Insensitivity of the superconducting gap to variations in the critical temperature of Zn-substituted Bi₂Sr₂CaCu₂O _8+δ superconductors

The phase diagram of the superconducting high-T_c cuprates is governed by two energy scales: T*, the temperature below which a gap is opened in the excitation spectrum, and T_c, the superconducting transition temperature. The way these two energy scales are reflected in the low-temperature energy gap is being intensively debated. Using Zn substitution and carefully controlled annealing we prepared a set of samples having the same T* but different T_c's, and measured their gap using angle-resolved photoemission spectroscopy (ARPES). We show that T_c is not related to the gap shape or size, but it controls the size of the coherence peak at the gap edge.