Straight and twisted open nodal-line phonon states in the CaI2 family of materials†
Abstract
Based on first-principles calculations and symmetry analysis, we propose that trigonal CaI2 with the space group P![[3 with combining macron]](https://www.rsc.org/images/entities/char_0033_0304.gif)
m1 possesses straight and twisted open nodal-line phonon states with linear dispersion. The symmetry analysis indicates that joint symmetry PT and rotational symmetry C3z protect the straight nodal lines along Γ–A and K–H while PT and mirror symmetry M010 (M110) maintain the twisted nodal lines that traverse Γ–M (Γ–K) and A–L (A–H). The calculated π Berry phase suggests that all the nodal lines are nontrivial and the corresponding drumhead-like surface states are clearly visible in the observation window, which is less than 6 THz, suggesting a significant chance for them to be measured using meV-resolution inelastic X-ray scattering. The distribution of the nodal lines in the Brillouin zone is also confirmed by the phononic tight-binding model. Furthermore, the isostructural compounds MgBr2 and MgI2 show similar phonon spectra and topological nontrivial surface states. This work provides promising candidates for investigating straight and twisted open nodal-line phonon states in a single material, which will facilitate future experimental observation.
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