The Hund-metal path to strong electronic correlations

Atomic physics has a profound impact on the physical properties of correlated electron materials. This article describes a prime example of this phenomenon. We provide a non-technical introduction to the physics of Hund metals, a broad class of materials which include in particular iron pnictides and chalcogenides, as well as oxides of the 4d transition-metal series such as ruthenates. We highlight experiments which reveal distinctive signatures of Hund physics in selected materials. A key property of Hund metals is a clear separation between the energy and temperature scales associated with spin and orbital degrees of freedom. We emphasize the conceptual and practical importance of the non-perturbative renormalization group flow which identifies the relevant degrees of freedom at each energy scale. The flow begins with local atomic degrees of freedom at high energy, displays a universal behavior in the intermediate regime of spin-orbital separation and ends into a broad diversity of possible ordered phases. Ordering can take place either as an instability of the Fermi liquid regime with coherent quasiparticles or directly from the intermediate regime before Fermi liquid quasiparticles had a chance to emerge. Dynamical mean-field theory provides a natural conceptual framework as well as a powerful computational method to explain, calculate and predict many physical properties of correlated materials such as Hund metals. A copy-edited version of this article has been published (April, 2024) in Physics Today, 77(4), 46-53 (2024) \url{https://doi.org/10.1063/pt.wqrz.qpjx}