High-Tc superconductor near the S–I transition

We report on the density of states measurements of Bi2212 (Bi2+xSrxCaCu2O8+δ) near the superconductivity-insulator transition using a low temperature scanning tunneling microscope. We prepared highly underdoped Bi rich Bi2212 single crystals (Tc ∼ 32 K). The energy gap distribution did not provide an energy scale proportional to Tc. Averaged tunnel spectra with various doping levels were scaled into a single line if energy was normalized by their respective gap values. This indicated there was no crossover energy, which separates a pseudogap and a superconducting gap.