Abstract

Superconductivity emerges from the collective coherence of Cooper pairs. In lattice systems, increasing the pairing strength tends to strongly localize electrons and weaken phase stiffness, thereby limiting critical temperatures – a phenomenon known as the (lattice) BCS –BEC crossover. We demonstrate that strongly correlated multiorbital systems can achieve high critical temperatures beyond the usual (single-band) lattice BCS –BEC crossover. Using a model of alkali-doped fullerides (A3C60), we find that the interplay of strong correlations and multiorbital effects enables a localized superconducting state with short coherence length but robust stiffness, where the critical temperature rises in a “domeless” fashion with increasing pairing interaction. To derive these insights, we introduce a new theoretical framework [1] that allows calculations of the fundamental superconducting length scales, namely the coherence length (ξ ) and the London penetration depth (λ_L), even in the presence of strong electron correlations.

[1] N. Witt et al., npj Quantum Mater. 9, 100 (2024)