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Description
The synthesis of superconducting fullerides is conventionally performed by intercalation of alkali ions via thermal gradients [1-4] or from solution [5-6]. Both methods have poor stoichiometry control, and thermal intercalation requires sample heating. Here we introduce electro-intercalation to directly drive Rb$^{+}$ into C$_{60}$ thin films with high stoichiometric precision. We use a new ultra-high vacuum (UHV) setup that combines \textit{in-situ} film preparation with in-operando Raman spectroscopy and four-terminal electronic transport [7].
The precise stoichiometry of the superconducting Rb$_3$C$_{60}$ phase is identified by the $A_g(2)$ Raman mode at about 1449 cm$^{-1}$, clearly distinct from pristine C$_{60}$. By varying the Rb/C$_{60}$ ratio we show that the intensity ratio of the Rb$_3$C$_{60}$ and C$_{60}$ Raman peaks correlates with the superconducting transition temperature T$_{\mathrm{C}}$. We demonstrate on-chip synthesis of superconducting Rb$_x$C$_{60}$ films and tuning of T$_{\mathrm{C}}$ from about 7 K up to nearly 26 K when the stoichiometry is adjusted from Rb$_{2.7}$C$_{60}$ to Rb$_3$C$_{60}$. Time-dependent Raman and transport data show that the Rb$^{+}$ electro-intercalation follows Butler–Volmer-type kinetics driven by the applied current.
Electro-intercalation thus provides a powerful route for precise, on-chip control of superconductivity in ultra-thin fulleride films and is extendable to other layered and porous materials.
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[7] Shchukin, K. P., Hell, M., & Grüneis, A. (2024). Combined Raman spectroscopy and electrical transport measurements in ultra-high vacuum down to 3.7 K. Review of Scientific Instruments, 95(12).