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 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 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 La 2- x Sr x CuO 4 , demonstrating that the route to a high- T c superconductor is not unique.