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The Rigakubu News

Disclaimer: machine translated by DeepL which may contain errors.

Matter, Elementary particles, and Technology

Satoru Nakatsuji
(Professor, Department of Physics)

A few Avogadro electrons in matter. The macroscopic quantum phenomena of these electrons have fascinated many people. One of them is the Hall effect. The Hall effect is a phenomenon in which a voltage is generated in a direction perpendicular to the direction of the current when a conductor or semiconductor in which a current flows is placed in a magnetic field. One hundred years later, in 1980, the German physicist Klaus von Klitzing discovered the quantum Hall effect, which is a Hall effect that appears in proportion to magnetization in ferromagnetic materials. The quantum Hall effect is a phenomenon in which the Hall resistance of a two-dimensional electron system is quantized to an integer multiple of h/e2 in a magnetic field. In recent years, a great leap forward has been made in the study of this phenomenon.

The quantum Hall effect has been known as a phenomenon that appears in two low dimensions and in special environments such as cryogenic temperatures and high magnetic fields. On the other hand, its study has brought about a great leap forward in physical concepts. For example, there is the Berry phase, a quantity considered in mathematical geometry and topology, which occurs as a result of the phase shift of a wave function in response to changes in the parameters of a system. It was in the study of the quantum Hall effect that the Berry phase, which is a mathematical object, was shown to appear in the physics of conduction phenomena. In fact, the quantum Hall effect appears when the sum of the Berry curvatures of the electronic structure is an integer multiple of 2π.

The fascination of physics is that the same concept can be used to explain various phenomena across space-time. In 1929, a year after Paul Dirac derived the equation describing the electron, Hermann Weyl proposed a solution for relativistic particles without mass. The Weyl particle, once thought to describe neutrinos, has now been identified as a quasiparticle in matter, nearly 100 years after its proposal, and has been found to be the source of the magnetic charge in momentum space and the Berry curvature it generates.

In fact, the anomalous Hall effect, like the quantum Hall effect, arises from the Berry curvature of momentum space, which governs the electronic structure. If so, the anomalous Hall effect may appear not only in ferromagnetic materials but also in materials without magnetization. If we can realize the Weil particle in a zero field at room temperature, the quantum Hall effect may appear in a three-dimensional material that is familiar to us, in a zero field at room temperature.


Anomalous Hall effect first appeared in antiferromagnetic materials
Giant Hall effect appearing at room temperature (left); schematic diagram of a magnetic structure that flips fast when a spin current is applied (right). Since its discovery by Edwin Hall in 1880, the Hall effect had been thought to appear only in a magnetic field or in ferromagnetic materials. In our laboratory, it was experimentally confirmed for the first time that the Hall effect, which is much larger than that of ferromagnetic materials, appears in antiferromagnetic materials at room temperature and zero magnetic field (left figure). The antiferromagnet Mn3Sn, in which this phenomenon was discovered, is also the first example of a magnetic material in which Weil particles were found as quasiparticles. In recent years, spin structure inversion by electric current (right) and tunnel magnetoresistance effect have been discovered, and as a typical example of antiferromagnetic spintronics research, it is being actively studied all over the world.

A phenomenon that has realized all of these expectations has been discovered in our laboratory and is attracting researchers. It was discovered in a material called antiferromagnet, which has been thought to be of no use because it does not have magnetization. In fact, the presence of Weil particles increases the Berry curvature, and a huge anomalous Hall effect appears even in antiferromagnets. It has also become clear that this can be regarded as a three-dimensional, room-temperature, zero-field version of the quantum Hall effect.

Weil particles in materials are not single particles as in the case of elementary particles, but rather are deeply bound to each other by quantum entanglement, which is thought to create both magnetism and conduction phenomena. The Weil particles are now giving rise to various research trends. One of them is antiferromagnetic spintronics. Although antiferromagnetic materials were once thought to be useless, researchers around the world are now pinning high hopes on them as the next generation of semiconductors that have supported the information society. This is because it has been discovered that the absence of magnetization makes it possible to process information at ultra-high speeds in picoseconds.

Understanding the quantum effects created by the myriad of electrons in matter is a major theme in physics. In the field of physical properties experiments, various phenomena have been discovered one after another all over the world. It is even more interesting to solve the riddle of how nature makes them possible due to the many-body effects that lie beneath the surface. There are still many new phenomena to be discovered that will overturn 100 years of common knowledge. Let's try to solve the riddle!

Published in the September 2024 issue of Faculty of Science News

Mysteries of Science>.


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