Electrostatic modulation of conductivity in Nd1.2Ba1.8Cu3Oy thin films

HTS properties can be modified by changing the carrier number n_h per CuO₂ plane. The astonishing generality of the HTS phase diagram (temperature vs. number of holes), poses many questions concerning the fundamental aspects of the physics of such materials and, at the same time, offers an interesting instrument for the control of their transport properties and for the realization of new devices. Using field effect devices, we have modified the number of carriers in the surface layers of Nd_1.2Ba_1.8Cu₃O₇ epitaxial films, grown on SrTiO₃ substrate, having thickness ranging between 4 and 10 unit cells (u.c.). The films investigate below 9 u.c. present a Variable Range Hopping Mott-insulator temperature dependence of the resistivity analogous to other insulators belonging to the families of strongly correlated systems. The gate can be applied both on the top layer through an Al₂O₃ layer (gate up configuration) or by applying the electrical field on the back side of the SrTiO₃ substrate (gate down configuration). The devices are fabricated by a completely in situ technique, avoiding superface and inter-layers impurities. Here we report on the I-V characteristics and on resistive measurements obtained mainly with gate down configuration on insulating samples. Particular interest was devoted to the understanding of the maximum modulation achievable in this compound as well. Considerations on superconducting samples (10 u.c.) are also reported as well as on the possibility on the possibilities of superconductor-insulator transitions.