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

Ferroelectric-superconductor heterostructures consisting of GdBa2Cu3O7-x and YBa2Cu3O7-δ channels and Pb(ZrxTi1-x)O3 (PZT) gate insulators were studied theoretically. We have shown that the remnant polarization Pr of the PZT gate induces a change in the carrier concentration nh of the superconductor, supposing direct action of the Coulomb field on the carriers. The dependences of the superconducting transition temperature Tc on nh were calculated in two cases: in the two-band model and in the phenomenological model with a parabolic dependence of Tc on nh. The ferroelectric field effects are maximum in the superconducting film sample at a doping level where the transition temperature Tc as a function of nh has the largest slope dTc/dnh. In agreement with the experiment, the shifts of Tc in the ferroelectric field |ΔTc(±Pr)| decrease when the film thickness d increases. The form of the calculated Tc(Vg) dependences on the gate voltage Vg can be related to the ferroelectric hysteresis loop of the PZT gate insulator.