Formation and consequences of heavy d-electron quasiparticles in Sr3Ru2O7

We report angle-resolved photoelectron spectroscopy measurements of the quantum critical metal Sr3Ru2O7 revealing itinerant Ru 4d-states confined over large parts of the Brillouin zone to an energy range of < 6 meV, nearly three orders of magnitude lower than the bare band width. We show that this energy scale agrees quantitatively with a characteristic thermodynamic energy scale associated with quantum criticality and illustrate how it arises from the hybridization of light and strongly renormalized, heavy quasiparticle bands. For the largest Fermi surface sheet we find a marked k-dependence of the renormalization and show that it correlates with the Ru 4d - O 2p hybridization.