New Research Figures Out Peculiar Spin Transport in Quantum Spin Liquids

Researchers at Tokyo Institute of Technology (Tokyo Tech) and Yokohama National University (YNU) have revealed the odd mechanism by which spin perturbations travel via an apparently unpassable area of a quantum spin liquid system.

The new discovery may be another step in the development of next-generation electronics and even quantum computers. Electronic devices, as we know them, are close to attaining their theoretical limits, which means that radically new technology will be needed to achieve better performance or higher miniaturization.

Spintronics to Advance the Development of Electronics

The issue is that modern electronics is focused around manipulating electric currents; hence, it is mainly centered on the collective charge of moving electrons. However, what if signals and data could be coded and sent in a more effective way?

That’s where spintronics comes into place – an emerging technology field believed to be able to revolutionize electronics and, probably, become a key player in the development of quantum computers.

In spintronic devices, the most significant feature of electrons is their spin, an intrinsic property that can be widely seen as their angular momentum, and that is the basic cause of magnetic phenomena in solids. Still, physicists all over the world are struggling to find practical methods to produce and transport ‘spin packets’ via materials.

In a recent study, scientists at Tokyo Tech and YNU, Japan, carried out a theoretical analysis of the weird spin transport properties of a particular system known as the ‘Kitaev model.’

The Kitaev model. [Image: Tokyo Tech]
This 2D model showcases a honeycomb network where each vortex has a spin. What is special about the Kitaev mechanism is that, because of the odd engagements between spins, it behaves as a quantum spin liquid (QSL). This mainly means that it is impossible in this system for spins to be places in a unique optimal way that ‘keeps every spin happy.’ This phenomenon, known as spin frustration, makes spins behave in a particularly confused way.

Professor Akihisa Koga, who led the study, says: “The Kitaev model is an interesting playground for studying QSLs. However, not much is known about its intriguing spin transport properties.”​

Majorana Fermions​

An important feature of the Kitaev model is that it has local symmetries. Such symmetries mean that spins are linked only with their nearest neighbors and not with distant spins, therefore, suggesting that there should be a barrier to spin transport.

Still, in reality, small magnetic disturbances on the margin of a Kitaev system do show up as changes in the spins at the opposite edge, although the perturbations do not appear to cause any changes in the magnetization of the central, more symmetrical area of the material.

This mechanism is what the team of researchers detailed in their study, which is published in Physical Review Letters and titled “Majorana-mediated spin transport in Kitaev quantum spin liquids.”

A schematic representation of Majorana-mediated spin transport. [Image: Tokyo Tech]
The image above details the result the team saw when it applied an impulse magnetic field on one margin of a Kitaev QSL to trigger ‘spin packet’ transport and numerically simulated the real-time mechanisms that continually unfolded.

They figured that the magnetic disturbance is moved through the central area of the material by traveling ‘Majorana fermions,’ as depicted in the image above. These are quasiparticles, which are not real particles but accurate approximations of the collective behavior of the system.

More so, Majorana-mediated spin transport cannot be described by classic spin-wave theory and, therefore, deserves further experimental analyses. But Koga is hopeful of the implementation potential of the results of this study.

He said: “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.”​

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