We examine key aspects of the theory of the Bardeen–Cooper–Schrieffer (BCS) to Bose–Einstein condensation (BEC) crossover, focusing on the temperature dependence of the chemical potential, μ. We identify an accurate method of determining the change of μ in the cuprate high temperature superconductors from angle-resolved-photoemission data (along the ‘nodal’ direction), and show that μ varies by less than a few percent of the Fermi energy over a range of temperatures from far below to several times above the superconducting transition temperature, T_c. This shows, unambiguously, that not only are these materials always on the BCS side of the crossover (which is a phase transition in the d-wave case), but are nowhere near the point of the crossover (where the chemical potential approaches the band bottom).

我们研究了 Bardeen-Cooper-Schrieffer (BCS) 到玻色-爱因斯坦凝聚 (BEC) 交叉理论的关键方面，重点关注化学势 μ 的温度依赖性。我们确定了一种从角度分辨光电子发射数据（沿“节点”方向）确定铜酸盐高温超导体中μ变化的准确方法，并表明μ的变化小于费米能量的百分之几温度范围从远低于超导转变温度Tc到_高几倍。这清楚地表明，不仅这些材料总是在交叉的 BCS 侧（这是 d 中的相变-wave 情况），但远不靠近交叉点（化学势接近带底）。