Spin freezing and the Sachdev-Ye model.

Spin-freezing is the origin of bad-metal physics and non-Fermi liquid (non-FL) properties in a broad range of correlated compounds. In a multi-orbital lattice system with Hund coupling, doping of the half-filled Mott insulator results in a highly incoherent metal with frozen magnetic moments. These moments fluctuate and collapse in a crossover region that is characterized by unusual non-Fermi liquid properties such as a self-energy whose imaginary part varies $\propto \sqrt{\omega}$ over a wide energy range. At low enough temperature, the local moment fluctuations induce electron pairing and this mechanism may be the unifying principle of unconventional superconductivity. While this physics has been discovered in numerical studies of multi-orbital Hubbard systems, it exhibits a striking similarity to the analytically solvable Sachdev-Ye (SY) model, and its recent fermionic extensions. Here, we clarify the relation between spin-freezing and SY physics, and thus shed light on fundamental properties of Hund metals.