Chemical Substitution as a Route to Anomalous Surface Superconductivity in NiBi$_3$

Overview & Synopsis

Stabilizing two-dimensional superconductivity in a bulk superconducting material provides a unique route to exploring the interplay between superconducting condensates of different dimensionalities. Here, we present evidence for a distinct surface-associated superconducting state in Pb-substituted NiBi$_3$. NiBi$_{2.8}$Pb$_{0.2}$ exhibits a resistive onset at $T_{c,\mathrm{onset}}^{*}\approx8.1$~K, nearly twice the bulk superconducting transition temperature of approximately 4.1~K. The absence of a corresponding Meissner response or specific-heat anomaly at the higher-temperature transition supports its non-bulk origin. A cusp-like angular dependence of the critical field, well described by the two-dimensional Tinkham model, together with nonlinear current--voltage characteristics and signatures of a Berezinskii--Kosterlitz--Thouless transition, establishes the low-dimensional character of this superconducting state. Remarkably, the surface-associated superconductivity persists well beyond the bulk upper critical field, reaching a maximum ratio of $H_{c3}/H_{c2}\approx4.3$, far exceeding the conventional Saint-James--de Gennes value of 1.695. These results demonstrate that Pb substitution can stabilize an anomalously enhanced low-dimensional superconducting state in NiBi$_3$, providing a platform for investigating superconductivity at the boundary of a bulk superconductor.