Evolution of the Fermi Surface with Carrier Concentration inBi2Sr2CaCu2O8+δ

We show, by use of angle-resolved photoemission spectroscopy, that underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ 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 $(\pi /2,\pi /2\mathrm{})$ has been found. These conclusions are based on a determination of the minimum gap locus in the pseudogap regime $T_c<T<\mathrm{}{T}^{*}$, 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.