Evidence of strong correlations and coherence-incoherence crossover in the iron pnictide superconductor KFe₂As₂

Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of KFe₂As ₂. Both the Sommerfeld coefficient (γ≈103 mJ mol ^-1 K^-2) and the Pauli susceptibility (χ≈4× 10^-4) are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas-van Alphen and angle-resolved photoemission spectroscopy experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which shows the proximity of KFe₂As₂ to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence- incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hund's coupling between orbitals.