Electric--field effect on electron-doped infinite-layer   Sr$_{0.88}$La$_{0.12}$CuO$_{2+x}$ thin films
  We have used the electric--field effect to modulate the resistivity of the surface of underdoped Sr$_{0.88}$La$_{0.12}$CuO$_{2+x}$ thin films, allowing opposite modifications of the electron and hole density in the CuO$_2$ planes, an original situation with respect to conventional chemical doping in electron-doped materials. When the Hall effect indicates a large contribution of a hole band, the electric--field effect on the normal state resistivity is however dominated by the electrons, and the superconducting transition temperature increases when carriers are transfered from holes to electrons. 
L. Fruchter, F. Bouquet and Z.Z. Li
cond-mat.supr-con cond-mat.str-el