Angle-Resolved Photoemission Spectroscopy of (Ca,Na)₂CuO₂Cl₂ Crystals: Fingerprints of a Magnetic Insulator in a Heavily Underdoped Superconductor

Abstract

Electronic evolution from an antiferromagnet to a high-T_c superconductor is revealed by angle-resolved photoemission experiments on tetragonal Ca_1.9Na_0.1CuO₂Cl₂ 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 La_2−xSr_xCuO₄, demonstrating that the route to a high-T_c superconductor is not unique.