Discovery could provide a path to smaller, faster electronic devices – ScienceDaily

In the world of particles, sometimes two are better than one. Take, for example, pairs of electrons. When two electrons are bonded together, they can slide through material without friction, giving the material special superconducting properties. These electron pairs, or Cooper pairs, are a kind of hybrid particle – a composite of two particles that behave as one, with properties greater than the sum of its parts.

Today, physicists at MIT have detected another type of hybrid particle in an unusual two-dimensional magnetic material. They determined that the hybrid particle is a mashup of an electron and a phonon (a quasi-particle that is produced from the vibrating atoms of a material). When they measured the force between the electron and the phonon, they found that the glue, or bond, was 10 times stronger than any other electron-phonon hybrid known to date.

The particle’s exceptional bond suggests that its electron and phonon could be tuned in tandem; for example, any change in the electron should affect the phonon, and vice versa. In principle, an electronic excitation, such as voltage or light, applied to the hybrid particle could stimulate the electron as it normally would, and also affect the phonon, which influences the structural or magnetic properties of a material. Such dual control could allow scientists to apply voltage or light to a material to tune not only its electrical properties, but also its magnetism.

The results are particularly relevant, as the team identified the hybrid particle in nickel phosphorus trisulfide (NiPS3), a two-dimensional material that has sparked recent interest in its magnetic properties. If these properties could be manipulated, for example through the newly detected hybrid particles, scientists believe that the material could one day be useful as a new type of magnetic semiconductor, which could be turned into smaller, more electronic electronics. faster and more energy efficient.

“Imagine if we could stimulate an electron and make magnetism react,” says Nuh Gedik, professor of physics at MIT. “Then you could create devices that are very different from how they work today. “

Gedik and his colleagues today published their results in the journal Nature Communication. His co-authors include Emre Ergeçen, Batyr Ilyas, Dan Mao, Hoi Chun Po, Mehmet Burak Yilmaz and Senthil Todadri at MIT, as well as Junghyun Kim and Je-Geun Park at Seoul National University in Korea.

Particle sheets

The field of modern condensed matter physics focuses, in part, on the search for interactions in matter at the nanoscale. Such interactions, between atoms of a material, electrons and other subatomic particles, can lead to surprising results, such as superconductivity and other exotic phenomena. Physicists research these interactions by condensing chemicals on surfaces to synthesize sheets of two-dimensional materials, which could be as thin as an atomic layer.

In 2018, a research group in Korea discovered unexpected interactions in synthesized sheets of NiPS3, a two-dimensional material which becomes an antiferromagnetic at very low temperatures of the order of 150 Kelvin, or -123 degrees Celsius. The microstructure of an antiferromagnetic resembles a honeycomb network of atoms whose spins are opposite to that of their neighbor. In contrast, a ferromagnetic material is made up of atoms whose spins are aligned in the same direction.

By probing NiPS3, this group found that exotic excitation becomes visible when the material is cooled below its antiferromagnetic transition, although the exact nature of the interactions responsible for this is unclear. Another group found signs of a hybrid particle, but its exact constituents and relationship to this exotic arousal were also unclear.

Gedik and his colleagues wondered if they could detect the hybrid particle and unravel the two particles that make up the set, capturing their signature movements with an ultra-fast laser.

Magnetically visible

Normally, the movement of electrons and other subatomic particles is too fast to image, even with the world’s fastest camera. The challenge, Gedik says, is similar to taking a picture of a person running. The resulting image is blurry because the camera’s shutter, which lets in light to capture the image, is not fast enough and the person is still running in the frame before the shutter can take a shot. sharp photo.

To work around this problem, the team used an ultra-fast laser that emits pulses of light lasting only 25 femtoseconds (one femtosecond is 1 millionth of 1 billionth of a second). They split the laser pulse into two separate pulses and aimed them at a sample of NiPS3. The two pulses were set with a slight delay from each other so that the first one stimulates or ‘kicks’ the sample, while the second picks up the sample’s response. with a temporal resolution of 25 femtoseconds. In this way, they were able to create ultra-fast “films” from which the interactions of the different particles within the material could be deduced.

In particular, they measured the precise amount of light reflected from the sample as a function of the time between the two pulses. This reflection should change in some way if hybrid particles are present. This has been found to be the case when the sample has been cooled below 150 Kelvin, when the material becomes antiferromagnetic.

“We discovered that this hybrid particle was only visible below a certain temperature, when magnetism is activated,” Ergeçen explains.

To identify the specific constituents of the particle, the team varied the color, or frequency, of the first laser and found that the hybrid particle was visible when the frequency of the reflected light was around a particular type of transition. known to occur when an electron moves between two orbitals d. They also looked at the spacing of the visible periodic pattern in the spectrum of reflected light and found that it corresponded to the energy of a specific type of phonon. This clarified that the hybrid particle is made up of d-orbital electron excitations and this specific phonon.

They performed further modeling based on their measurements and found that the force binding the electron to the phonon is about 10 times stronger than what has been estimated for other known electron-phonon hybrids.

“One potential way to harness this hybrid particle is that it could allow you to pair with one of the components and indirectly tune the other,” Ilyas said. “That way you could change the properties of a material, like the magnetic state of the system.”

This research was funded, in part, by the US Department of Energy and the Gordon and Betty Moore Foundation.

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