Compound geometric resonances in Bi₂Sr₂CaCu₂O_x single crystals

Single crystals of Bi₂Sr₂CaCu₂O_x with transition temperatures of 85 K have been studied using a low temperature scanning tunneling microscope. Spectra of very weakly coupled point contact configuration have been obtained and reproducible periodic conductance structures in the bias voltage have been found. A model based on geometrical resonances in superconducting superlattice is proposed and found to provide a satisfactory explanation of the data.