The high-T_c superconductor Bi₂Sr₂CaCu₂O_x as a superconducting superlattice

Structures in the conductance which are periodic in bias voltage have been observed in very weakly coupled Bi₂Sr₂CaCu₂O_x/W point-contact tunneling junctions. These reproducible features appear to result from geometrical resonances of the order parameter, as would be found in superconductor-normal multilayers, rather than from Coulomb-staircase or multiple-gap effects. The data and its analysis strongly suggest the validity of treating even microscopic properties of Bi₂Sr₂CaCu₂O_x using multilayer models with geometrical parameters inferred from the placement and thicknesses of the CuO₂ conductive sheets within the unit cell.