Angle-resolved photoemission spectroscopy of (Ca,Na)2CuO2Cl2 crystals:: Fingerprints of a magnetic insulator in a heavily underdoped superconductor

Electronic evolution from an antiferromagnet to a high-T-c superconductor is revealed by angle-resolved photoemission experiments on tetragonal Ca1.9Na0.1CuO2Cl2 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 pseudogap. 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 La2-xSrxCuO4, demonstrating that the route to a high-T-c superconductor is not unique.