We utilize Angle Resolved Photoemission Spectroscopy (ARPES) to uncover heavy electron (∼ 25m_e) behavior in the unconventional superconductor YFe₂Ge₂ (T_c = 1.2 K), from bands < 10 meV of the Fermi level, around the X point of the Brillouin zone. We obtain exceptional agreement between the experimental data and a many-body Dynamical Mean Field Theory (DMFT) calculation. We attribute the high level of agreement to the fact that we build the DMFT upon a more accurate single-particle treatment (Quasiparticle Self-Consistent GW or QSGW) rather than the typical Density Functional Theory (DFT) starting point. We clarify the degree of correlations and the source of the heavy electron behavior: they originate initially from one-body effects (interference among electron hopping matrix elements) but are enhanced by spin fluctuations. The one-particle spectrum of the flat bands around the X point provides a source for Kondo physics, observed both in ARPES and by the theory, with a coherence temperature of ~100 K.

Participation in the M²S 2026 conference has been supported by a grant from the National Synchrotron Radiation Centre SOLARIS under the Strategic Programme Excellence Initiative at the Jagiellonian University.