Two-dimensional superconductivity of epitaxial tantalum films

Abstract

The recently realized single crystalline α-phase tantalum (Ta) films, mainly explored for their use in quantum computing, are yet another elemental superconductor with strong spin-orbit interactions. In this work, we address the superconducting properties of the α-Ta films from the two-dimensional (2D) limit to the 2D-3D crossover regime. We epitaxially grow α-Ta thin films with thicknesses ranging from 6.5 nm to over 160 nm on Al2O3 substrates. Interestingly, the in-plane upper critical magnetic fields Bc2,∥ of our Ta films always exhibit a parabolic temperature dependence—one defining character of 2D superconductivity. The extracted superconducting thickness dsc can reach 1.7–2.4 times the in-plane coherence length ξab in the relatively thick films (90–160 nm), drawing a sharp contrast to the usual expectation of dsc<ξab for 2D superconductors. Furthermore, we observe deviation from the 2D Tinkham behavior in the angular dependence of the upper-critical field only in the thickest sample (about 160 nm). It suggests that Ta enters the 2D-3D crossover regime at a very large sample thickness. Finally, we analyze the enhancement of Bc2,∥ over the Pauli limit in the thinnest sample. The systematic evolution of Bc2,∥ as a function of thickness suggests the involvement of intrinsic spin-orbit interactions.

Source-reported materials — not catalogue approval

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