Evolution of the Fermi Surface with Carrier Concentration in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>Bi</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>Sr</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>CaCu</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>8</mml:mn><mml:mo>+</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>

We show, by use of angle-resolved photoemission spectroscopy, that underdoped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ appears to have a large Fermi surface centered at $(\ensuremath{\pi},\ensuremath{\pi})$, even for samples with a ${T}_{c}$ as low as 15 K. No clear evidence of a Fermi surface pocket around $(\ensuremath{\pi}/2,\ensuremath{\pi}/2)$ has been found. These conclusions are based on a determination of the minimum gap locus in the pseudogap regime ${T}_{c}&lt;T&lt;{T}^{*}$, 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.

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