Intrinsic anharmonicity tuned by in-plane rotational phonon mode in two-dimensional group-IB chalcogenides

Anharmonicity as a fundamental issue inspires numerous interesting phenomena in phase transition, electronic structure, thermal transport, and so on. Here, we find that the peculiar A1u phonon mode of in-plane rotational vibration of group-IB-atom ring introduces the anharmonicity into the s(I) and s(II) phases of two-dimensional group-IB chalcogenides. Compared to the high-symmetry s(I) phase, the A1u phonon mode is always active and the anharmonicity is stronger in the symmetry-breaking s(II) phase by releasing the strain energy. The temperature-hardened A1u mode stabilizes the s(I) phase and reduces the lattice thermal conductivity by strengthening the anharmonicity. The strain-softened A1u mode drives the s(II)-to-s(I) phase transition and enhances the lattice thermal conductivity by weakening the anharmonicity. We also establish the relationships of the anharmonicity vs the band structure and Poisson's ratio. As the anharmonicity is weakened during the strain-induced s(II)-to-s(I) phase transition, the bandgap significantly increases. Meanwhile, the weaker anharmonicity implies the lower Poisson's ratio, which further drops much faster with the strain. Our work realizes the tuning of anharmonicity by the peculiar phonon mode in 2D group-IB chalcogenides, which provides a useful guidance for further understanding the anharmonic effect.