Scanning tunneling spectroscopy study of c-axis proximity effect in epitaxial bilayer manganite/cuprate thin films

摘要

Recent experimental studies have indicated novel superconducting proximityeffects in thin-film heterostructures comprising ferromagnetic manganites andsuperconducting cuprates. To look for such effects microscopically, weperformed scanning tunneling spectroscopy onLa$_{2/3}$Ca$_{1/3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ (LCMO/YBCO) bilayerthin films. $\emph{c}$-axis oriented films of varying thickness were grown onSrTiO$_{3}$ substrates using pulsed laser-ablated deposition.Heteroepitaxiality of the films was confirmed by cross-sectional transmissionelectron microscopy. Tunneling spectra were measured at 4.2 K, and analyzed forsignatures of a pairing gap on the LCMO layer. For bilayer samples with LCMOthickness down to 5nm, asymmetric conductance spectra characteristic ofsingle-layer LCMO films were observed, showing no clear gap structures. Theseobservations are consistent with a very short-range proximity effect involvingspin-singlet pairs, and difficult to reconcile with longer-range proximityscenarios involving spin-triplet pairs.