New quantum boundary discovered: Spin size determines how the Kondo effect behaves

(Press-News.org) Collective behavior is an unusual phenomenon in condensed-matter physics. When quantum spins interact together as a system, they produce unique effects not seen in individual particles. Understanding how quantum spins interact to produce this behavior is central to modern condensed-matter physics.

Among these phenomena, the Kondo effect—the interaction between localized spins and conduction electrons—plays a central role in many quantum phenomena.

Yet in real materials, the presence of additional charges and orbital degrees of freedom make 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 behaviors mix together, it becomes hard to focus only on the spin interactions responsible for the Kondo effect.

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 realization had been an open challenge for nearly half a century.

One of the key questions is whether the Kondo effect and the resultant behavior fundamentally change depending on the size of the localized spin. Understanding this property would be universally important in quantum material research.

A research team led by Associate Professor Hironori Yamaguchi of the Graduate School of Science at Osaka Metropolitan University successfully realized 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 realization of a spin-1/2 Kondo necklace, the researchers demonstrated that the behavior of the Kondo effect changes qualitatively when the localized 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, thereby stabilizing long-range magnetic order.

This result overturns the traditional view that the Kondo effect primarily suppresses magnetism by binding free spins into singlets, a maximally entangled state whose total spin is zero. Instead, the study shows that when the localized 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 realizations 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 stabilizes magnetic order for spin-1 and higher.

This discovery provides the first direct experimental evidence that the function of the Kondo effect fundamentally depends on spin size.

“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,” Yamaguchi said. “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”

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.

Controlling whether a Kondo lattice becomes magnetic or non-magnetic is highly relevant for future quantum technologies because it offers a way to control important behaviors like entanglement, magnetic noise, and quantum critical behaviors. 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.

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