Topological Quantum Materials Research Articles

Weyl, Dirac and high-fold chiral fermions in topological quantum matter

Quantum materials hosting Weyl fermions have opened a new era of research in condensed matter physics. First proposed in 1929 in particle physics, Weyl fermions have yet to be observed as elementary particles. In 2015, Weyl fermions were detected as collective electronic excitations in the strong spin-orbit coupled material tantalum arsenide, TaAs. This discovery was followed by a flurry of experimental and theoretical explorations of Weyl phenomena in materials. Weyl materials naturally lend themselves to the exploration of the topological index associated with Weyl fermions and their divergent Berry curvature field, as well as the topological bulk-boundary correspondence giving rise to protected conducting surface states. Here, we review the broader class of Weyl topological phenomena in materials, starting with the observation of emergent Weyl fermions in the bulk and of Fermi arc states on the surface of the TaAs family of crystals by photoemission spectroscopy. We then discuss some of the exotic optical and magnetic responses observed in these materials, as well as the progress in developing some of the related chiral materials. We discuss the conceptual development of high-fold chiral fermions, which generalize Weyl fermions, and we review the observation of high-fold chiral fermion phases by taking the rhodium silicide, RhSi, family of crystals as a prime example. Lastly, we discuss recent advances in Weyl-line phases in magnetic topological materials. With this Review, we aim to provide an introduction to the basic concepts underlying Weyl physics in condensed matter, and to representative materials and their electronic structures and topology as revealed by spectroscopic studies. We hope this work serves as a guide for future theoretical and experimental explorations of chiral fermions and related topological quantum systems with potentially enhanced functionalities.

Microwave response in a topological superconducting quantum interference device

Photon detection at microwave frequency is of great interest due to its application in quantum computation information science and technology. Herein are results from studying microwave response in a topological superconducting quantum interference device (SQUID) realized in Dirac semimetal Cd3As2. The temperature dependence and microwave power dependence of the SQUID junction resistance are studied, from which we obtain an effective temperature at each microwave power level. It is observed the effective temperature increases with the microwave power. This observation of large microwave response may pave the way for single photon detection at the microwave frequency in topological quantum materials.

Enhancing electrocatalytic hydrogen evolution of WTe2 by formation of amorphous phosphate nanoshells

Recently, designing new catalysts based on topological quantum materials has attracted much research interest. Here, we have explored WTe2, a type-II Weyl semimetal, as an effective electrocatalyst for hydrogen evolution reaction (HER). New nanostructure am-WPO@WTe2 has been constructed based on WTe2, in which the crystalline P-doped WTe2 core is covered by amorphous phosphate species. Due to the unique core-shell structure, the HER catalytic performance of am-WPO@WTe2 has been greatly enhanced, delivering a low overpotential of 220 mV and 280 mV at the current density of 10 mA/cm2 in acidic and alkaline electrolytes, respectively, with outstanding durability. The improved HER electrocatalytic performance could be attributed to the abundant active sites provided by the amorphous phosphate on the surface of the catalyst and the enhanced electron transfer due to the crystalline P-doped WTe2 core. This strategy of surface modification of Weyl semimetal may open new opportunities in utilizing topological quantum materials for application in catalysis.

First-principles calculations for topological quantum materials

Discoveries of topological states and topological materials reshape our understanding of physics and materials over the last 15 years. First-principles calculations have been playing a significant role in bridging the theory of topology and experiments by predicting realistic topological materials. In this article, we overview the first-principles methodology on topological quantum materials. First, we unify different concepts of topological states in the same band inversion scenario. Then, we discuss the topology using first-principles band structures and newly-established topological materials databases. We stress challenges in characterizing symmetry-independent Weyl semimetals and calculating topological surface states, closing with an outlook on the exciting transport and optical phenomena induced by the topology.

Coexistence of topological Weyl and nodal-ring states in ferromagnetic and ferrimagnetic double perovskites

Magnetic topological quantum materials have attracted great attention due to their exotic topological quantum physics induced by the interplay among crystalology, magnetism, and topology, which is of profound importance to fundamental research and technology applications. However, limited materials are experimentally available, most of whom are realized by magnetic impurity doping or heterostructural constructions. In this work, based on the first-principles calculations, we predict that double perovskite Ba2CdReO6 is an intrinsic ferromagnetic topological semi-half-metal, while the ferrimagnetic double perovskite with space group symmetry Fm-3m, such as Ba2FeMoO6, belongs to a topological half-metal. One pair of Weyl points and fully spin-polarized nodal-ring states are found in the vicinity of the Fermi level in Ba2CdReO6. Its two-dimensional nearly flat drumhead surface states are fully spin-polarized. In Ba2FeMoO6, however, there exist four pairs of Weyl points and two fully spin-polarized nodal-rings near the Fermi level. These topological properties are stable in the presence of spin-orbit coupling. This makes these materials be an appropriate platform for studying the emerging intriguing properties, especially for the applications in spintronics, information technology, and topological superconductivity.

Evidence for a higher-order topological insulator in a three-dimensional material built from van der Waals stacking of bismuth-halide chains.

Low-dimensional van der Waals materials have been extensively studied as a platform with which to generate quantum effects. Advancing this research, topological quantum materials with van der Waals structures are currently receiving a great deal of attention. Here, we use the concept of designing topological materials by the van der Waals stacking of quantum spin Hall insulators. Most interestingly, we find that a slight shift of inversion centre in the unit cell caused by a modification of stacking induces a transition from a trivial insulator to a higher-order topological insulator. Based on this, we present angle-resolved photoemission spectroscopy results showing that the real three-dimensional material Bi4Br4 is a higher-order topological insulator. Our demonstration that various topological states can be selected by stacking chains differently, combined with the advantages of van der Waals materials, offers a playground for engineering topologically non-trivial edge states towards future spintronics applications.

Electronic Transport Properties of Nb1–xTaxSb2 Single-Crystal Semimetals Grown by a Chemical Vapor Transport Based High-Throughput Method

In quantum materials, the Fermi surface and physical behaviors are particularly sensitive to the crystal quality and composition, while it is difficult to obtain a large quantity of quasi-continuous stoichiometric ratio high-quality single crystals quickly, which seriously restricts the research efficiency and scientific achievement transformation. In the past decade, the rise of high-throughput technologies has brought new opportunities to the exploration and mechanism cognition of topological quantum materials. Taking Nb1–xTaxSb2 (NTS) topological semimetals as an example, here we propose a high-throughput single-crystal growth and design method based on chemical vapor transport (CVT) for high-melting-point transition-metal dichalcogenide materials, which intends to accelerate the crystal synthesis and the research efficiency by the influence of different proportions on the properties of the quantum systems. We have successfully grown Nb1–xTaxSb2 single crystals of various proportions (x = 0, 0.1, 0.2, 0.3, 0.5, 0.7, 0.8, 0.9, 1) in one pot at one time with the help of a CVT-based high-throughput method. Then, we systematically study the effect of composition on its crystallographic and magnetotransport properties. First, the excellent crystallographic quality and element uniformity of the produced NTS were verified via X-ray diffraction, energy dispersive X-ray spectroscopy, and transmission electron microscopy. Later, we performed temperature- and angle-dependent resistivity measurements on the as-grown NTS crystals under magnetic fields up to B = 9 T and found the following. (1) The resistivity of all NTS crystals decreases monotonically with decreasing temperature from T = 300 K under zero magnetic field and displays resistivity plateaus below T = 10 K. (2) Upon application of a magnetic field larger than 3 T the resistivity shows a significant upturn in the low-temperature regions for NbSb2 and TaSb2. This resistivity upturn, however, is rapidly suppressed with an increase in x from 0 to 0.5, completely disappears for x = 0.5, and recovers with a further increase in x from 0.5 to 1. (3) All NTS crystals show anisotropic parabolic-like magnetoresistance behaviors with no sign of saturation. (4) The magnetoresistance highly depends on the Nb:Ta ratio and reaches a minimum value when the Nb:Ta ratio is 1. An analysis of the results reveals that the degree of Nb:Ta atomic disorder and the electron–hole compensation could account for the doping-dependent magnetotransport behaviors. In conclusion, our method does have high-throughput characteristics, and the results not only provide further insight into the electronic transport properties of the NTS family but also give a reliable reference for other similar systems, which may promote the opening of a new chapter of a high-throughput research mode in quantum materials.

Electrically and magnetically switchable nonlinear photocurrent in \u0420\u0422-symmetric magnetic topological quantum materials

Nonlinear photocurrent in time-reversal invariant noncentrosymmetric systems such as ferroelectric semimetals sparked tremendous interest of utilizing nonlinear optics to characterize condensed matter with exotic phases. Here we provide a microscopic theory of two types of second-order nonlinear direct photocurrents, magnetic shift photocurrent (MSC) and magnetic injection photocurrent (MIC), as the counterparts of normal shift current (NSC) and normal injection current (NIC) in time-reversal symmetry and inversion symmetry broken systems. We show that MSC is mainly governed by shift vector and interband Berry curvature, and MIC is dominated by absorption strength and asymmetry of the group velocity difference at time-reversed ±k points. Taking {cal{P}}{cal{T}}-symmetric magnetic topological quantum material bilayer antiferromagnetic (AFM) MnBi2Te4 as an example, we predict the presence of large MIC in the terahertz (THz) frequency regime which can be switched between two AFM states with time-reversed spin orderings upon magnetic transition. In addition, external electric field breaks {cal{P}}{cal{T}} symmetry and enables large NSC response in bilayer AFM MnBi2Te4, which can be switched by external electric field. Remarkably, both MIC and NSC are highly tunable under varying electric field due to the field-induced large Rashba and Zeeman splitting, resulting in large nonlinear photocurrent response down to a few THz regime, suggesting bilayer AFM-z MnBi2Te4 as a tunable platform with rich THz and magneto-optoelectronic applications. Our results reveal that nonlinear photocurrent responses governed by NSC, NIC, MSC, and MIC provide a powerful tool for deciphering magnetic structures and interactions which could be particularly fruitful for probing and understanding magnetic topological quantum materials.

Topological Quantum Materials from the Viewpoint of Chemistry.

Topology, a mathematical concept, has recently become a popular and truly transdisciplinary topic encompassing condensed matter physics, solid state chemistry, and materials science. Since there is a direct connection between real space, namely atoms, valence electrons, bonds, and orbitals, and reciprocal space, namely bands and Fermi surfaces, via symmetry and topology, classifying topological materials within a single-particle picture is possible. Currently, most materials are classified as trivial insulators, semimetals, and metals or as topological insulators, Dirac and Weyl nodal-line semimetals, and topological metals. The key ingredients for topology are certain symmetries, the inert pair effect of the outer electrons leading to inversion of the conduction and valence bands, and spin–orbit coupling. This review presents the topological concepts related to solids from the viewpoint of a solid-state chemist, summarizes techniques for growing single crystals, and describes basic physical property measurement techniques to characterize topological materials beyond their structure and provide examples of such materials. Finally, a brief outlook on the impact of topology in other areas of chemistry is provided at the end of the article.

2D Nanosheets of Topological Quantum Materials from Homologous (Bi2)m(Bi2Se3)n Heterostructures: Synthesis and Ultralow Thermal Conductivity

Topological quantum materials with layered heterostructure hold great promise for exhibiting low thermal conductivity. Homologous (Bi2)m(Bi2Se3)n (m, n: integers) series hosts different layered top...

Research progress on topological nodal line semimetals

Topological quantum materials, such as topological insulators, topological semimetals, and topological superconductors, are the research focus of modern condensed matter physics and materials science. The purpose of this article is to give a brief introduction on the research progress of topological nodal line semimetal materials in recent years, and to focus on the latest research progress of our group in this field. Topological semimetals can be classified mainly by two aspects: energy band degeneracy and dispersion. In terms of energy band degeneracy, this article will introduce nodal line semimetals protected by different symmetries including nodal line semimetals protected by symmorphic space groups and non-symmorphic space groups in the system with time reversion symmetry, and nodal line semimetals coupled with magnetic and topological orders without time reversion symmetry, respectively. In terms of energy band dispersion, this article will introduce high-order nodal line semimetals and type-II nodal line semimetals.

Unconventional Charge-Spin Conversion in Weyl-Semimetal WTe2.

An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge-to-spin conversion efficiency. Here, a charge-current-induced spin polarization in the type-II Weyl semimetal candidate WTe2 and efficient spin injection and detection in a graphene channel up to room temperature are reported. Contrary to the conventional spin Hall and Rashba-Edelstein effects, the measurements indicate an unconventional charge-to-spin conversion in WTe2 , which is primarily forbidden by the crystal symmetry of the system. Such a large spin polarization can be possible in WTe2 due to a reduced crystal symmetry combined with its large spin Berry curvature, spin-orbit interaction with a novel spin-texture of the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization using both charge-to-spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing topological Weyl materials as nonmagnetic spin sources in all-electrical van der Waals spintronic circuits and for low-power and high-performance nonvolatile spintronic technologies.

Persistent surface states with diminishing gap in MnBi2Te4/Bi2Te3 superlattice antiferromagnetic topological insulator

Magnetic topological quantum materials (TQMs) provide a fertile ground for the emergence of fascinating topological magneto-electric effects. Recently, the discovery of intrinsic antiferromagnetic (AFM) topological insulator MnBi2Te4 that could realize quantized anomalous Hall effect and axion insulator phase ignited intensive study on this family of TQM compounds. Here, we investigated the AFM compound MnBi4Te7 where Bi2Te3 and MnBi2Te4 layers alternate to form a superlattice. Using spatial- and angle-resolved photoemission spectroscopy, we identified ubiquitous (albeit termination dependent) topological electronic structures from both Bi2Te3 and MnBi2Te4 terminations. Unexpectedly, while the bulk bands show strong temperature dependence correlated with the AFM transition, the topological surface states with a diminishing gap show negligible temperature dependence across the AFM transition. Together with the results of its sister compound MnBi2Te4, we illustrate important aspects of electronic structures and the effect of magnetic ordering in this family of magnetic TQMs.

Electronic structure of the Si-containing topological Dirac semimetal CaAl2Si2

There has been an upsurge in the discovery of topological quantum materials, where various topological insulators and semimetals have been theoretically predicted and experimentally observed. However, only very few of them contains silicon, the most widely used element in electronic industry. Recently, ternary compound CaAl2Si2 has been predicted to be a topological Dirac semimetal, hosting Lorentz-symmetry-violating quasiparticles with a strongly tilted conical band dispersion. In this work, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we investigated the comprehensive electronic structure of CaAl2Si2. A pair of topological Dirac crossings is observed along the kz direction, in good agreement with the ab initio calculations, confirming the topological Dirac semimetal nature of the compound. Our study expands the topological material family on Si-containing compounds, which have great application potential in realizing low-cost, nontoxic electronic device with topological quantum states.

Angle-Resolved Photoemission Spectroscopy Study of Topological Quantum Materials

The recently discovered topological quantum materials (TQMs) have electronic structures that can be characterized by certain topological invariants. In these novel materials, the unusual bulk and surface electrons not only give rise to many exotic physical phenomena but also foster potential new technological applications. To characterize the unusual electronic structures of these new materials, investigators have used angle-resolved photoemission spectroscopy (ARPES) as an effective experimental tool to directly visualize the unique bulk and surface electronic structures of TQMs. In this review, we first give a brief introduction of TQMs and ARPES, which is followed by examples of the application of ARPES to different TQMs ranging from topological insulators to Dirac and Weyl semimetals. We conclude with a brief perspective of the current development of ARPES and its potential application in the study of TQMs.

Rare-earth based half-Heusler topological quantum materials: A perspective

Topological insulator (TI) materials which are conductive at the surface but insulating in the bulk have drawn much attention in the past decade due to their fascinating properties and potential application in the field of spintronics, quantum computing, topological superconductivity and next generation electronics. In the search of three-dimensional TIs, half-Heusler compounds are the new entrants. Half-Heusler compounds are equiatomic ternary compounds with cubic symmetry. Due to the availability of a large pool of elements in the half Heusler family, the physical properties of these materials can be tuned by choosing the desired combination of elements. In this perspective, we have briefly discussed the development of structural relations, the quantum hall effect, Landau quantization, and topological properties of a few representative systems in the half-Heusler family, including methods by which they are studied and characterized such as Angle Resolved Photoemission Spectroscopy, Shubnikov-de-Hass Oscillations and Nuclear Magnetic Resonance.

Observation of sixfold degenerate fermions in PdSb2

Three types of fermions have been extensively studied in topological quantum materials: Dirac, Weyl, and Majorana fermions. Beyond the fundamental fermions in high energy physics, exotic fermions are allowed in condensed matter systems residing in three-, six- or eightfold degenerate band crossings. Here, we use angle-resolved photoemission spectroscopy to directly visualize three-doubly-degenerate bands in PdSb$_2$. The ultrahigh energy resolution we are able to achieve allows for the confirmation of all the sixfold degenerate bands at the R point, in remarkable consistency with first-principles calculations. Moreover, we find that this sixfold degenerate crossing has quadratic dispersion as predicted by theory. Finally, we compare sixfold degenerate fermions with previously confirmed fermions to demonstrate the importance of this work: our study indicates a topological fermion beyond the constraints of high energy physics.

Topological quantum materials

Abstract

Intrinsically Low Thermal Conductivity and High Carrier Mobility in Dual Topological Quantum Material, n‐Type BiTe

AbstractA challenge in thermoelectrics is to achieve intrinsically low thermal conductivity in crystalline solids while maintaining a high carrier mobility (μ). Topological quantum materials, such as the topological insulator (TI) or topological crystalline insulator (TCI) can exhibit high μ. Weak topological insulators (WTI) are of interest because of their layered hetero‐structural nature which has a low lattice thermal conductivity (κlat). BiTe, a unique member of the (Bi2)m(Bi2Te3)n homologous series (m:n=1:2), has both the quantum states, TCI and WTI, which is distinct from the conventional strong TI, Bi2Te3 (where m:n=0:1). Herein, we report intrinsically low κlat of 0.47–0.8 W m−1 K−1 in the 300–650 K range in BiTe resulting from low energy optical phonon branches which originate primarily from the localized vibrations of Bi bilayer. It has high μ≈516 cm2 V−1 s−1 and 707 cm2 V−1 s−1 along parallel and perpendicular to the spark plasma sintering (SPS) directions, respectively, at room temperature.

Intrinsically Low Thermal Conductivity and High Carrier Mobility in Dual Topological Quantum Material, n-Type BiTe.

A challenge in thermoelectrics is to achieve intrinsically low thermal conductivity in crystalline solids while maintaining a high carrier mobility (μ). Topological quantum materials, such as the topological insulator (TI) or topological crystalline insulator (TCI) can exhibit high μ. Weak topological insulators (WTI) are of interest because of their layered hetero-structural nature which has a low lattice thermal conductivity (κlat ). BiTe, a unique member of the (Bi2 )m (Bi2 Te3 )n homologous series (m:n=1:2), has both the quantum states, TCI and WTI, which is distinct from the conventional strong TI, Bi2 Te3 (where m:n=0:1). Herein, we report intrinsically low κlat of 0.47-0.8 W m-1 K-1 in the 300-650 K range in BiTe resulting from low energy optical phonon branches which originate primarily from the localized vibrations of Bi bilayer. It has high μ≈516 cm2 V-1 s-1 and 707 cm2 V-1 s-1 along parallel and perpendicular to the spark plasma sintering (SPS) directions, respectively, at room temperature.