In operando evidence of deoxygenation in ionic liquid gating of YBa2Cu3O7-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 VO₂ 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 YBa₂Cu₃O_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.

Keywords: electric double-layer techniques; first-principles density functional theory; high-temperature superconductivity; near-edge X-ray absorption spectroscopies; superconductor-insulator transition.

Fig. 1.

Schematic structure and R(T) measurements of the electric double-layer field-effect transistor using YBCO. (A) Schematic representation of the cuprate electric double-layer transistor device showing the configuration of the gating process and the geometry of the X-ray measurements. (B) Resistance versus temperature curves measured at different gate voltages: 0 V (black), 4 V (red), 5 V (green), and 10 V (blue).

Fig. 2.

Gate voltage, V_G, dependence of Cu K-edge spectra of YBCO. Evolution of the Cu K-edge NEXAFS spectra of YBCO (A) and the CuO_X deposited under the gate electrode (B) with increasing gate voltage [0 V (black), 4 V (red), 5 V (green), and 10 V (blue)]. The difference spectra determined by subtracting the 0-V curve from each of the other curves for YBCO and the CuO_X are shown in C and D, respectively. As expected the differences for the spectra of the CuO_X deposited on top of alumina and under the gold gate electrode are negligible.

Fig. 3.

NEXAFS spectra simulations for different oxygen stoichiometries of YBCO. (A) Calculated NEXAFS spectra of YBCO with oxygen stoichiometries corresponding to 7, 6.75, 6.5, and 6.25 (black, red, green, and blue) when the oxygen vacancies are exclusively generated in the CuO_X chains. (B) Calculated NEXAFS spectra of YBCO for the same stoichiometries used in A but with the oxygen vacancies generated exclusively in the CuO₂ planes. (C) Calculated NEXAFS spectra of YBCO when the compound is electrostatically charged with 0 (7), 0.25 (6.75), 0.5 (6.5), and 0.75 (6.25) electrons per unit cell (corresponding oxygen stoichiometry). D–F show the intensity difference between the calculated spectra for YBCO with oxygen stoichiometries 6.75, 6.5, and 6.25 and the calculated spectra of YBCO with oxygen stoichiometry 7 for the different cases of study. Note that B–F use the same color codes as displayed in A.

Fig. 4.

Molecular orbitals structure before and after the doping process. Schematic representation of the hybridized molecular orbitals for the two extreme compounds of the YBCO series: YBa₂Cu₃O₇ (A) and YBa₂Cu₃O₆ (B). The Cu coordination in the y-z plane of the CuO_X chain structure is also represented. (C) NEXAFS experimental data obtained with a voltage gate of 0 V (blue line) and 10 V (red line) show the spectral weight transfer from the main edge feature to the pre-edge, in agreement with the formation of oxygen vacancies in the Cu chains. (D) YBCO crystallographic structure with the Cu, O, Ba, and Y represented with the yellow, red, green, and blue colors, respectively. The coordination polyhedra of the CuO_X chains planes (Top and Bottom) and the superconducting CuO₂ planes (Middle) are highlighted as well as the superconducting CuO₂ planes. The black arrow illustrates the intraunit cell-charge transfer process between the CuO_X chains and the superconducting CuO₂ planes.

Fig. 5.

(A) Schematic structure and R(T) measurements of the electric double-layer field-effect transistor of YBCO with an interlayer. Schematic representation of the gating device showing the configuration with an interlayer between the YBCO and the IL. B and C show resistance versus temperature curves measured at different gate voltages (from 0 V to 1.8 V) when the interlayer is amorphous ALO and isostructural PBCO, respectively. Whereas the amorphous ALO is acting as a physical barrier preventing oxygen diffusion, the PBCO interlayer acts as an oxygen reservoir allowing oxygen migration from the YBCO. C shows a similar behavior of the resistance versus temperature curves of deoxygenated YBCO samples.

Fig. 6.

Reversibility of the doping process. Resistance versus temperature curves obtained after gating with positive (A) and negative (B) voltages. (A) Positive gate voltages applied in 0.1 V steps up to 0.9 V produce a systematic shift of T_C. Fixed gate voltage and increasing polarization time from 10 to 110 min leads to further decrease of the critical temperature underscoring the important oxygen migration role in the doping process. (B) Application of negative gate voltages recovers the initial T_C.