When does magnetism turn into gravity?
Weyl fermions in the spotlight
Jenny Witt Press and public relations
Max Planck Institute for the Structure and Dynamics of Matter
Researchers from the theory department of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg and the North Carolina State University in the U.S. have shown that the long sought-after semi-metallic, magnetic Weyl state can be generated with ultra-fast laser pulses in magnetic materials, the so-called pyrochlore iridates. Her results, published by Nature Communications, could pave the way for the development of high-speed magneto-optic topological switches for future electronic applications.
All previously known elementary particles fall into two categories: bosons and fermions. Bosons convey forces such as magnetism or gravity, while matter is made up of fermions, such as electrons. On a theoretical basis, it is predicted that three types of fermions can exist, named after the physicists Dirac, Weyl and Majorana.
In free space, electrons are Dirac fermions, but in solids they can change their properties. In the atomically thin carbon material graphene, they become massless Dirac fermions. In other recently discovered and produced materials, they can also become Weyl and Majorana fermions, which is why these materials are of interest for future technologies such as topological quantum computers and other innovative electronic devices.
In combination with a wave of bosons, namely the photons in a laser, fermions can be transformed from one type to another, as predicted by MPSD theorists back in 2016 (see Ref. 1 below).
A new study by doctoral student Gabriel Topp from Michael Sentef's Emmy Noether group has now shown that electron spins can be specifically manipulated with short light pulses, whereby magnetic Weyl fermions are generated in an insulator. Based on a previous study by Nicolas Tancogne-Déjean and the director of the theory department, Angel Rubio (see Ref. 2) below), the researchers used the idea of laser-altered electron-electron repulsion to exploit the magnetism in pyrochlore iridates suppress, in which electron spins are positioned on a tetrahedral lattice.
On this grid, all electron spins point like small compass needles inwards, towards the center of the tetrahedron, and on the neighboring tetrahedron they all point outwards. This all-in, all-out combination and the length of the compass needles cause the material to be insulating when it is not stimulated by light.
Using modern computer simulations in large computing clusters, however, the scientists found that the needles start to rotate when a flash of light hits the fabric and thus appear like shorter needles with a lower magnetic order. This reduction in magnetism makes the material semi-metallic and the Weyl fermions conduct the electrical current.
“Our research into how light can be used to manipulate materials on ultra-short time scales has made good progress,” says Michael Sentef. Gabriel Topp adds: “We were surprised by the fact that even a laser pulse that is too strong, which should completely suppress the magnetism, can lead to a magnetic Weyl state. This is because the matter has no opportunity to balance itself in thermal equilibrium in these short periods of time. When everything is shaken back and forth, it takes a little while until the additional energy of the laser flash is evenly distributed to all particles of the material. "
The scientists are confident that their work will stimulate further theoretical and experimental study. “We are only just beginning to understand that light and matter can be combined in many impressive ways, creating fantastic effects that we may not even be able to dream of today,” says Angel Rubio. “With our dedicated, highly motivated and talented young scientists at the MPSD, we are working hard to explore these almost unlimited possibilities so that society can benefit from our discoveries.
1) "Creating stable Floquet-Weyl semimetals by laser-driving of 3D Dirac materials" - https://www.nature.com/articles/ncomms13940
2) "Ultrafast modification of Hubbard U in a strongly correlated material: ab initio high-harmonic generation in NiO" - https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.097402
Dr. Michael Sentef: +49 (0) 40 8998-88350
http: // MPSD research report: http://www.mpsd.mpg.de/506762/2018-10-weyl-sentef
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