In operando evidence of deoxygenation in ionic liquid gating of YBa ₂ Cu ₃ O _7-X

Abstract

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...

Dominant role of processing temperature in electric field induced superconductivity in layered ZrNBr

“…A similar vacancy forming process was reported in VO 2 film, in which suppression of metal-insulator transition was caused by electric field induced oxygen vacancy formation [10,28,29]. As recently reported on YBa 2 Cu 3 O 7−x [30], the EDLT induced deoxygenation of the CuO chains causes the modification of Cu coordination, leading to a superconductor-to-insulator transition. Based on the observations in the present study, we reasonably consider that the temperature of 235 K is close to a critical temperature, above which the deintercalation of Br ions could occur when applying proper V G s. Thus, the irreversible superconductivity could be caused by the partial deintercalation of Br in the gating process.…”

ON the Nature of Ionic Liquid Gating of La2−xSrxCuO4

“…The diffusion constants extracted for this process support an interpretation in terms of the oxygen being provided from the lower layers of the film, rather than from the outside. These results are in line with previous IL gating and X-ray absorption measurements [11,13], and will be important in forming a proper understanding for further exploration of this rapidly developing field.…”

“…This behavior has been reported before in the IL-gating of YBa 2 Cu 3 O 7−x films [12,24], where it was interpreted as an electric field driven slow redistribution process of the oxygen atoms in the copper oxide planes of the film [13,24]. …”

“…However, when applied to correlated oxide systems [5,10] there is substantial evidence that electrochemical processes related to interstitial oxygen is crucial in the gating process [11]. As for cuprate high temperature superconductors such as YBaCu 3 O 7−x , several reports suggested that the superconducting transition can be induced by (de)oxygenation of these materials by means of IL gating [12,13], while others suggested an electrostatic mechanism [14]. Dubuis et al [11] argued that the electric field of the electric double layer drives a redistribution of the oxygen atoms in the basal planes of the cuprate in an electrostatic fashion, while others have suggested an electrochemical process leading to oxygenation of the cuprate [12,13].…”

“…As for cuprate high temperature superconductors such as YBaCu 3 O 7−x , several reports suggested that the superconducting transition can be induced by (de)oxygenation of these materials by means of IL gating [12,13], while others suggested an electrostatic mechanism [14]. Dubuis et al [11] argued that the electric field of the electric double layer drives a redistribution of the oxygen atoms in the basal planes of the cuprate in an electrostatic fashion, while others have suggested an electrochemical process leading to oxygenation of the cuprate [12,13]. In this work, we attempt to address the mechanism of the IL gating of the cuprate material of La 2−x Sr x CuO 4 .…”