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 commonly known as the BCS-BEC crossover. In this study [1], we report on an enhancement of superconductivity beyond the limits of the conventional BCS-BEC crossover present in a multi-orbital model of alkali-doped fullerides (A3C60). Our findings reveal a localized superconducting regime characterized by a robustly short coherence length and a domeless rise in critical temperature with increasing pairing interaction. We identify strong correlations and multiorbital effects as the underlying cause of this behavior. These insights are derived from the development of a theoretical framework to calculate the fundamental length scales of superconductors, namely the coherence length (ξ_0) and the London penetration depth (λ_L). Importantly, our approach allows for the determination of these scales in microscopic theories and from first principles, even in presence of strong electron correlations.

[1] N. Witt et al., arXiv:2310.09063 (2023)