Evolution of the Fermi surface with carrier concentration in Bi2Sr2CaCu2O8+delta

We show, by use of angle-resolved photoemission spectroscopy, that underdoped Bi2Sr2CaCu2O8+delta appears to have a large Fermi surface centered at (pi, pi), even for samples with a T-c as low as 15 K. No clear evidence of a Fermi surface pocket around (pi/2, pi/2) has been found. These conclusions are based on a determination of the minimum gap locus in the pseudogap regime T-c < T < T*, which is found to coincide with the locus of gapless excitations in momentum space (Fermi surface) determined above T*. These results suggest that the pseudogap is more likely of precursor pairing rather than magnetic origin.