Anisotropic interface-confined superconductivity in FeTe-based heterostructures

Abstract: Interface-confined superconductivity emerges from the interaction of electronic states across chemically distinct boundaries, providing a route to engineer superconducting phases where magnetism and topology coexist. Determining the intrinsic nature of such superconductivity, however, is challenging because the superconducting layer is only a few nanometers thick and buried beneath several normal layers. Here, we measure the whole-sample Meissner response of six $\mathrm{FeTe}$-based heterostructures in a uniform magnetic field using a frequency-domain tunnel-diode resonator. In the ultrathin limit, the conventional normalization of the measured susceptibility, $χ(T\to0)=-1$, fails by tens of percent. We establish the appropriate calibration and invert $χ(T)$ to determine the London penetration depth $λ(T)$. Two key results emerge. First, the broad transitions observed in $χ(T)$ arise naturally from the extreme geometry and large $λ$, without requiring chemical or structural inhomogeneity; the extracted $λ(T)$ closely tracks the resistive transition. Second, $λ(T)$ and the corresponding superfluid density are inconsistent with a fully gapped isotropic $s$-wave state and instead indicate a strongly anisotropic order parameter possibly with line nodes or deep gap minima. The inferred $λ(0)$ is of order $1\,μ$m, consistent with an independent analysis of the 2D phase stiffness. Despite the distinct chemical, magnetic, and topological character of the three overlayers, all six FeTe heterostructures exhibit similar low-temperature power-law behavior, with no systematic dependence of the superconducting response on overlayer identity. These results point to the interfacial $\mathrm{FeTe}$ layer as the common origin of superconductivity.