A range of of unusual emergent behaviors have been reported in the charge-density wave (CDW) state of the AV3Sb5 (A=K, Rb, Cs) kagome metals, including a CDW formation process without soft phonons, which points to an unconventional CDW mechanism. Here, we use inelastic x-ray scattering to show that the CDW in KV3Sb5 forms via phonons that soften to zero energy at the CDW ordering vector (L-point) around TCDW=78~K. The intensity of soft phonons exhibit a remarkable in-plane anisotropy, extending over a much larger momentum range along L-A relative to L-H, which leads to diffuse scattering common among AV3Sb5. Using first-principles calculations, we find that the momentum-dependent electron-phonon coupling (EPC) is peaked at L and exhibits the same in-plane anisotropy as the phonon softening. Conversely, the electronic susceptibility is not peaked at L and shows the opposite in-plane anisotropy. Our findings favor momentum-dependent EPC as the driving mechanism of the CDW in KV3Sb5, with a CDW formation process similar to that of transition metal dichalcogenides.
@article{arxiv.2510.20230,
title = {Soft Phonon Charge-Density Wave Formation in the Kagome Metal KV$_3$Sb$_5$},
author = {Yifan Wang and Chenchao Xu and Zhimian Wu and Huachen Rao and Zhaoyang Shan and Yi Liu and Guanghan Cao and Michael Smidman and Ming Shi and Huiqiu Yuan and Tao Wu and Xianhui Chen and Chao Cao and Yu Song},
journal= {arXiv preprint arXiv:2510.20230},
year = {2026}
}