Orbital-specific Itinerancy and Localization in a Kagome Magnet

TL;DR

This paper demonstrates that the kagome magnet YMn$_6$Sn$_6$ exhibits orbital-specific itinerant and localized electrons, revealing a new form of orbital selectivity driven by geometry and Hund's physics, with implications for exotic quantum phases.

Contribution

It introduces the first evidence of orbital differentiation in a kagome lattice, combining experimental and theoretical methods to reveal orbital-specific electron behavior.

Findings

Coexistence of itinerant and localized electrons within the same Mn 3d orbitals.

Orbital-directed anisotropy leads to non-Fermi-liquid behavior.

Hund's exchange stabilizes orbital differentiation and ferromagnetic coupling.

Abstract

The kagome lattice naturally hosts flat bands, Dirac fermions, and van Hove singularities, yet whether its geometry can stabilize orbital-selective phases - a hallmark of Hund's physics in multi-orbital correlated systems - has remained an open question. Here, we combine resonant inelastic X-ray scattering with density functional theory and dynamical mean-field theory to demonstrate that YMn$_6$Sn$_6$ exhibits a spontaneous orbital differentiation into coexisting itinerant and localized electrons within the same Mn $3d$ manifold. Orbitals directed along Mn-Mn bonds provide coherent quasiparticles and metallic bands, while those pointing toward ligands become strongly correlated and display non-Fermi-liquid behavior. Hund's intra-atomic exchange suppresses orbital fluctuations, stabilizing this dichotomy and providing a natural double-exchange-like mechanism for the observed ferromagnetic bilayer coupling. Our work establishes YMn$_6$Sn$_6$ as a kagome platform where orbital selectivity, flat-band topology, and Hund's metallicity converge - revealing that geometric frustration and correlation-driven orbital differentiation can cooperatively design exotic quantum phases beyond the canonical paradigms of Mott physics or band topology alone.