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Description
Growing thin films of the high-critical-temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi-2212) is essential for fabricating nanostructures in this material using a helium ion microscope (HIM). At the beam energy of 30 keV available in the HIM, helium ions only have limited penetration depth, and their path broadens with increasing film thickness. Consequently, the film thickness is limited to just a few unit cells, but performance of such thin films is limited by the interface with the substrate at one side and with air on the other.
The films under investigation were grown by pulsed laser deposition at JKU Linz [1], patterned into stripes suitable for electronic transport measurements by optical lithography and wet-chemical etching, and post-annealed to compensate for oxygen losses that occurred during the structuring process.
From electronic transport measurements, we can derive important superconducting metrics such as the critical temperature, and the critical current at different temperatures and magnetic fields, which proved to be exceptionally high for films of this thickness [2]. Due to the high anisotropy of Bi-2212, it is essential to perform measurements with different directions of the magnetic field relative to the film surface. From these data, we derived fundamental parameters such as the in-plane and out-of-plane coherence lengths, the London penetration depth, the anisotropy factor and the Ginzburg Landau parameter. Hall effect measurements further reveal the temperature dependence of the carrier mobility, and the Hall coefficient exhibits two sign reversals – an observation that remains to be fully explained.
This work can help to better understand how to optimize electrical properties in ultrathin Bi-2212 thin films, which can potentially be nanopatterned into interesting device geometries by helium ion microscopy.
[1] S. Keppert, B. Aichner, R. Adhikari, B. Faina, W. Lang, J. D. Pedarnig: Phase Purity and Surface Morphology of High-J$_c$ Superconducting Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Thin Films, Appl. Surf. Sci. 636, 157822 (2023)
[2] B. Aichner, S. Keppert, J. D. Pedarnig, W. Lang: Enhanced superconducting properties of Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ films with sub-50-nm thickness, Sci. Rep. 15, 11855 (2025)