Strain Unlocks Multiple Topological Phases in Cu₂SnS₃

The quest to uncover and manipulate exotic topological phases in materials is a central theme in modern condensed matter physics. Scientists are keen to find ways to transition between these phases by tuning various parameters, such as chemical composition, electric or magnetic fields, and mechanical strain. As lead author Pandey notes in the paper, "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." This highlights the significant challenge in achieving such control.

Addressing this challenge, a recent ab-initio computational study focused on the orthorhombic material Cu₂SnS₃ to explore the effects of uniaxial compressive strain (UCS). The research revealed that in its pristine state, without considering spin-orbit coupling (SOC), Cu₂SnS₃ exhibits a type-II nodal-ring. When SOC is factored in, the material transforms into a Weyl phase, characterized by seven Weyl points—three located at the Γ point and four at general positions—accompanied by nodal arcs. Crucially, the application of UCS proved to be a powerful knob for tuning its topological properties. For instance, without SOC, increasing UCS led to a progression from a type-II nodal-ring (below 5.5% strain) to a type-III nodal-ring (between 5.5% and 5.6% strain). Remarkably, at precisely 5.6% UCS, a Weyl phase with four Weyl nodes emerged even in the absence of SOC, a phenomenon attributed to a strain-driven topological flat band.

Further investigation into the SOC-enabled Weyl phases showed that all seven initial Weyl points persisted below 5% UCS. Between 5% and 5.6% UCS, four of these Weyl points (those at general positions) vanished, though the nodal arcs remained stable across all studied strain ranges. This research successfully demonstrates the intricate interplay between mechanical strain and fundamental electronic properties, leading to the realization of multiple topological phases and their controlled transitions within a single material. The discovery that topological flat bands, usually associated with specialized structures like kagome and Lieb lattices, can be induced by strain in Cu₂SnS₃ opens new avenues for designing and exploring materials with tunable topological characteristics.