This paper looks at why two families of kagome superconductors — AV3Sb5 (A = K, Rb, Cs) and ATi3Bi5 (A = Rb, Cs) — show unexpectedly similar and strongly correlated behavior even though their nominal electron counts differ. The authors use a theoretical look at the local, high-energy electronic structure and paired experiments that add nonmagnetic tin (Sn) atoms to the crystals. They report that the transition-metal ions (vanadium and titanium) behave like 2+ ions. That implies the number of electrons that can move freely through the lattice (itinerant carriers) is similar in the two families, while the main difference appears as quantum-fluctuating local magnetic moments on the ions.

The theoretical part focuses on the Hartree-scale local electronic structure. “Hartree-scale” here refers to effects set by strong Coulomb repulsion inside the transition-metal atoms, a high-energy aspect of their electronic state. The calculations suggest that strong intra-atomic repulsion suppresses charge changes on the metal ions and fixes their valence at about 2+. As a result, the ligand atoms (the atoms around the metals) carry similar densities of mobile electrons in both V- and Ti-based materials. The remaining difference is a different size of the ionic spin moment that can fluctuate quantum mechanically.

Because the atoms sit on a kagome lattice, their spins are geometrically frustrated. Frustration means the lattice geometry prevents simple, stable magnetic order and lets ionic spins fluctuate very rapidly. The paper points out that these fast, short-range fluctuations can be hard to see with standard experimental probes. In particular, in pristine (undoped) samples the fluctuations may be too rapid or too strongly coupled to moving electrons for techniques like muon spin rotation or relaxation (μSR) to detect a clear local moment signal.