Distinct Fermi-Momentum-Dependent Energy Gaps in Deeply Underdoped Bi2212

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Abstract
We used angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi 2 Sr 2 Ca (1– x ) Y x Cu 2 O 8 (Bi2212) to reveal the presence of two distinct energy gaps exhibiting different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increased with underdoping, a behavior known for more than a decade and considered as the general gap behavior in the underdoped regime. The other gap, associated with the near-nodal regime where a coherent peak in the spectrum can be observed, did not increase with less doping, a behavior not previously observed in the single particle spectra. We propose a two-gap scenario in momentum space that is consistent with other experiments and may contain important information on the mechanism of high–transition temperature superconductivity.
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