Charge density wave in a band insulator

Charge-density-wave implies a periodic modulation of the charge density. Typically observed in metallic systems, charge-density-wave arise from Fermi surface instabilities, resulting in the total or partial gapping of the Fermi surface. Here, we present experimental evidence for a charge-density-wave state emerging in a band insulator which has no Fermi surface. The bulk and surface of our material platform, $\alpha$ -Bi₄Br₄, is gapped over the entire Brillouin zone. Through topographic and spectroscopic imaging at low temperatures, we unveil an unexpected unidirectional charge modulation in $\alpha$ -Bi₄Br₄, breaking the lattice translation symmetry. The charge-density-wave develops at temperatures below 40 K and adds an energy gap atop the existing insulating gap of $\alpha$ -Bi₄Br₄. Furthermore, our transport measurements reveal nonlinear electrical conduction, a phenomenon conventionally associated with the sliding or phason mode of incommensurate charge-density-waves. These unusual observations represent a different type of charge-density-wave.