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Study Unravels How Spin Perturbations Pass via Quantum Spin Liquid Barrier

The unique mechanism behind the passage of spin perturbations across an apparently impenetrable region of a quantum spin liquid system has been unraveled by researchers at the Tokyo Institute of Technology and Yokohama National University.

The Kitaev model. The vertices of this honeycomb network represent sites with two possible spin states. One intriguing property of this model is that a magnetic pulse applied in the left shaded region causes spin changes in the right shaded regions but not in the middle portion. Until now, the mechanism by which the spin perturbation crossed the middle region was unclear. Image Credit: Tokyo Institute of Technology.

The new discovery could prove to be another building block in next-generation electronics, as well as quantum computers.

It is a well-known fact that electronic devices are almost reaching their theoretical limits. This means that completely new technology will be needed to achieve higher miniaturization or better performance.

The difficulty is that modern electronics focuses on the manipulation of electric currents. Therefore, it mainly relates to the collective charge of moving electrons. However, what if there exists a more efficient method to code and send signals and data?

Spintronics—an emerging technological field thought to transform electronics—could prove to be a key player in the development of quantum computers. In spintronic devices, the spin of electrons forms their most crucial characteristic. It is an inherent property that can be widely observed as their angular momentum and the fundamental cause of magnetic phenomena in solids.

But physicists across the globe have been striving to discover viable means to produce and transport “spin packets” through materials. As part of the new study, researchers at Tokyo Tech and YNU, Japan, performed a theoretical examination of the unique spin transport properties of a specific system known as the “Kitaev model.”

This two-dimensional (2D) model includes a honeycomb network, each vertex of which hosts a spin. The unique aspect of the Kitaev system is that due to the peculiar interactions among spins, it acts as a quantum spin liquid (QSL).

In general, this means that in this system, it is not viable for spins to be ordered in an exclusive optimal way that “keeps every spin happy.” Dubbed spin frustration, this phenomenon makes spins to act in a specifically disordered way.

The Kitaev model is an interesting playground for studying QSLs. However, not much is known about its intriguing spin transport properties.

Akihisa Koga, Professor and Study Lead, Tokyo Institute of Technology

A main feature of the Kitaev model is that it consists of local symmetries. The occurrence of such symmetries indicates that spins are correlated only with their closest neighbors and not with distant spins, which in turn implies that a barrier to spin transport must exist.

But, in reality, small magnetic perturbations occurring on one edge of a Kitaev system do show up as variations in the spins at the opposite edge, although the perturbations do not appear to induce any variations in the magnetization of the central, more symmetrical region of the material.

The research team clarified precisely this intriguing mechanism in their study published in Physical Review Letters.

An impulse magnetic field was applied to one edge of a Kitaev QSL to activate “spin packet” transport, and the real-time dynamics that emerged, as a result, were numerically simulated.

As it turns out, the magnetic perturbation passes through the material’s central region with the help of traveling “Majorana fermions.” These quasiparticles are not real particles but accurate approximations of the system’s collective behavior.

It should be pointed out that it is not feasible to explain spin transport mediated by Majorana fermions using classic spin-wave theory. Thus, this topic requires further experimental studies. However, Koga believes the results of this study could find potential applications.

Our theoretical results should be relevant in real materials as well, and the setup of our study could be implemented physically in certain candidate materials for Kitaev systems.

Akihisa Koga, Professor and Study Lead, Tokyo Institute of Technology

In the published paper, the research team describes potential materials, techniques to produce the spin perturbations, and methods to experimentally discover the evidence for the travel of Majorana fermions through the bulk of the material to reach the other edge.

Eventually, it could be feasible to manipulate the motion of the static (non-traveling) Majorana fermions in the system, which could be of potential use. Over time, physicists could solve many more mysteries of the quantum realm and learn how humans can benefit from them.

Journal Reference:

Minakawa, T., et al. (2020) Majorana-Mediated Spin Transport in Kitaev Quantum Spin Liquids. Physical Review Letters.


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