Quasiparticle interference and superconducting gap in Ca _2-x Na _x CuO ₂ Cl ₂

High-transition-temperature (high-Tc) superconductivity is ubiquitous in the cuprates containing CuO2 planes, but each cuprate has its own character. The study of the material dependence of the d-wave superconducting gap (SG) should provide important insights into the mechanism of high-Tc superconductivity. However, because of the pseudogap phenomenon, it is often unclear whether the energy gaps observed by spectroscopic techniques really represent the SG. Here, we use scanning tunnelling spectroscopy to image nearly optimally doped Ca2xNaxCuO2Cl2(Na-CCOC) with Tc≤25-28K. It enables us to observe the quasiparticle interference effect in this material, through which we obtain unambiguous information on the SG. Our analysis of quasiparticle interference in Na-CCOC reveals that the SG dispersion near the gap node is almost identical to that of Bi2Sr2CaCu2Oy (Bi2212) at the same doping level, despite the Tc of Bi2212 being three times higher than that of Na-CCOC. We also find that the SG in Na-CCOC is confined in narrower energy and momentum ranges than Bi2212, which explainsat least in partthe remarkable material dependence of Tc.