Electric field effect and superconducting-insulating transition in '123' cuprate superconductors

The physics of high critical temperature superconductors (HTS) remains a fascinating but undisclosed issue in condensed matter. One of the most interesting topics is the transition from the insulating phase of the parent compound, having long range antiferromagnetic order, to the superconducting phase. A method to investigate in detail the superconducting to insulating (SIT) transition in HTS is to control the doping of the CuO2 planes in a fine way. Here, by using the electric field effect on thin Nd1Ba2Cu3O7 films, we present a study of the HTS phase diagram close to the SIT with unprecedented detail. By virtue of these data, we will show that doping of holes in samples located at the boundary separating the superconducting and insulating regions produces changes in the transport characteristic consistent with an electronic phase separation scenario. Some consequences of these data are the failure of standard 2D quantum scaling theory and the possible coexistence of superconducting and weakly insulating phases in this region of the phase diagram. A continuous transition between the two competing phases as a function of doping place evident constraints on the mechanism of superconductivity.