Transport properties of Nd1.2Ba1.8Cu3OZ ultrathin films by field-effect doping

We report a study on the transport properties of ultrathin Nd1.2Ba1.8Cu3Oz (NdBCO) films by using field effect devices. Very high quality NdBCO films, having thickness ranging between 5 and 130 nm, have been prepared using diode high oxygen pressure sputtering. The temperature dependence of the resistivity has been studied as a function of the number of layers and of doping induced by field effect. An insulating-superconducting transition is observed in these films when the thickness is increased above 9 unit cells (u.c.). Below 9 u.c. the resistivity follows a 2D Mott variable range hopping temperature dependence and the localization length, estimated from a fit, is found to increase when holes are injected in the sample by field effect. A similar trend is observed when the number of layers in the film increases as a result of the changes of doping. The analysis suggests that hole density plays a major role in the transport properties of NdBCO ultrathin films.