Evidence for Charge-Density-Wave in Underdoped Bi2201 from ARPES and LEED

While there is mounting evidence for a broken symmetry in the pseudogap state of the high-$T_c$ cuprates, the identification of a specific phase remains elusive. Through the combination of electronic (ARPES) and structural (LEED) probes, we uncover a temperature dependent evolution of the CuO$_2$ plane band dispersion in highly-ordered Bi2201, which is directly associated with a hitherto-undetected evolution of the incommensurate superstructure. The quasilinear, continuous variation of the modulation wavelength $2\pi/Q_2$ from $\sim\!66$ to $43$\AA, below a characteristic $T_{Q_2}\!\simeq\!130$\,K, provides evidence for an electronically-driven charge-density-wave ordering. This points to a remarkable electron-lattice coupling, in which the footprint of the BiO-layer-induced superstructure is found in the modulated electronic structure of the CuO$_2$ plane.