Indications of an Electronic Phase Transition in 2D YBa2Cu3O7-x Induced   by Electrostatic Doping
  We successfully tuned an underdoped ultrathin YBa2Cu3O7-x film into the overdoped regime by means of electrostatic doping using an ionic liquid as a dielectric material. This process proved to be reversible. Transport measurements showed a series of anomalous features compared to chemically doped bulk samples and a different two-step doping mechanism for electrostatic doping was revealed. The normal resistance increased with carrier concentration on the overdoped side and the high temperature (180 K) Hall number peaked at a doping level of p$\sim$0.15. These anomalous behaviors suggest that there is an electronic phase transition in the Fermi surface around the optimal doping level. 
Xiang Leng, Javier Garcia-Barriocanal, Boyi Yang, Yeonbae Lee and   Allen M. Goldman
cond-mat.supr-con cond-mat.str-el