Superfluid condensation can fundamentally be different from that predicted by the Bardeen-Cooper-Schrieffer (BCS) theory. In a broad class of low-carrier-density superconductors, such as granular aluminum, doped nitrides, and high-$T_c$ cuprates, tunneling experiments reveal strong rather than weak coupling, as well as a conductance that does not return to that of the normal state upon approaching the critical temperature $T_c$. Here, we show that this behavior is in quantitative agreement with a tunneling theory that takes into account the large pairing-fluctuation effects that occur in the crossover region from weak-coupling BCS to strong-coupling Bose-Einstein condensation, provided the coherence energy scale rather than the single-particle energy gap is used to evaluate the coupling ratio. We also propose that the tendency toward strong coupling is a generic property of quasi-2D low-carrier-density superconductors.

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• Superconductors
• Tunnel junctions
• Unconventional superconductors
• Diagrammatic methods
• Green's function methods