Quantum critical point and phase separation at finite doping in Hund metals.*

"Hund metals”  are multi-orbital paramagnetic metals with sizeable effects due to the intra-atomic exchange energy or Hund’s coupling, and are characterised by strong, orbital-selective correlations and large fluctuating local magnetic moments. Their physics is relevant for iron-based superconductors and other materials like transition metal oxides.

A general feature found in models and realistic simulations of these materials, and corroborated by experimental data, is a frontier crossing the doping-interaction strength plane, and originating from the Mott transition point of the half-filled system, across which the aforementioned defining features are strongly enhanced.

This frontier is a cross-over at large doping while approaching half-filling it becomes a first-order transition between two metals. It features a phase separation zone ending in a quantum critical point at finite doping.

I will show that all this phenomenology is due to the first-order nature of the Mott transition and can be back-tracked to a small energy scale splitting the atomic ground-state multiplet, in this case the Hund’s coupling. 

I will highlight the perfect parallel with a leading scenario for the physics of the cuprates, and thus possibly a universal one for materials showing high-Tc superconductivity.