Enhancement of electronic inhomogeneities due to out-of-plane disorder in Bi2 Sr2 Cu O6+δ superconductors observed by scanning tunneling spectroscopy

Nanoscale electronic inhomogeneity in optimally doped single-layer Bi2 Sr1.6 L0.4 Cu O6+δ, (L -Bi2201, L=La and Gd) as well as bilayer Bi2 Sr2 Ca Cu2 O8+δ (Bi2212), superconductors has been investigated by the low-temperature scanning-tunneling-microscopy and spectroscopy. The inhomogeneous gap structures are observed above and below Tc on Gd-Bi2201. Increasing the out-of-plane disorder which is controlled by mismatch of the ionic radius between Sr and Ln results in the enhancement of pseudogap region with suppressed coherence peaks in the conductance spectrum, making the gap average larger and the gap distribution wider. The degree of electronic inhomogeneity measured by the distribution width of the gap amplitude is shown to have a correlation with Tc in optimally doped Bi2201 and Bi2212.