Electronic structure of the ingredient planes of the cuprate superconductorBi2Sr2CuO6+δ: A comparison study withBi2Sr2CaCu2O8+δ

By means of low-temperature scanning tunneling microscopy, we report on the electronic structures of the BiO and SrO planes of the ${\mathrm{Bi}}₂{\mathrm{Sr}}₂{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ (Bi-2201) superconductor prepared by argon-ion bombardment and annealing. Depending on post annealing conditions, the BiO planes exhibit either a pseudogap (PG) with sharp coherence peaks and an anomalously large gap magnitude of 49 meV or van Hove singularity (vHS) near the Fermi level, while the SrO is always characteristic of a PG-like feature. This contrasts with the ${\mathrm{Bi}}₂{\mathrm{Sr}}₂{\mathrm{CaCu}}₂{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi-2212) superconductor where vHS occurs solely on the SrO plane. We disclose the interstitial oxygen dopants ($\ensuremath{\delta}$ in the formulas) as a primary cause for the occurrence of vHS, which are located dominantly around the BiO and SrO planes, respectively, in Bi-2201 and Bi-2212. This is supported by the contrasting structural buckling amplitude of the BiO and SrO planes in the two superconductors. Our findings provide solid evidence for the irrelevance of PG to the superconductivity in the two superconductors, as well as insights into why Bi-2212 can achieve a higher superconducting transition temperature than Bi-2201, and by implication, the mechanism of cuprate superconductivity.