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

Recent experimental studies have indicated novel superconducting proximity effects in thin-film heterostructures comprising ferromagnetic manganites and superconducting cuprates. To look for such effects microscopically, we performed scanning tunneling spectroscopy on La2/3Ca1/3MnO3/YBa2Cu3O7−δ (LCMO/YBCO) bilayer thin films. c-axis-oriented films of varying thickness were grown on SrTiO3 substrates using pulsed laser-ablated deposition. Heteroepitaxiality of the films was confirmed by cross-sectional transmission electron microscopy. Tunneling spectra were measured at 4.2 K and analyzed for signatures of a pairing gap on the LCMO layer. For bilayer samples with LCMO thickness down to 5 nm, asymmetric conductance spectra characteristic of single-layer LCMO films were observed, showing no clear gap structures. These observations are consistent with a very short-range proximity effect involving spin-singlet pairs, and difficult to reconcile with longer-range proximity scenarios involving spin-triplet pairs.