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.

中文翻译：

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非常规准二维超导体的隧道谱


超流体凝结可能与 Bardeen-Cooper-Schrieffer (BCS) 理论预测的有根本不同。在广泛的低载流子密度超导体中，例如粒状铝、掺杂氮化物和高载流子密度超导体中， $T_c$ 铜酸盐，隧道实验揭示了强耦合而不是弱耦合，以及在接近临界温度时电导不会恢复到正常状态 $T_c$ 。在这里，我们证明这种行为与隧道理论在数量上一致，隧道理论考虑了从弱耦合 BCS 到强耦合玻色-爱因斯坦凝聚的交叉区域中发生的大配对涨落效应，前提是相干能量尺度而不是使用单粒子能隙来评估耦合比。我们还提出，强耦合的趋势是准二维低载流子密度超导体的通用属性。