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Electrostatic Tuning of the Properties of High Temperature Superconductors Using Ionic Liquids

Ultrathin high temperature superconducting films have been grown on epitaxially compatible substrates using the techniques of high-pressure oxygen sputtering and ozone assisted molecular beam epitaxy. Films were then incorporated in field effect transistor configurations, which facilitated the control of superconductivity by electrostatic charging. While devices using SrTiO 3 as both the substrate and gate dielectric have produced only relatively small shifts in film electrical properties, very large changes were observed using electronic double layer transistor configurations employing ionic liquids as the dielectric. By varying the nominal hole concentration an apparent electrostatically tuned superconductor-insulator transition was realized in films of YBa 2 Cu 3 O 7-x and La 2 CuO 4+x by our group, and in La 2-x Sr x CuO 4 films by others. Very weak re-entrant superconductivity was also realized in YBa 2 Cu 3 O 7-x films when electron doped.  Also by accumulating holes an underdoped YBa 2 Cu 3 O 7-x film was tuned into the overdoped regime. Hall effect measurements suggest the presence of electronic phase transitions or changes in the Fermi surface at carrier concentrations close to those corresponding to both optimal doping and to the superconductor–insulator transition. In effect the full phase diagrams of these systems can be mapped including regions of phase space not accessible by convention chemical doping, and presumably not equilibrium configurations.  In most, but not all cases, the changes brought about by gating were reversible, suggesting, but not proving, that the processes are physical rather than chemical. However the precise processes involved in electrostatic gating remain largely unknown.