An X-ray absorption study of the electric field effect mechanism in "123" cuprates

X-ray absorption spectroscopy in the presence of an electric field has been used to study the doping mechanism of the CuO(2) planes in Nd(1+x)Ba(2-x)Cu(3)O(7) compounds. The electric field effect doping is well-known as a method to modify the electrical properties of a thin film using an external gate voltage and in the copper based high critical temperature superconductors (HTS), it has been used to shift the critical temperature and even to induce phase transitions. Field effect experiments in ultra-thin HTS are usually interpreted by supposing that the induced charges develop into carriers in the CuO(2) conducting planes, thus changing the filling of the Zhang-Rice (ZR) band. Here we show that the polarization charges in both insulating and superconducting films, are mainly confined in the charge reservoir, and in particular in the CuO chains. The characteristics of the charge reservoir layer determine the doping of the CuO(2) planes, achieved by transfer of a fraction of the total injected charges. Moreover we found that holes injection in the CuO(2) planes is reduced in oxygen deficient NdBCO films.