Charge density wave (CDW) implies a periodic modulation of the charge density. Typically observed in metallic systems, CDWs arise from Fermi surface instabilities, resulting in the total or partial gapping of the Fermi surface. Here, we present experimental evidence for a CDW state emerging in a band insulator which has no Fermi surface. The bulk and surface of our material platform, Bi4Br4, is gapped over the entire Brillouin zone. Through topographic and spectroscopic imaging at low temperatures, we unveil an unexpected unidirectional charge modulation in Bi4Br4, breaking the lattice translation symmetry. The CDW develops at temperatures below 40 K and adds an energy gap atop the existing insulating gap of Bi4Br4. Furthermore, our transport measurements reveal nonlinear electrical conduction, a phenomenon conventionally associated with the sliding or phason mode of incommensurate CDWs. These highly unusual observations represent a new type of CDW and demand a new theoretical framework for CDWs.
@article{arxiv.2605.24153,
title = {Charge density wave in a band insulator},
author = {Md Shafayat Hossain and Wenhao Liu and Yuqi Zhang and Qi Zhang and Chao Lei and Nana Shumiya and Kouta Dagnino and Maksim Litskevich and Yu-Xiao Jiang and Jia-Xin Yin and Nikhil Dhale and Zi-Jia Cheng and Byunghoon Kim and Yongkai Li and Tyler A. Cochran and Xian P. Yang and Fan Zhang and Yugui Yao and Zhiwei Wang and Bing Lv and Titus Neupert and Luis Balicas and M. Zahid Hasan},
journal= {arXiv preprint arXiv:2605.24153},
year = {2026}
}