The coexistence and transformation of topological phonon states in NiAsS with strain regulation

The exploration and manipulation of topological states have long been a hotspot in condensed matter physics and materials science. In electronic topological systems, related research has reached a relatively mature stage, while in topological phononic systems, such investigations are just emerging. Based on first-principles calculations and symmetry analysis, we identify NiAsS as an ideal topological phonon material in space group 198, which features the coexistence of spin-1 Weyl, charge-2 Dirac, and nodal-surface phonons within two isolated frequency ranges: 9.0-9.8 THz and 8.1-8.8 THz. On (001), (100) and (010) surfaces, topologically protected surface states and surface arcs connect the projections of spin-1 Weyl and charge-2 Dirac points, enabling experimental observation. Under strain regulation, the spin-1 Weyl point transforms into two single Weyl points along specific directions relative to the Γ point (e.g., Γ-Z, Γ-Y and Γ-X), while the charge-2 Dirac point and nodal surfaces persist. Moreover, the charge-2 Dirac point bridges these two newly generated Weyl points, thus giving rise to triangular Weyl complex phonons. The resulting topological evolution and connection, validated by clearly visible surface states and surface arcs on the corresponding surfaces, establish NiAsS as a paradigm platform for strain-tunable topological phononics, offering critical insights into symmetry-controlled topological phase transitions.