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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.
To overcome that, the team added nonmagnetic Sn impurities on purpose. The idea is that a nonmagnetic impurity locally breaks the perfect frustration and lets nearby ionic moments slow down and show more classical, detectable behavior. In experiments, the authors found that as they increased the Sn level the magnetic susceptibility went up in a systematic way. They also used μSR (muon spin rotation or relaxation) measurements and report evidence consistent with the emergence of well-defined local magnetic moments near the impurities. This experimental trend supports the theoretical picture of 2+ valence ions with fluctuating local moments.
The authors argue this shifts the picture away from models that treat only itinerant electrons and no local spins. Instead, they suggest a combined view in which strong local ionic spins and their interaction with mobile electrons help drive the rich phenomena seen in these materials, such as charge density waves and broken time-reversal symmetry. Important caveats remain: the Hartree-scale results concern high-energy structure and do not by themselves determine low-energy properties like superconducting pairing. Also, the direct experimental detection relied on impurity-doped samples, so the behavior seen around defects may not fully represent the pure compounds. Finally, geometric frustration and fast fluctuations make quantitative measurement and interpretation challenging, so further work will be needed to map how these local moments influence the low-energy physics.