Angle-resolved photoemission spectroscopy of Na-doped Ca2CuO2Cl2 single   crystals: Fingerprints of a magnetic insulator in a heavily underdoped   superconductor
  Electronic evolution from an antiferromagnet to a high-Tc superconductor is revealed by angle-resolved photoemission experiments on tetragonal Ca(1.9)Na(0.1)CuO(2)Cl(2) single crystals, which were successfully grown for the first time under high pressures. In this underdoped superconductor, we found clear fingerprints of the parent insulator: a shadow band and a large pseudo-gap. These observations are most likely described by a "chemical potential shift", which contrasts clearly with the prevailing wisdom of the "pinned chemical potential" learned from the prototype La(2-x)Sr(x)CuO(4), demonstrating that the route to a high-Tc superconductor is not unique. 
Y. Kohsaka, T. Sasagawa, F. Ronning, T. Yoshida, C. Kim, T. Hanaguri,   M. Azuma, M. Takano, Z.-X. Shen, and H. Takagi