Electric double layer transistor configurations have been employed to electrostatically dope single crystals of insulating SrTiO3. Here we report on the results of such doping over broad ranges of temperature and carrier concentration employing an ionic liquid as the gate dielectric. The surprising results are, with increasing carrier concentration, an apparent carrier-density dependent conductor-insulator transition, a regime of anomalous Hall effect, suggesting magnetic ordering, and finally the appearance of superconductivity. The possible appearance of magnetic order near the boundary between the insulating and superconducting regimes is reminiscent of effects associated with quantum critical behavior in some complex compounds.

Using an electronic double layer transistor we have systematically studied the superconductor to insulator transition in La2CuO 4+δ thin films growth by ozone assisted molecular beam epitaxy. We have confirmed the high crystalline quality of the cuprate films and have demonstrated the suitability of the electronic double layer technique to continuously vary the charge density in a system that is otherwise characterized by the presence of miscibility gaps. The transport and magneto-transport results highlight the role of electron-electron interactions in the mechanism of the transition due to the proximity of the Mott insulating state.

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Field-effect experiments on cuprates using ionic liquids have enabled the exploration of their rich phase diagrams [Leng X, et al. (2011) Phys Rev Lett 107(2):027001]. Conventional understanding of the electrostatic doping is in terms of modifications of the charge density to screen the electric field generated at the double layer. However, it has been recently reported that the suppression of the metal to insulator transition induced in VO 2 by ionic liquid gating is due to oxygen vacancy formation rather than to electrostatic doping [Jeong J, et al. (2013) Science 339(6126):1402-1405]. These results underscore the debate on the true nature, electrostatic vs. electrochemical, of the doping of cuprates with ionic liquids. Here, we address the doping mechanism of the high-temperature superconductor YBa 2 Cu 3 O 7-X (YBCO) by simultaneous ionic liquid gating and X-ray absorption experiments. Pronounced spectral changes are observed at the Cu K-edge concomitant with the superconductor-to-insulator transition, evidencing modification of the Cu coordination resulting from the deoxygenation of the CuO chains, as confirmed by first-principles density functional theory (DFT) simulations. Beyond providing evidence of the importance of chemical doping in electric double-layer (EDL) gating experiments with superconducting cuprates, our work shows that interfacing correlated oxides with ionic liquids enables a delicate control of oxygen content, paving the way to novel electrochemical concepts in future oxide electronics.electric double-layer techniques | high-temperature superconductivity | near-edge X-ray absorption spectroscopies | first-principles density functional theory | superconductor-insulator transition T he use of an electric field to modulate the charge density of a material in a controlled way is extensively used in metal oxide semiconductor technology and is also a promising technique to manipulate the properties of complex oxides to generate new devices with exciting functionalities. Many correlated oxides exist in the vicinity of a metal to insulator transition and, as such, their rich phase diagrams are critically controlled by the charge density (1). Gating experiments allowing its controlled modification have attracted much attention (2-8), and in recent years, unprecedented changes in the carrier concentration of complex materials have been achieved by using electric double-layer (EDL) techniques that use ionic liquids (ILs) as gate dielectrics (9-11). Superconductivity has been induced in otherwise band-insulating materials such as SrTiO 3 (STO) (12), ZrNCl (13), KTaO 3 (14), or MoS 2 (15), and the superconducting properties of several cuprates have been tuned to the insulating state (16)(17)(18)(19)(20). These results have focused much attention on the EDL technique not only from an applied point of view but also for providing an opportunity to obtain fundamental knowledge about the phase diagrams.Specifically, in the case of high-temperature cuprate superconductors (16-20), a striking similarity betw...