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. Credit: Akihisa Koga.
Scientists
at Tokyo Institute of Technology (Tokyo Tech) and Yokohama National University
(YNU) uncover the peculiar mechanism by which spin perturbations travel through
a seemingly unpassable region of a quantum spin liquid system. This new insight
may represent another building block in next-generation electronics and even
quantum computers.
Electronic
devices as we know them are close to reaching their theoretical limits, meaning
that radically new technology will be required to obtain better performance or
higher miniaturization. The problem is that modern electronics is centered
around manipulating electric currents and is therefore mainly concerned about
the collective charge of moving electrons. But what if signals and data could
be coded and sent in a more efficient way?
Enter spintronics, an emerging technological field envisioned to revolutionize electronics and, hopefully, become a key player in the development of quantum computers. In spintronic devices, the most important characteristic of electrons is their spin, an intrinsic property that can be broadly seen as their angular momentum and that is the underlying cause of magnetic phenomena in solids. However, physicists worldwide are struggling to find practical ways to generate and transport ‘spin packets’ through materials. In a recent study, scientists at Tokyo Tech and YNU, Japan, conducted a theoretical analysis of the peculiar spin transport characteristics of a particular system called the ‘Kitaev model.’
A magnetic impulse at the left end causes spin excitations there owing to the time variability of their spins. This converts to the motion of the majorana particles, that are then transmitted through the material to its opposite edge. Credit: Akihisa Koga.
This
two-dimensional model comprises a honeycomb network where each vertex hosts a
spin. What’s special about the Kitaev system is that, because of the peculiar
interactions between spins, it behaves as a quantum spin liquid (QSL). This
broadly means that it is impossible in this system for spins to be arranged in
a unique optimal way that ‘keeps every spin happy.’ This phenomenon, called
spin frustration, causes spins to behave in a particularly disordered fashion.
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.â€
An
important characteristic about the Kitaev model is that it has local
symmetries; such symmetries mean that spins are correlated only with their
nearest neighbors and not with far-away spins, thus implying that there should
be a barrier to spin transport. However, in reality, small magnetic
perturbations on one edge of a Kitaev system do manifest as changes in the
spins at the opposite edge, even though the perturbations do not seem to cause
any changes in the magnetization of the central, more symmetrical region of the
material. This intriguing mechanism is what the team of scientists clarified in
their study, which is published in Physical Review Letters.
They
applied an impulse magnetic field on one edge of a Kitaev QSL to trigger ‘spin
packet’ transport and numerically simulated the real-time dynamics that
consequently unfolded. It turns out that the magnetic perturbation is carried
across the central region of the material by traveling ‘Majorana fermions.’
These are quasiparticles; they are not real particles but precise
approximations of the collective behavior of the system.
Notably,
Majorana-mediated spin transport cannot be explained by classic spin-wave
theory and therefore warrants further experimental studies. But Koga is hopeful
of the application potential of the results of this study. He says: “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.â€
In their
article, the scientists discuss possible materials, ways of creating the spin
perturbations, and ways to experimentally find evidence of the Majorana
fermions traveling through the bulk of the material to reach the other edge. It
may even be possible to control the motion of the static (non-traveling)
Majorana fermions in the system, which could be of practical use. Only time
will tell how many more mysteries of the quantum world physicists will solve
and how we will benefit from them.