Hund’s metals and insulators - effects caused by J

In addition to the U term, the effect of the electron exchange J is also the source of interesting electronic behavior.

While the effect of the U term is to push away incoming electrons by creating a “Coulomb Blockade” on the atomic sites, the J term tries to pull electrons away from other atoms in order to maximise the spin on a single site. Ultimately, a large enough J would also drive the system towards an insulating state in the form of a charge-disproportionated phase, where the electronic charge preferentially localizes on certain sites, leaving its neighbors positively charged. We study this phenomenon on the nickelates ANiO₃ [1, 2] and CaFeO₃ [3].
Perhaps unintuitively, in a system with large U and J, the competition of the two resulting insulating phases can result in a correlated metallic phase known as Hund’s Metal, where we focus on SrCrO₃.

1. O. E. Peil, A. Hampel, C. Ederer, and A. Georges, external pageMechanism and control parameters of the coupled structural and metal-insulator transition in nickelatescall_made, Phys. Rev. B 99 (2019)
2. A. Hampel, P. Liu, C. Franchini, and C. Ederer, external pageEnergetics of the coupled electronic–structural transition in the rare-earth nickelatescall_made, npj Quant Mater 4 (2019)
3. M. E. Merkel and C. Ederer, external pageCharge disproportionation and Hund’s insulating behavior in a five-orbital Hubbard model applicable to d4 perovskitescall_made, Phys. Rev. B 104, 165135 (2021)