The influence of ferroelectric polarization on the superconductivity in ultrathin high-T-c films

Ferroelectric-superconductor heterostructures consisting of GdBa2Cu3O7-x, a nd YBa2Cu3O7-delta channels and Pb(ZrxTi1-x)O-3 (PZT) gate insulators were studied theoretically. We have shown that the remnant polarization P-r of t he PZT gate induces a change in the carrier concentration nh of the superco nductor, supposing direct action of the Coulomb field on the carriers. The dependences of the superconducting transition temperature T-c on n(h) were calculated in two cases: in the two-band model and in the phenomenological model with a parabolic dependence of T-c on n(h). The ferroelectric field e ffects are maximum in the superconducting film sample at a doping level whe re the transition temperature T-c as a function of nh has the largest slope dT(c)/dn(h). In agreement with the experiment, the shifts of T-c in the fe rroelectric field \DeltaT(c)(+/-P-r)\ decrease when the film thickness d in creases. The form of the calculated T-c(V-g) dependences on the gate voltag e V-g can be related to the ferroelectric hysteresis loop of the PZT gate i nsulator.