Design A Family of 2D Nb-Based Multilayer Kagome Semimetals with High Fermi Velocity and Low Thermal Conductivity

Although two-dimensional (2D) multilayer kagome materials have opened up new windows of opportunity for exploring novel physical properties, their development has been constrained by the scarcity of available material systems. In light of this, in this study, relying on our previously proposed innovative "1+3" design strategy for multilayer kagome materials, we have successfully designed nine stable 2D niobium-based multilayer kagome monolayers with tunable compositions: Nb₆Cl₂S₃Br₆, Nb₆Cl₂S₄Br₆, Nb₆Cl₂Se₃Br₆, Nb₆Cl₂Se₄Br₆, Nb₆Cl₂S₁Se₃Br₆, Nb₆Cl₂S₃Se₁Br₆, Nb₆S₄Cl₈, Nb₆Se₄Br₈, and Nb₆Br₂S₃Se₁Cl₆. These nine new materials all belong to the category of Dirac semimetals, with their Dirac cone structures primarily arising from the dz² orbitals based on Nb-based kagome lattice. Hybrid functional calculations reveal that these materials boast Fermi velocities as high as 2.36-3.04*10⁵ m/s. Moreover, these materials generally exhibit characteristics of relatively low phonon group velocities and shorted phonon lifetimes. Under room temperature conditions, they possess comparatively low lattice thermal conductivities, with values ranging from 1.704-8.149 Wm^-1K^-1 . Our research not only robustly confirms the feasibility of the "1+3" multilayer kagome lattices design strategy in the realm of kagome material development but also sets an exemplary benchmark for the study of Nb-based multilayer kagome materials.