Quasiparticle interference and superconducting gap in Ca(2-x)Na(x)CuO(2)Cl(2)

High-transition-temperature (high-T(c)) superconductivity is ubiquitous in the cuprates containing CuO(2) 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-T(c) 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 Ca(2-x)Na(x)CuO(2)Cl(2)(Na-CCOC) with T(c)=25-28 K. 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 Bi(2)Sr(2)CaCu(2)O(y) (Bi2212) at the same doping level, despite the T(c) 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 explains-at least in part-the remarkable material dependence of T(c).