Topological and chiral phonons in two-dimensional transitional metal monocarbide monolayers

Since the work of Zhao et al. in 2014, two-dimensional (2D) transition-metal monocarbides (TMMCs) have emerged as atomically thin materials. With unique metal–carbon bonding properties, 2D TMMCs exhibit enhanced structural stability and excellent electronic properties, making them ideal candidates for spintronics and flexible mechanical applications. However, the structural diversity and the phonon characteristics of 2D TMMCs remain underexplored. Here, we carry out a systematic first-principles investigation of five prototypical TMMC lattices, revealing 27 dynamically stable monolayers—seven of which host clean topological phonons and six of which host chiral phonons. The honeycomb-RuC and tetragonal-RuC (H-RuC and T-RuC) are selected as typical examples for detailed phonon spectral analyses and to assess their structural stabilities. Both H-RuC and T-RuC support topological phonons and phononic edge states, whereas chiral phonons arise exclusively at the K points in H-RuC due to its threefold rotational symmetry. Through phonon free-energy calculations coupled with thermodynamic modeling, T-RuC is preferred in the 2000 K temperature range and in biaxial compression, while H-RuC is more stable under tensile strain conditions. Our findings not only expand the 2D TMMC materials family but also illuminate the relationship between structural phases and phononic functionality, laying the groundwork for their integration into phononic applications in the future.