Evolution of the Fermi surface with carrier concentration in   Bi_2Sr_2CaCu_2O_{8+\delta}
  We show, by use of angle-resolved photoemission spectroscopy, that underdoped Bi_2Sr_2CaCu_2O_{8+\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. 
H. Ding, M. R. Norman, T. Yokoya, T. Takeuchi, M. Randeria, J. C.   Campuzano, T. Takahashi, T. Mochiku, and K. Kadowaki