  The superconductor-to-insulator transition (SIT) induced by means such as
external magnetic fields, disorder or spatial confinement is a vivid
illustration of a quantum phase transition dramatically affecting the
superconducting order parameter. In pursuit of a new realization of the SIT by
interfacial charge transfer, we developed extremely thin superlattices composed
of high $T_c$ superconductor YBa$_2$Cu$_3$O$_7$ (YBCO) and colossal
magnetoresistance ferromagnet La$_{0.67}$Ca$_{0.33}$MnO$_3$ (LCMO). By using
linearly polarized resonant X-ray absorption spectroscopy and magnetic circular
dichroism, combined with hard X-ray photoelectron spectroscopy, we derived a
complete picture of the interfacial carrier doping in cuprate and manganite
atomic layers, leading to the transition from superconducting to an unusual
Mott insulating state emerging with the increase of LCMO layer thickness. In
addition, contrary to the common perception that only transition metal ions may
response to the charge transfer process, we found that charge is also actively
compensated by rare-earth and alkaline-earth metal ions of the interface. Such
deterministic control of $T_c$ by pure electronic doping without any hindering
effects of chemical substitution is another promising route to disentangle the
role of disorder on the pseudo-gap and charge density wave phases of underdoped
cuprates.
