Effects of out-of-plane disorder on the nodal quasiparticle and   superconducting gap in single-layer Bi$_2$Sr$_{1.6}Ln_{0.4}$CuO$_{6+\delta}$   ($Ln$ = La, Nd, Gd)
  How out-of-plane disorder affects the electronic structure has been investigated for the single-layer cuprates Bi$_2$Sr$_{1.6}$$Ln$$_{0.4}$CuO$_{6+\delta}$ ($Ln$ = La, Nd, Gd) by angle-resolved photoemission spectroscopy. We have observed that, with increasing disorder, while the Fermi surface shape and band dispersions are not affected, the quasi-particle width increases, the anti-nodal gap is enhanced and the superconducting gap in the nodal region is depressed. The results indicate that the superconductivity is significantly depressed by out-of-plane disorder through the enhancement of the anti-nodal gap and the depression of the superconducting gap in the nodal region. 
M. Hashimoto, T. Yoshida, A. Fujimori, D.H. Lu, Z.-X. Shen, M. Kubota,   K. Ono, M. Ishikado, K. Fujita, and S. Uchida