  Much of what is known about high-temperature cuprate superconductors stems
from studies based on two surface analytical tools, angle-resolved
photoemission spectroscopy (ARPES) and spectroscopic imaging scanning tunneling
microscopy (SI-STM). A question of general interest is whether and when the
surface properties probed by ARPES and SI-STM are representative of the
intrinsic properties of bulk materials. We find this question is prominent in
thin films of a rarely studied cuprate DBCO. We synthesize DBCO films by oxide
molecular beam epitaxy and study them by in situ ARPES and SI-STM. Both ARPES
and SI-STM show that the surface DBCO layer is different from the bulk of the
film. It is heavily underdoped, while the doping level in the bulk is close to
optimal doping evidenced by bulk-sensitive mutual inductance measurements.
ARPES shows the typical electronic structure of a heavily underdoped CuO2 plane
and two sets of one-dimensional bands originating from the CuO chains with one
of them gapped. SI-STM reveals two different energy scales in the local density
of states, with one corresponding to the superconductivity and the other one to
the pseudogap. While the pseudogap shows large variations over the length scale
of a few nanometers, the superconducting gap is very homogeneous. This
indicates that the pseudogap and superconductivity are of different origins.
