Realization of strain induced multiple topological phases in Cu ₂ SnS ₃ : an <i>ab-initio</i> study

The search of multiple topological phases (TPs) and their transitions by tuning different parameters through chemical substitutions, electric field, magnetic field, strain and Floquet engineering, etc has garnered a widespread attention in recent time. In spite of great effort, the observations of multiple TPs in a single material and multiple TP transitions in the presence of one parameter remain elusive. Here we demonstrate the presence of multiple TPs and their transitions with uniaxial compressive strain (UCS) in orthorhombic Cu₂SnS₃by using<i>state-of-the-art</i><i>ab-initio</i>calculations. In the absence of spin-orbit coupling (SOC), the Cu₂SnS₃exhibits only one (type-II) nodal-ring and in the presence of SOC, it hosts Weyl phase with seven Weyl points (three at Γ and four at general positions) along with nodal arcs. On the application of UCS, the system exhibits a type-II nodal ring for UCS<5.5%, which further evolves into type-III nodal-ring for5.5%⩽UCS<5.6%, thereby marking a TP transition in the nodal topology. Interestingly, at 5.6% of UCS, it shows Weyl phase with four Weyl nodes even in the absence of SOC. All the above-mentioned seven Weyl points persist below 5% of UCS. For5%⩽UCS<5.6%, four Weyl points (at general positions) disappear and nodal-arcs remain intact in all the studied range of UCS. The TPs observed in the absence of SOC appears to arise due to the presence of strain driven topological flat band, which is typically reported to be seen in kagome and Lieb lattices.