The discovery of quantum oscillations in the normal-state electrical resistivity of YBa2Cu3O6.5 provides the first evidence for the existence of Fermi surface (FS) pockets in an underdoped cuprate. However, the pockets electron vs. hole character, an
d the very interpretation in terms of closed FS contours, are the subject of considerable debate. Angle-resolved photoemission spectroscopy (ARPES), with its ability to probe electronic dispersion as well as the FS, is ideally suited to address this issue. Unfortunately, the ARPES study of YBa2C3O7-d (YBCO) has been hampered by the techniques surface sensitivity. Here we show that this stems from the polarity and corresponding self-doping of the YBCO surface. By in-situ deposition of potassium atoms on the cleaved surface, we are able to continuously tune the doping of a single sample from the heavily overdoped to the underdoped regime. This reveals the progressive collapse of the normal-metal-like FS into four disconnected nodal FS arcs, or perhaps into hole but not electron pockets, in underdoped YBCO6.5.
