Topological Metal Research Articles

Correction: Nonequilibrium Electrical, Thermal and Spin Transport in Open Quantum Systems of Topological Superconductors, Semiconductors and Metals

Correction: Nonequilibrium Electrical, Thermal and Spin Transport in Open Quantum Systems of Topological Superconductors, Semiconductors and Metals

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.

Topological superconductor candidates PdBi2Te4 and PdBi2Te5 from a generic ab initio strategy

Superconducting topological metals (SCTMs) have recently emerged as a promising platform of topological superconductivity (TSC) and Majorana zero modes for quantum computation. Despite their importance in both fundamental research and applications, SCTMs are very rare in nature. Here, we propose a strategy to design SCTMs by intercalating the superconducting units into the topological insulators. A program that characterizes the superconducting BdG Chern number of 2D BdG Hamiltonian from ab initio calculations is also developed. Following this strategy, PdBi2Te5 and PdBi2Te4 are found to be experimentally synthesizable and ideal SCTMs. Chiral TSC could be realized in such SCTMs by incorporating topological surface states with Zeeman effect, which can be realized by an external magnetic field or in proximity to ferromagnetic insulator. Our strategy provides a new method for identifying the SCTMs and TSC candidates, and the program makes it possible to design and modulate the TSC candidates from ab initio calculations.

Emergent metallicity at the grain boundaries of higher-order topological insulators

Topological lattice defects, such as dislocations and grain boundaries (GBs), are ubiquitously present in the bulk of quantum materials and externally tunable in metamaterials. In terms of robust modes, localized near the defect cores, they are instrumental in identifying topological crystals, featuring the hallmark band inversion at a finite momentum (translationally active type). Here we show that the GB superlattices in both two-dimensional and three-dimensional translationally active higher-order topological insulators harbor a myriad of dispersive modes that are typically placed at finite energies, but always well-separated from the bulk states. However, when the Burgers vector of the constituting edge dislocations points toward the gapless corners or hinges, both second-order and third-order topological insulators accommodate self-organized emergent topological metals near the zero energy (half-filling) in the GB mini Brillouin zone. We discuss possible material platforms where our proposed scenarios can be realized through the band-structure and defect engineering.

Chiral topological metals with multiple types of quasiparticle fermions and large spin Hall effect in the SrGePt family materials

Chiral topological metals with multiple types of quasiparticle fermions and large spin Hall effect in the SrGePt family materials

Anisotropic proximity–induced superconductivity and edge supercurrent in Kagome metal, K 1− x V 3 Sb 5

Materials with Kagome nets are of particular importance for their potential combination of strong correlation, exotic magnetism, and electronic topology. KV 3 Sb 5 was discovered to be a layered topological metal with a Kagome net of vanadium. Here, we fabricated Josephson Junctions of K 1− x V 3 Sb 5 and induced superconductivity over long junction lengths. Through magnetoresistance and current versus phase measurements, we observed a magnetic field sweeping direction–dependent magnetoresistance and an anisotropic interference pattern with a Fraunhofer pattern for in-plane magnetic field but a suppression of critical current for out-of-plane magnetic field. These results indicate an anisotropic internal magnetic field in K 1− x V 3 Sb 5 that influences the superconducting coupling in the junction, possibly giving rise to spin-triplet superconductivity. In addition, the observation of long-lived fast oscillations shows evidence of spatially localized conducting channels arising from edge states. These observations pave the way for studying unconventional superconductivity and Josephson device based on Kagome metals with electron correlation and topology.

NiB monolayer: A topological metal with high NORR electrocatalytic perfomance

Electrochemical reduction reaction of NO (NORR) offers a promising strategy for effective removal of NO coupled with synthesis of NH3. In this study, the newly reported NiB monolayer with topological metal properties is proved to have excellent catalytic activity and high selectivity toward NO-to-NH3 conversion by using density functional theory (DFT) and the constant potential method (CPM). Calculations reveal that the peculiar building blocks of Ni-B triangles and tetragons can facilitate NO adsorption and activation via an electron donation-back-donation interaction mechanism. The predicted free energy profiles of the four considered NORR pathways toward synthesis of NH3 are generally going downhill under the applied potential of 0 V, and an optimal reduction pathway was screened out with a maximum kinetic barrier of 0.16 eV during the hydrogenation process. The low adsorption free energy of H atom together with the existing relatively higher energy barriers for the formation of N2O/N2 guarantee the hydrogen evolution reaction (HER) and the partial reduction channel are less competitive than NH3 production. These findings enrich the potential application of two dimensional topological materials in electrocatalysis.

Revealing topology in metals using experimental protocols inspired by K-theory

Topological metals are conducting materials with gapless band structures and nontrivial edge-localized resonances. Their discovery has proven elusive because traditional topological classification methods require band gaps to define topological robustness. Inspired by recent theoretical developments that leverage techniques from the field of C∗-algebras to identify topological metals, here, we directly observe topological phenomena in gapless acoustic crystals and realize a general experimental technique to demonstrate their topology. Specifically, we not only observe robust boundary-localized states in a topological acoustic metal, but also re-interpret a composite operator—mathematically derived from the K-theory of the problem—as a new Hamiltonian whose physical implementation allows us to directly observe a topological spectral flow and measure the topological invariants. Our observations and experimental protocols may offer insights for discovering topological behaviour across a wide array of artificial and natural materials that lack bulk band gaps.

Topological metals constructed by sliding quantum wire arrays

A general strategy of alternated slide construction to craft topological metals is proposed, where there is a relative slide between the odd and even chains in the trivial spinless quantum wire array. Firstly, taking the three-leg ladder as an example, we find that alternated slide can induce a topological phase transition from the normal metal to topological metal phases, which are protected by inversion symmetry. Remarkably, topological metal without nontrivial edge states is found, and the bulk-boundary correspondence breaks down. Secondly, the two-dimensional quantum wire arrays with alternated slide manifests similar physical behaviors. Two types of topological metal phases emerge, where there are gapless bulk bands with and without nontrivial edge states. These results could be confirmed by current experimental techniques.

Flat band separation and robust spin Berry curvature in bilayer kagome metals

AbstractKagome materials have emerged as a setting for emergent electronic phenomena that encompass different aspects of symmetry and topology. It is debated whether the XV6Sn6 kagome family (where X is a rare-earth element), a recently discovered family of bilayer kagome metals, hosts a topologically non-trivial ground state resulting from the opening of spin–orbit coupling gaps. These states would carry a finite spin Berry curvature, and topological surface states. Here we investigate the spin and electronic structure of the XV6Sn6 kagome family. We obtain evidence for a finite spin Berry curvature contribution at the centre of the Brillouin zone, where the nearly flat band detaches from the dispersing Dirac band because of spin–orbit coupling. In addition, the spin Berry curvature is further investigated in the charge density wave regime of ScV6Sn6 and it is found to be robust against the onset of the temperature-driven ordered phase. Utilizing the sensitivity of angle-resolved photoemission spectroscopy to the spin and orbital angular momentum, our work unveils the spin Berry curvature of topological kagome metals and helps to define its spectroscopic fingerprint.

A General Preparation of Solid Solution-Oxide Heterojunction Photocatalysts through Metal-Organic Framework Transformation Induced Pre-nucleation.

Solid solution-oxide heterostructures combine the advantages of solid solution and heterojunction materials to improve electronic structure and optical properties by metal doping, and enhance charge separation and transfer in semiconductor photocatalysts by creating a built-in electric field. Nevertheless, the effective design and synthesis of these materials remains a significant challenge. Here, we develop a generally applicable strategy that leverages the transformable properties of metal-organic frameworks (MOFs) to prepare solid solution-oxide heterojunctions with controllable structural and chemical compositions. The process consists of three main steps. First, MOFs with different topological structures and metal centers are transformed, accompanied by pre-nucleation of a metal oxide. Second, solid solution is prepared through calcination of the transformed MOFs. Finally, a heterojunction is formed by combining solid solution with another metal oxide group through endogenous overflow. DFT calculations and study on carrier dynamics show that the structure of the material effectively prevents electrons from returning to the bulk phase, exhibiting superior photocatalytic reduction performance of CO2 . This study is expected to promote the controllable synthesis and research of MOF-derived heterojunctions.

A General Preparation of Solid Solution‐Oxide Heterojunction Photocatalysts through Metal–Organic Framework Transformation Induced Pre‐nucleation

AbstractSolid solution‐oxide heterostructures combine the advantages of solid solution and heterojunction materials to improve electronic structure and optical properties by metal doping, and enhance charge separation and transfer in semiconductor photocatalysts by creating a built‐in electric field. Nevertheless, the effective design and synthesis of these materials remains a significant challenge. Here, we develop a generally applicable strategy that leverages the transformable properties of metal–organic frameworks (MOFs) to prepare solid solution‐oxide heterojunctions with controllable structural and chemical compositions. The process consists of three main steps. First, MOFs with different topological structures and metal centers are transformed, accompanied by pre‐nucleation of a metal oxide. Second, solid solution is prepared through calcination of the transformed MOFs. Finally, a heterojunction is formed by combining solid solution with another metal oxide group through endogenous overflow. DFT calculations and study on carrier dynamics show that the structure of the material effectively prevents electrons from returning to the bulk phase, exhibiting superior photocatalytic reduction performance of CO2. This study is expected to promote the controllable synthesis and research of MOF‐derived heterojunctions.

Realization of Z2 Topological Metal in Single‐Crystalline Nickel Deficient NiV2Se4

AbstractTemperature‐dependent electronic and magnetic properties are reported for nickel‐deficient NiV2Se4. Single‐crystal X‐ray diffraction shows it to crystallize in the monoclinic Cr3S4 structure type with space group and vacancies on the Ni site, resulting in the composition Ni0.85V2Se4 in agreement with our electron‐probe microanalysis. Structural distortions are not observed down to 1.5 K. Nevertheless, the electrical resistivity shows metallic behavior with a broad anomaly around 150–200 K that is also observed in the heat capacity data. This anomaly indicates a change of state of the material below 150 K. It is believed that this anomaly could be due to spin fluctuations or charge‐density‐wave fluctuations, where the lack of long‐range order is caused by vacancies at the Ni site of Ni0.85V2Se4. The non‐linear temperature dependence of the resistivity as well as an enhanced value of the Sommerfeld coefficient mJ mol−1 K−2 suggest strong electron–electron correlations in this material. First‐principles calculations performed for NiV2Se4, which are also applicable to Ni0.85V2Se4, classify this material as a topological metal with and coexisting electron and hole pockets at the Fermi level. The phonon spectrum lacks any soft phonon mode, consistent with the absence of periodic lattice distortion in the present experiments.

Topological and superconducting properties in bilayer kagome metals YT6Sn6 ( T =V, Nb, Ta)

Kagome materials exhibit fascinating properties and hold important research significance. Inspired by the extensive studies on the $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ family of materials, here, we investigate the topological and superconducting properties of the bilayer kagome metal materials $\mathrm{Y}{T}_{6}{\mathrm{Sn}}_{6}$ ($T$=V, Nb, Ta) using first-principles calculations. These kagome materials exhibit the ${\mathrm{MgFe}}_{6}{\mathrm{Ge}}_{6}$-prototype structure and have no magnetism. The calculated results for formation energy and the phonon dispersion spectrum demonstrate their stability. Based on the calculation of topologically invariant and surface states, they can be categorized as ${\mathbb{Z}}_{2}$ topological metals. The Van Hove singularity and Dirac points are also observed near the Fermi level. According to electron-phonon coupling (EPC) calculations, $\mathrm{Y}{T}_{6}{\mathrm{Sn}}_{6}$ systems are all predicted to be weak superconductors. Using the Allen-Dynes modified McMillan formula, the superconducting critical temperatures ${T}_{c}$ of ${\mathrm{YV}}_{6}{\mathrm{Sn}}_{6}, {\mathrm{YNb}}_{6}{\mathrm{Sn}}_{6}$, and ${\mathrm{YTa}}_{6}{\mathrm{Sn}}_{6}$ are predicted to be 0.65, 1.17, and 0.89 K, respectively. The EPC is mainly contributed by the vibrations of V atoms and partially by the out-of-plane vibrational modes of Sn atoms. The coexistence of nontrivial topological properties and superconducting properties in these bilayer kagome systems is helpful to study the relationship between different physical properties and to design new topological superconductors.

Topological half-metallic features in alkali metal doped CrCl3 monolayers

Two-dimensional (2D) topological half metals, namely, empowering nontrivial topological states with 100% spin-polarized 2D systems, may find important applications in low-dissipation spintronics. However, exploration of such intriguing materials is still very challenging. The recently discovered 2D magnetic materials provide unprecedented opportunities to realize 2D topological half-metallic state. Using first-principles calculations, we first find that the ferromagnetism of $\mathrm{Cr}{\mathrm{Cl}}_{3}$ monolayer can be well reserved by doping alkali metals, in fact, whose Curie temperature can be enhanced from 23 to 66 K after doping K. Then, we notice that semiconducting to Dirac half-metallic phase transition is achieved via Na and K doping, where the linear Dirac dispersion originates from the honeycomb lattice formed by Cr atoms. Most strikingly, Na- and K doped $\mathrm{Cr}{\mathrm{Cl}}_{3}$ exhibit quantum anomalous Hall effect (QAHE) with Chern number $C=1$, while the Li-doped one harbors high Chern number QAHE with $C=\ensuremath{-}2$. Thus, our work provides a theoretical guideline to realize 2D topological half-metallic state, rendering it as a promising platform for future applications.

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.

Intrinsic topological metal state in T-graphene

An intrinsic topological metal (TM) state is found in the T-graphene, a monolayer with both the time-reversal symmetry and the four-fold symmetry. The state distinguishes itself by the nontrivial electric polarization from the ordinary metals and features with two local edge states in the corresponding nanoribbons. The TM state is confirmed as a transition state bridging the ordinary metal state and the topological insulator state when the relative neighboring hoppings change in the lattice. The topological nature is further verified by checking the robustness of transport property against randomly-introduced strong disorders. The fact that the multiple topological states indexed by different parameters coexist in such a practical system shows a broad prospect in versatile topological transport devices.

Strain induced metal-semiconductor transition in two-dimensional topological half metals

Spintronic applications of two-dimensional (2D) magnetic half metals and semiconductors are thought to be very promising. Here, we suggest a family of stable 2D materials (X= Cl, Br, and I). The monolayer exhibits an in-plane ferromagnetic (FM) ground state with a Curie temperature of 118K, which is unveiled to be a 2D Weyl half semimetal with two Weyl points of opposite chirality connected by a remarkable Fermi arc. In addition, it appears that a biaxial tensile strain can lead to a metal-semiconductor phase transition as a result of the increased anomalous Jahn-Teller distortions, which raise the degeneracy of the energy level and cause a significant energy splitting. A 10% biaxial tensile strain also increases the Curie temperature to about 159K, which originates from the enhanced Mn-Cl-Mn FM superexchange. Moreover, the metal-semiconductor transition can also be induced by a uniaxial strain. Our findings provide an idea to create 2D magnetic semiconductors through metal-semiconductor transition in half metals.

Intrinsic magnetic topological materials

Topological states of matter possess bulk electronic structures categorized by topological invariants and edge/surface states due to the bulk-boundary correspondence. Topological materials hold great potential in the development of dissipationless spintronics, information storage and quantum computation, particularly if combined with magnetic order intrinsically or extrinsically. Here, we review the recent progress in the exploration of intrinsic magnetic topological materials, including but not limited to magnetic topological insulators, magnetic topological metals, and magnetic Weyl semimetals. We pay special attention to their characteristic band features such as the gap of topological surface state, gapped Dirac cone induced by magnetization (either bulk or surface), Weyl nodal point/line and Fermi arc, as well as the exotic transport responses resulting from such band features. We conclude with a brief envision for experimental explorations of new physics or effects by incorporating other orders in intrinsic magnetic topological materials.

Topological Metal MoP Nanowire for Interconnect.

The increasing resistance of copper (Cu) interconnects for decreasing dimensions is a major challenge in continued downscaling of integrated circuits beyond the 7nm technology nodeas it leads to unacceptable signal delays and power consumption in computing. The resistivity of Cu increases due to electron scattering at surfaces and grain boundaries at the nanoscale. Topological semimetals, owing to their topologically protected surface states and suppressed electron backscattering, are promising candidates to potentially replace current Cu interconnects. Here,we report the unprecedented resistivity scaling of topological metal molybdenum phosphide (MoP) nanowires, and it is shown that the resistivity values are superior to those of nanoscale Cu interconnects <500nm2 cross-section areas. The cohesive energy of MoP suggests better stability against electromigration, enabling a barrier-free design . MoP nanowires are more resistant to surface oxidation than the 20nm thick Cu. The thermal conductivity of MoP is comparable to those of Ru and Co. Most importantly, it is demonstrated that the dimensional scaling of MoP, in terms of line resistance versus total cross-sectional area, is competitive to those of effective Cu with barrier/liner and barrier-less Ru, suggesting MoP is an attractive alternative for the scaling challenge of Cu interconnects.