On the nature of the electric-field effect on YBa2Cu3O7−δ grain boundary junctions employing epitaxial SrTiO3 gate insulators

We have fabricated Josephson field-effect transistors based on YBa2Cu3O7−δ bicrystal grain-boundary junctions (GBJs) and epitaxial SrTiO3 films as gate insulators. The SrTiO3 gate insulator shows high products of the breakdown field Ebd and the dielectric constant εr up to Ebdεr=1.3×1010 V/m allowing measurements over a wide range of applied gate electric-field Eg. The critical current Ic of the GBJs is found to depend highly nonlinear on Eg. Remarkably, the measured Ic(Eg) are very similar to the εr(Eg) 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.