Size of ferroelectric field effects in oxide heterostructures*

Ferroelectric field-effects allow the manipulation of the physical properties of strongly correlated oxides. This is achieved in heterostructures that combine these materials with ferroelectrics. Similarly, as in a transistor, a strong accumulation of charge is induced in the correlated oxide channel to screen the electric field from the ferroelectric gate, which results in a significant doping of the correlated material. Here we investigate the extrinsic factors that may limit the size of ferroelectric field-effects, which include among other the nanoscale structural and ferroelectric properties near the interface. The investigation is done via a combination of transport measurements (Hall effect and T_C measurements), piezoresponse force microscopy (PFM), high-resolution scanning transmission electron microscopy (HR-STEM) and electron energy loss spectroscopy (EELS) in BiFeO₃ (ferroelectric)/YBa₂Cu₃O₇ (superconductor) heterostructures. We find that incomplete ferroelectric switching, probably associated to the interface atomic terminations, and changes in the carrier mobility that accompany the carrier concentration modulation, conspire to reduce the size of the field effects.