Quantum spin size determines whether the Kondo effect suppresses or preserves magnetism
The size of the spin crucially affects how the system behaves. At spin-1/2, fully quantum spins pair up and cancel each other, so no magnetism appears. At spin > 1/2, larger spins can’t fully cancel, leaving leftover spins that can interact and create magnetic order.
Image Credit: Osaka Metropolitan University
Scientific Frontline: "At a Glance" Summary
- Main Discovery: The Kondo effect fundamentally changes function based on spin size; while it suppresses magnetism in spin-1/2 systems by forming singlets, it conversely promotes and stabilizes long-range magnetic order in systems with spin greater than 1/2.
- Methodology: Researchers synthesized a precise organic-inorganic hybrid "Kondo necklace" material containing organic radicals and nickel ions using the RaX-D molecular design framework, then utilized thermodynamic measurements and quantum analysis to compare spin-1/2 and spin-1 behaviors.
- Key Data: Increasing the localized spin from 1/2 to 1 triggered a clear phase transition to a magnetically ordered state, challenging the established view where Kondo interactions typically bind free spins into non-magnetic singlets.
- Significance: This finding overturns the traditional understanding that the Kondo effect primarily suppresses magnetism, establishing a new quantum boundary where spin magnitude acts as a determinative switch between non-magnetic and magnetic regimes.
- Future Application: Development of next-generation quantum materials with tunable magnetic properties, specifically for managing entanglement and magnetic noise in quantum computing and information devices.
- Branch of Science: Condensed-Matter Physics / Quantum Materials Science
- Additional Detail: The study provides a rare experimental realization of the "Kondo necklace model," a theoretical platform proposed by Sebastian Doniach in 1977 to isolate spin degrees of freedom.
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 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 behaviors mix, 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.
Funding: This research was partly supported by JST PRESTO Grant No. JPMJPR2599. A part of this work was performed under the interuniversity cooperative research program of the joint-research program of ISSP, the University of Tokyo.
Published in journal: Communications Materials
Title: Emergence of Kondo-assisted Néel order in a Kondo necklace model
Authors: Hironori Yamaguchi, Shunsuke C. Furuya, Yu Tominaga, Takanori Kida, Koji Araki, and Masayuki Hagiwara
Source/Credit: Osaka Metropolitan University
Reference Number: qs012026_02