  Neutron and x-ray scattering experiments have provided mounting evidence for
spin and charge ordering phenomena in underdoped cuprates. These range from
early work on stripe correlations in Nd-LSCO to the latest discovery of
charge-density-waves in YBCO. Both phenomena are characterized by a pronounced
dependence on doping, temperature, and an externally applied magnetic field.
Here we show that these electron-lattice instabilities exhibit also a
previously unrecognized bulk-surface dichotomy. Surface-sensitive electronic
and structural probes uncover a temperature-dependent evolution of the CuO2
plane band dispersion and apparent Fermi pockets in underdoped Bi2201, which is
directly associated with an hitherto-undetected strong temperature dependence
of the incommensurate superstructure periodicity below 130K. 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 and other complex
oxides as a rich playground for the study of electronically soft phases.
