Heavy holes as a precursor to superconductivity in antiferromagnetic CeIn₃

Numerous phenomenological parallels have been drawn between f- and d- electron systems in an attempt to understand their display of unconventional superconductivity. The microscopics of how electrons evolve from participation in large moment antiferromagnetism to superconductivity in these systems, however, remains a mystery. Knowing the origin of Cooper paired electrons in momentum space is a crucial prerequisite for understanding the pairing mechanism. Of special interest are pressure-induced superconductors CeIn ₃ and CeRhIn₅ in which disparate magnetic and superconducting orders apparently coexist - arising from within the same f-electron degrees of freedom. Here, we present ambient pressure quantum oscillation measurements on CeIn₃ that crucially identify the electronic structure - potentially similar to high-temperature superconductors. Heavy hole pockets of f-character are revealed in CeIn₃, undergoing an unexpected effective mass divergence well before the antiferromagnetic critical field. We thus uncover the softening of a branch of quasiparticle excitations located away from the traditional spin fluctuation-dominated antiferromagnetic quantum critical point. The observed Fermi surface of dispersive f-electrons in CeIn₃ could potentially explain the emergence of Cooper pairs from within a strong moment antiferromagnet.