Strain Unlocks Complex Topological Phonons in NiAsS

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. This context highlights the importance of identifying ideal materials that can exhibit and allow control over these exotic phonon behaviors.

Based on first-principles calculations and symmetry analysis, NiAsS has been identified as an ideal topological phonon material in space group 198. It 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 the (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 material exhibits a significant topological transformation. 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). Throughout this process, 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.

As lead author Lu notes in the paper, "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."