Shallow core levels, or how to determine the doping and T_c of Bi_2Sr_2CaCu_2O_8+δ and Bi_2Sr_2CuO_6+δ without cooling

Determining the doping level in high-temperature cuprate superconductors is crucial for understanding the origin of superconductivity in these materials and for unlocking their full potential. However, accurately determining the doping level remains a significant challenge due to a complex interplay of factors and limitations in various measurement techniques. In particular, in Bi_2Sr_2CuO_6+δ and Bi_2Sr_2CaCu_2O_8+δ, where the mobile carriers are introduced by non-stoichiometric oxygen δ, the determination has been extremely problematic. Here, we study the doping dependence of the electronic structure of these materials in angle-resolved photoemission and find that both the doping level, p, and the superconducting transition temeprature, T_c can be precisely determined from the binding energy of the Bi 5d core-levels. The measurements can be performed at room temperature, enabling the determination of p and T_c without cooling the samples. This should be very helpful for further studies of these materials.