Quantum effects in Kondo lattices can determine whether a system behaves magnetically or non-magnetically, opening new avenues for designing future quantum materials and technologies.
The Kondo effect – the interaction between localised spins and conduction electrons –plays a central role in many quantum phenomena.
However, in real materials, the presence of additional charges and orbital degrees of freedom makes it difficult to isolate the essential quantum mechanism behind the Kondo effect. In these materials, electrons don’t just have spin; they also move around and can occupy different orbitals.
When all these extra behaviours mix together, it becomes hard to focus only on the spin interactions responsible for the Kondo effect.
A research team led by Associate Professor Hironori Yamaguchi of the Graduate School of Science at Osaka Metropolitan University have sought to overcome this barrier.
The Kondo necklace model and its potential for exploring new states
The Kondo necklace model, proposed in 1977 by Sebastian Doniach, simplifies the Kondo lattice by focusing exclusively on spin degrees of freedom.
This model has long been regarded as a promising conceptual platform for exploring new quantum states; however, its experimental realisation has remained an open challenge for nearly half a century.
One of the key questions is whether the Kondo effect and the resulting behaviour fundamentally depend on the size of the localised spin.
Understanding this property would be universally important in quantum material research.
The Kondo effect changes based on spins
The researchers successfully realised a new type of Kondo necklace using a precisely designed organic-inorganic hybrid material composed of organic radicals and nickel ions.
This achievement was made possible by RaX-D, an advanced molecular design framework that enables precise control over the molecular arrangement within the crystal and the resulting magnetic interactions.
Building on their earlier realisation of a spin-1/2 Kondo necklace, the researchers demonstrated that the behaviour of the Kondo effect changes qualitatively when the localised spin (decollated spin) is increased from 1/2 to 1. Thermodynamic measurements revealed a clear phase transition to a magnetic ordered state.
Through quantum analysis, the team clarified that the Kondo coupling mediates an effective magnetic interaction between spin-1 moments, therefore stabilising long-range magnetic order.
Overturning traditional theories
This result overturns the traditional view that the Kondo effect primarily suppresses magnetism by binding free spins into singlets, a maximally entangled state with zero total spin.
Instead, the study shows that when the localised spin is larger than 1/2, the same Kondo interaction works in the opposite direction, promoting magnetic order.
By comparing the spin-1/2 and spin-1 realisations side-by-side in a clean spin-only platform, the researchers identified a new quantum boundary: the Kondo effect inevitably forms local singlets for spin-1/2 moments but stabilises magnetic order for spin-1 and higher.
This discovery provides the first direct experimental evidence that the Kondo effect’s function fundamentally depends on spin size.
Innovating new quantum materials
Discovering that the Kondo effect operates in fundamentally different ways depending on spin size offers a fresh perspective on our understanding of quantum matter and establishes a new conceptual basis for the engineering of spin-based quantum devices.
Professor Yamaguchi explained: “The discovery of a quantum principle dependent on spin size in the Kondo effect opens up a whole new area of research in quantum materials.
“The ability to switch quantum states between nonmagnetic and magnetic regimes by controlling the spin size represents a powerful design strategy for next-generation quantum materials.”
Next steps
Controlling whether a Kondo lattice becomes magnetic or non-magnetic is highly relevant for future quantum technologies because it enables manipulation of key behaviours such as entanglement, magnetic noise, and quantum criticality.
The researchers are hopeful that their findings will help to innovate new quantum materials and may ultimately contribute to the development of emerging quantum technologies, including quantum information devices and quantum computing.
