Indications of an electronic phase transition in two-dimensional superconducting YBa ₂Cu ₃O _7-x thin films induced by electrostatic doping

We successfully tuned an underdoped ultrathin YBa ₂Cu ₃O _7-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∼0.15. These anomalous behaviors suggest that there is an electronic phase transition in the Fermi surface around the optimal doping level.