, to confirm the topological nature of selected materials, we plot their surface states along with calculation of Z_2 invariants. Our results show that GaBiSe_2 is a strong Topological Insulator, besides, we report six weak Topological Insulators, viz., PbBiSe_2, SnBiSe_2, SbBiSe_2, Bi_2Se_2, TlSnSe_2 and PbSbSe_2. We have further verified that all the reported TIs are dynamically stable, showing all real phonon modes of vibration.

Weak quantization of noninteracting topological Anderson insulator

We study the transition between the two-dimensional topological insulator (TI) featuring quantized edge conductance and the trivial Anderson insulator (AI) induced by strong disorder. We discover a distinct scaling behavior of TI near the phase transition where the longitudinal conductance approaches the quantized value by a power law with system size, instead of an exponential law in clean TI. This region is thus called the weak quantization topological insulator (WQTI). By using the self-consistent Born approximation, we associate the emergence of the weak quantization with the imaginary part of the effective self-energy acquiring a finite value at strong disorder. We use our analytical theory, supported by direct numerical simulations, to study the effect of disorder range on the topological Anderson insulator. Interestingly, while this phase is quite generic for uncorrelated or short-range disorder, it is strongly suppressed by long-range disorder, perhaps explaining why it has never been seen in solid state systems.

Nontrivial gapless electronic states at the stacking faults of weak topological insulators

Lattice defects such as stacking faults may obscure electronic topological features of real materials. In fact, defects are a source of disorder that can enhance the density of states and conductivity of the bulk of the system and they break crystal symmetries that can protect the topological states. On the other hand, in recent years it has been shown that lattice defects can act as a source of nontrivial topology. Motivated by recent experiments on three-dimensional (3D) topological systems such as Bi$_2$TeI and Bi$_{14}$Rh$_3$I$_9$, we examine the effect of stacking faults on the electronic properties of weak topological insulators (WTIs). Working with a simple model consisting of a 3D WTI formed by weakly-coupled two-dimensional (2D) topological layers separated by trivial spacers, we find that 2D stacking faults can carry their own, topologically nontrivial gapless states. Depending on the WTI properties, as well as the way in which the stacking fault is realized, the latter can form a topologically protected 2D semimetal, but also a 2D topological insulator which is embedded in the higher-dimensional WTI bulk. This suggests the possibility of using stacking faults in real materials as a source of topologically nontrivial, symmetry-protected conducting states.