In condensed matter physics, some of the most unusual behavior occurs only when many quantum particles interact as a group. A single quantum spin behaves in a relatively simple way by itself, but when the spins interact across a material, entirely new effects can emerge. Explaining how these collective interactions arise is a central challenge of modern physics.
One of the most important collective phenomena is the Kondo effect. It describes how localized quantum spins interact with mobile electrons in a material and plays a major role in shaping the behavior of many quantum systems.
Why is studying the Kondo effect so difficult?
In real materials, it is not easy to isolate the underlying physics of the Kondo effect. Electrons don’t just carry spin. They also move through the material and occupy different orbitals, introducing charge motion and additional degrees of freedom. When all these effects appear at once, it is difficult to separate the spin interactions that drive the Kondo effect from everything else going on in the system.
Physicists have long relied on simplified theoretical models to deal with this complexity. One of the most influential is the Kondo necklace model, introduced in 1977 by Sebastian Doniach. This model removes electron motion and orbital effects, leaving behind a system made up entirely of interacting spins. Although widely regarded as a powerful framework for exploring new quantum states, its experimental detection remained an open challenge for nearly fifty years.
It will change the amount of rotation of the quantum behavior
The basic question has persisted for decades. Does the Kondo effect behave the same for all spin sizes, or does changing the localized spin size change the result? The answer to this question is crucial for a broader understanding of quantum materials.
A research team led by Associate Professor Hironori Yamaguchi from the Graduate School of Science at Osaka Metropolitan University has now provided the answer. The team created a new type of Kondo necklace using a carefully designed organic-inorganic hybrid material made from organic radicals and nickel ions. This precise design was achieved using RaX-D, a molecular design framework that enables fine control of crystal structure and magnetic interactions.
From Spin One Half to Spin One
Scientists have previously succeeded in constructing a 1/2 Kondo necklace. In their latest work, they extended the system by increasing the localized spin (decollated spin) from 1/2 to 1. Thermodynamic measurements revealed a clear phase transition that showed the system entered a magnetically ordered state.
A detailed quantum analysis explained the origin of this change. Kondo coupling creates an effective magnetic interaction between spin-1 moments that stabilizes the long-range magnetic order across the material.
Challenging the long-held view of magnetism
For many years it was thought that the Kondo effect mainly suppresses magnetism by locking spins into singlets, a maximally entangled state with zero total spin. The new results overturn this traditional picture. When the localized spin exceeds 1/2, the same Kondo interaction no longer weakens the magnetism. Instead, it actively supports the magnetic order.
By directly comparing the spin-1/2 and spin-1 systems within a pure spin-only platform, the researchers identified a clear quantum boundary. The Kondo effect always creates local singlets for spin-1/2 moments, but stabilizes the magnetic order for spin-1 and higher.
This work provides the first direct experimental evidence that the role of the Kondo effect depends fundamentally on the magnitude of the rotation.
Implications for quantum materials and technology
“The discovery of the spin size-dependent quantum principle in the Kondo effect opens up a whole new area of quantum materials research,” Yamaguchi said. “The ability to switch quantum states between non-magnetic and magnetic regimes by controlling the amount of spin represents a powerful design strategy for next-generation quantum materials.”
Showing that the Kondo effect can work in the opposite way depending on the amount of spin offers a new insight into quantum matter and creates a new conceptual basis for designing spin-based quantum devices.
The ability to control whether the Kondo lattice becomes magnetic or non-magnetic is particularly important for future quantum technologies. Such control could affect key properties such as entanglement, magnetic noise, and quantum critical behavior. The researchers hope that their findings will lead to the development of new quantum materials and ultimately contribute to emerging technologies, including quantum information devices and quantum computers.
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