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Fractional power-law behavior and its origin in iron-chalcogenide and ruthenate superconductors: Insights from first-principles calculations

We perform realistic first-principles calculations of iron chalcogenides and ruthenate based materials to identify experimental signatures of Hund's coupling induced correlations in these systems. We find that FeTe and K$_x$Fe$_{2-y}$Se$_2$ display unusual orbital dependent fractional powerlaw behavior in their quasiparticle self energy and optical conductivity, a phenomena first identified in SrRuO$_3$. Strong incoherence in the paramagnetic state of these materials results in electronic states hidden to angle-resolved photoemission spectroscopy which reemerge at low temperatures. We identify the effective low energy Hamiltonian describing these systems and show that these anomalies are not controlled by the proximity to a quantum critical point but result from coexistence of fast quantum mechanical orbital fluctuations and slow spin fluctuations.