Modified doping at cuprate / lanthanum manganite interfaces

Oxide heterostructures allow combining materials with similar structure but with very different ground states, which may compete at the interface to yield novel behaviors and functionalities. We explore the YBa$_{2}$Cu$_{3}$O$_{7}$ (YBCO) / La$_{1-x}$Ca$_{x}$MnO$_{3}$ (LCMO) interface in thin film heterostructures. For x=0.3 the manganite is ferromagnetic which causes a strong depression of the superconductivity at the YBCO side. There is also a depression of the ferromagnetic moment at the interface suggesting electron transfer from the manganite into the YBCO. This is confirmed from superlattices alternating YBCO and LaMnO$_{3}$ (LMO), an A- type AF insulator. While for thin LMO layers ($<$ 6 unit cells) there is little effect on YBCO superconductivity, thicker LMO layers result in reduced Tc values and induced ferromagnetism at the interface, thus providing a firm indication of charge transfer. The occurrence of charge transfer over length scales much longer than the Thomas Fermi screening length (1 nm) is a novel behavior which, we hope, will stimulate future theoretical studies. Work supported by CICYT MAT2005 06024 C02-02.