  Ferroelectric field-effect doping has emerged as a powerful approach to
manipulate the ground state of correlated oxides, opening the door to a new
class of field-effect devices. However, this potential is not fully exploited
so far, since the size of the field-effect doping is generally much smaller
than expected. Here we study the limiting factors through magneto-transport,
scanning transmission electron and piezo-response force microscopy in
ferroelectric/superconductor (YBa2Cu3O7-{\delta} /BiFeO3) heterostructures, a
model system showing very strong field-effects. Still, we find that they are
limited in the first place by an incomplete ferroelectric switching. This can
be explained by the existence of a preferential polarization direction set by
the atomic terminations at the interface. More importantly, we also find that
the field-effect carrier doping is accompanied by a strong modulation of the
carrier mobility. Besides making quantification of field-effects via Hall
measurements not straightforward, this finding suggests that ferroelectric
poling produces structural changes (e.g. charged defects or structural
distortions) in the correlated oxide channel. Those findings have important
consequences for the understanding of ferroelectric field-effects and for the
strategies to further enhance them.
