Distinct Fermi-Momentum Dependent Energy Gaps in Deeply Underdoped   Bi2212
  We use angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi2Sr2(Ca,Y)Cu2O8 (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, increases 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, does not increase with less doping - a behavior not observed in the single particle spectra before. 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-Tc superconductivity. 
Kiyohisa Tanaka, W.S. Lee, D.H. Lu, A. Fujimori, T. Fujii, Risdiana,   I. Terasaki, D.J. Scalapino, T.P. Devereaux, Z. Hussa...