Surface-enhanced charge-density-wave instability in underdoped Bi2201

Neutron and x-ray scattering experiments have provided mounting evidence for spin and charge ordering phenomena in underdoped cuprates, ranging from stripe correlations in Nd-LSCO to the recently discovered charge-density-waves in YBCO. Here we show that these electron-lattice instabilities also exhibit a previously unrecognized bulk-surface dichotomy. Surface-sensitive electronic and structural probes uncover a temperature-dependent evolution of the CuO$_2$ plane band dispersion and apparent Fermi pockets in underdoped Bi2201, which is associated with a strong temperature dependence of the incommensurate superstructure periodicity below 130 K. In stark contrast, the structural modulation revealed by bulk-sensitive probes is temperature independent. These findings point to a surface-enhanced incipient charge-density-wave instability, driven by Fermi surface nesting. This discovery is of critical importance in the interpretation of single-particle spectroscopy data and establishes the surface of cuprates as a rich playground for the study of electronically soft phases.