On the nature of the electric-field effect on YBa₂Cu₃O_7-δ grain boundary junctions employing epitaxial SrTiO₃ gate insulators

We have fabricated Josephson field-effect transistors based on YBa₂Cu₃O_7-δ bicrystal grain-boundary junctions (GBJs) and epitaxial SrTiO₃ films as gate insulators. The SrTiO₃ gate insulator shows high products of the breakdown field E_bd and the dielectric constant ∈_r up to E_bd∈_r= 1.3 × 10₁₀ V/m allowing measurements over a wide range of applied gate electric-field E_g. The critical current I_c of the GBJs is found to depend highly nonlinear on E_g. Remarkably, the measured I_c(E_g) are very similar to the ∈_r(E_g) curves. This strongly suggests that the observed electric-field effect is not due to a field-induced change in carrier concentration but is related to the dielectric properties of the SrTiO₃ gate insulator.