Gap and sub-gap stuctures of intrinsic Josephson tunnel junctions in Bi2Sr2CaCu2O8+x single crystals

摘要

Abstract: Stacked series arrays of intrinsic Josephson (IJ) tunnel junctions fabricated on the surfaces of Bi$-2$/Sr$-2$/CaCu$- 2$/O$-8$PLU@x$/ (Bi2212) single crystals by photolithography have been studied. Advanced technology with in-situ control of stack heights allows us to make a specified number (3- 2000) of junctions in the stack. The quasi-particle branch of an individual IJ junction has a well-developed gap structure. The gap value is approximately 12-13 meV at low temperature, which is approximately two times smaller than reported elsewhere. The temperature dependence of the gap deviates strongly from the BCS one. Proximity induced superconductivity of the Vi-O layers is one probable explanation for the reduced gap observed in the experiments. True superconducting contact obtained between the outermost Bi-O layer of a freshly cleaved Bi2212 single crystal and a Pb thin film is a supporting evidence for the Bi-O layers being superconducting. Multiple peaks are seen in the first derivative of the current-voltage characteristics at sub-gap voltages for all samples studied. The peak positions in voltage do not change with temperature and do not depend on the dimensions of the stack. Dynamic modulation of the tunnel density of states by low energy optical phonons and/or resonant tunneling through localized electronic states of the Bi-O and Sr-O layers are possible reasons for these observations. !36