Flat bands, strange metals and the Kondo effect

• Published in: Nature reviews. Materials Vol. 9; no. 7; pp. 509 - 526
• Main Authors: Checkelsky, Joseph G., Bernevig, B. Andrei, Coleman, Piers, Si, Qimiao, Paschen, Silke
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.07.2024; Nature Publishing Group
• Subjects: 639/766/119/2792; 639/766/119/2795; 639/766/119/995; 639/766/119/997; Biomaterials; Chemistry and Materials Science; Commonality; Condensed Matter Physics; Correlation; Electrons; Fermions; Group 5A compounds; Kondo effect; Lattices; Materials Science; Nanotechnology; Optical and Electronic Materials; Orbitals; Perspective; Physical properties; Physics; Platforms; Science & Technology - Other Topics; Superconductors; Superlattices; Topology; Unconventional superconductivity
• ISSN: 2058-8437; 2058-8437
• DOI: 10.1038/s41578-023-00644-z

Flat-band materials such as the kagome metals or moiré superlattices are of intense current interest. Flat bands can result from the electron motion on numerous (special) lattices and usually exhibit topological properties. Their reduced bandwidth proportionally enhances the effect of Coulomb interaction, even when the absolute magnitude of the latter is relatively small. Seemingly unrelated to these materials is the large family of strongly correlated electron systems, which include the heavy-fermion compounds, and cuprate and pnictide superconductors. In addition to itinerant electrons from large, strongly overlapping orbitals, they frequently contain electrons from more localized orbitals, which are subject to a large Coulomb interaction. The question then arises as to what commonality in the physical properties and microscopic physics, if any, exists between these two broad categories of materials. A rapidly increasing body of strikingly similar phenomena across the different platforms — from electronic localization–delocalization transitions to strange-metal behaviour and unconventional superconductivity — suggests that similar underlying principles could be at play. Indeed, it has recently been suggested that flat-band physics can be understood in terms of Kondo physics. Inversely, the concept of electronic topology from lattice symmetry, which is fundamental in flat-band systems, is enriching the field of strongly correlated electron systems, in which correlation-driven topological phases are increasingly being investigated. In this Perspective article, we elucidate this connection, survey the new opportunities for cross-fertilization across platforms and assess the prospect for new insights that may be gained into correlation physics and its intersection with electronic topology. Flat-band materials such as kagome and moiré lattices and strongly correlated electron systems including heavy-fermion compounds exhibit strikingly similar phenomena of topology and strong correlations. This Perspective article discusses Kondo physics as the underlying theme and a route to a unified understanding.