Probing the interface and individual layers in cuprate/manganite heterostructures by Raman spectroscopy

We report a comprehensive set of polarized Raman spectra on thin-film multilayers of the high-$T_c$ superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ and electrically insulating manganites $R{\mathrm{MnO}}_3$ ($R=$ rare-earth partially substituted with group-II elements) hosting magnetic, charge and orbital order (COO). Such multilayers have been shown to exhibit a unique insulating-to-superconducting transition that is induced by magnetic field, electric field, or by tailoring the chemical composition of the $R$ site of the manganite. The Raman spectra show significant Jahn-Teller distortions of the manganite structure, which correlate with COO, approximately 90 K above the magnetic ordering temperature of 140 K. Based on the Raman data and earlier electrical transport studies of single-layer manganite films, we argue that the manganite layers in our heterostructures remain electrically insulating across the range of investigated temperatures, dopings, and magnetic fields. The Raman spectra show a pronounced redshift and broadening of lattice vibrations around 200 ${\mathrm{cm}}^{-1}$ in the multilayers compared to those of manganite films, which may indicate hybridization with ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ phonons. We also observe additional excitations at 690 ${\mathrm{cm}}^{-1}$ and 830 ${\mathrm{cm}}^{-1}$ that are absent in the single films or bulk responses, which we discuss to originate from the cuprate/manganite interface. These observations demonstrate that the phonon spectrum is significantly modified in our multilayer samples. This is expected to play an important role in the mechanism of the insulating-to-superconducting transition found in these cuprate-manganite multilayers.