Superconductivity emerges from the spatial coherence of a macroscopic condensate of Cooper pairs. Increasingly strong binding and localization of electrons into these pairs compromises the condensate’s phase stiffness, thereby limiting critical temperatures – a phenomenon known as the BCS–BEC crossover in lattice systems. In this study, we demonstrate enhanced superconductivity in a multiorbital model of alkali-doped fullerides (A₃C₆₀) that goes beyond the limits of the lattice BCS–BEC crossover. We identify that the interplay of strong correlations and multiorbital effects results in a localized superconducting state characterized by a short coherence length but robust stiffness and a domeless rise in critical temperature with increasing pairing interaction. To derive these insights, we introduce a new theoretical framework allowing us to calculate the fundamental length scales of superconductors, namely the coherence length (ξ₀) and the London penetration depth (λ_L), even in presence of strong electron correlations.

超导性源于库珀对的宏观凝聚态的空间相干性。电子越来越强的结合和定位到这些对中，这损害了凝聚态的相刚度，从而限制了临界温度——这种现象在晶格系统中被称为 BCS-BEC 交叉。在这项研究中，我们在碱掺杂富勒化物 （A₃C₆₀） 的多轨道模型中证明了增强的超导性，这超出了晶格 BCS-BEC 交叉的限制。我们发现，强相关性和多轨道效应的相互作用导致了局域超导状态，其特征是相干长度短但刚度强，临界温度无圆顶上升，配对相互作用增加。为了获得这些见解，我们引入了一个新的理论框架，使我们能够计算超导体的基本长度尺度，即相干长度 （ξ₀） 和伦敦穿透深度 （λ_L），即使在存在强电子相关性的情况下也是如此。