Bulk Fermi Surfaces of the Dirac Type-II Semimetallic Candidates MAl_{3} (Where M=V, Nb, and Ta)

Quantum oscillations evidence for topological bands in kagome metal ScV6Sn6

Metals with kagome lattice provide bulk materials to host both the flat-band and Dirac electronic dispersions. A new family of kagome metals is recently discovered inAV6Sn6. The Dirac electronic structures of this material needs more experimental evidence to confirm. In the manuscript, we investigate this problem by resolving the quantum oscillations in both electrical transport and magnetization in ScV6Sn6. The revealed orbits are consistent with the electronic band structure models. Furthermore, the Berry phase of a dominating orbit is revealed to be aroundπ, providing direct evidence for the topological band structure, which is consistent with calculations. Our results demonstrate a rich physics and shed light on the correlated topological ground state of this kagome metal.

Development of a Ni-Doped VAl3 Topological Semimetal with a Significantly Enhanced HER Catalytic Performance

Topological materials with robust topological surface states appear to be well-suited as electrochemical catalysts. However, few studies have been published on the development of non-noble metal topological catalysts, most likely because the topological properties tend to be attributed to the s and p orbital electrons, while transition-metal catalysis mainly involves d orbital electrons. Herein, we proposed a topological semimetallic (TSM) compound, VAl₃, with a surface state consisting mainly of d orbital electrons, as an electrocatalyst for the hydrogen evolution reaction (HER). Density functional theory (DFT) calculations showed that the surface state electrons enhanced the adsorption of H atoms. Moreover, the transfer of surface state electrons between the surface and adsorbed H atoms was optimized through nickel doping. We experimentally prepared single-crystals VAl₃ and V_0.75Ni_0.25Al₃ alloys. Electrochemical analysis showed that not only did V_0.75Ni_0.25Al₃ outperform VAl₃ but also it was among the best non-noble metal topological HER electrocatalysts currently available.

Bond-breaking induced Lifshitz transition in robust Dirac semimetal VAI3

Understanding the relation between crystal structure and electronic properties is crucial for designing new quantum materials with desired functionality. So far, controlling a chemical bond is less considered as an effective way to manipulate the topological electrons. In this paper, we show that the V–Al bond acts as a shield for protecting the topological electrons in Dirac semimetal VAl3. The Dirac electrons remain intact in the V1−xTixAl3 solid solutions, even after a substantial part of V atoms have been replaced. A Lifshitz transition from Dirac semimetal to trivial metal occurs as long as the V–Al bond is completely broken. Our finding highlights a rational approach for designing new quantum materials via controlling their chemical bond.

Topological electrons in semimetals are usually vulnerable to chemical doping and environment change, which restricts their potential application in future electronic devices. In this paper, we report that the type-II Dirac semimetal VAl3 hosts exceptional, robust topological electrons which can tolerate extreme change of chemical composition. The Dirac electrons remain intact, even after a substantial part of V atoms have been replaced in the V1−xTixAl3 solid solutions. This Dirac semimetal state ends at x=0.35, where a Lifshitz transition to p-type trivial metal occurs. The V–Al bond is completely broken in this transition as long as the bonding orbitals are fully depopulated by the holes donated from Ti substitution. In other words, the Dirac electrons in VAl3 are protected by the V–Al bond, whose molecular orbital is their bonding gravity center. Our understanding on the interrelations among electron count, chemical bond, and electronic properties in topological semimetals suggests a rational approach to search robust, chemical-bond-protected topological materials.

Deterministic Scheme for Two-Dimensional Type-II Dirac Points and Experimental Realization in Acoustics

Low-energy electrons near Dirac/Weyl nodal points mimic massless relativistic fermions. However, as they are not constrained by Lorentz invariance, they can exhibit tipped-over type-II Dirac/Weyl cones that provide highly anisotropic physical properties and responses, creating unique possibilities. Recently, they have been observed in several quantum and classical systems. Yet, there is still no simple and deterministic strategy to realize them since their nodal points are accidental degeneracies, unlike symmetry-guaranteed type-I counterparts. Here, we propose a band-folding scheme for constructing type-II Dirac points, and we use a tight-binding analysis to unveil its generality and deterministic nature. Through realizations in acoustics, type-II Dirac points are experimentally visualized and investigated using near-field mappings. As a direct effect of tipped-over Dirac cones, strongly tilted kink states originating from their valley-Hall properties are also observed. This deterministic scheme could serve as a platform for further investigations of intriguing physics associated with various strongly Lorentz-violating nodal points.