In the realm of particles, two is sometimes better than one. Take electron pairs, for example. Two electrons bonded together can glide through a material without friction, giving it superconducting qualities. Cooper pairs, or paired electrons, are a type of hybrid particle made up of two particles that act as one and have qualities that are greater than the sum of their parts.
A new type of hybrid particle has been discovered by MIT physicists in an unusual two-dimensional magnetic substance. They discovered that the hybrid particle is a cross between an electron and a phonon (a quasiparticle formed by vibrating atoms in a substance). Scientists discovered that the glue, or bond, between the electron and the phonon was 10 times stronger than any other electron-phonon hybrid known to date when they tested the force between the two.
The particle’s unusual bond suggests that its electron and phonon are tuned in lockstep; any change to the electron, for example, should influence the phonon, and vice versa. In theory, an electronic stimulation applied to the hybrid particle, such as voltage or light, may stimulate the electron as it typically would while simultaneously affecting the phonon, which impacts the structural or magnetic properties of the material.
Scientists may use voltage or light to alter not only a material’s electrical properties, but also its magnetism, using this dual control. The findings are especially significant because the scientists discovered the hybrid particle in nickel phosphorus trisulfide (NiPS3), a two-dimensional material with magnetic characteristics that has recently piqued curiosity.
Scientists believe that if these qualities can be managed, such as through the newly discovered hybrid particles, the material could one day be used as a new type of magnetic semiconductor, allowing for smaller, faster, and more energy-efficient electronics.
“Imagine if we could stimulate an electron, and have magnetism respond,” says Nuh Gedik, professor of physics at MIT. “Then you could make devices very different from how they work today.”
The findings of Gedik and his colleagues were published today in the journal Nature Communications. Emre Ergeçen, Batyr Ilyas, Dan Mao, Hoi Chun Po, Mehmet Burak Yilmaz, and Senthil Todadri from MIT, as well as Junghyun Kim and Je-Geun Park from Seoul National University in Korea, are among his co-authors.
Imagine if we could stimulate an electron, and have magnetism respond. Then you could make devices very different from how they work today.
Nuh Gedik
Particle sheets
Modern condensed matter physics is primarily concerned with the hunt for nanoscale interactions in matter. Interactions between atoms, electrons, and other subatomic particles in a material can produce unexpected results, such as superconductivity and other exotic phenomena.
Physicists search for these interactions by condensing chemicals onto surfaces to create two-dimensional materials sheets as thin as one atomic layer.
In 2018, a Korean research team observed some surprising interactions in synthesized sheets of NiPS3, a two-dimensional material that becomes antiferromagnetic at temperatures as low as 150 kelvins (-123 degrees Celsius).
An antiferromagnet’s microstructure resembles a honeycomb lattice of atoms with opposing spins to their neighbors. A ferromagnetic material, on the other hand, is made up of atoms with spins aligned in the same direction.
When they investigated NiPS3, they observed that when the material was cooled below its antiferromagnetic transition, an unusual excitation appeared, while the specific nature of the interactions responsible for this was unknown.
Another group discovered evidence of a hybrid particle, but the specific composition of the particle and its relationship to the exotic excitation were unknown.
Gedik and his colleagues wondered if using a super-fast laser to catch the hallmark motions of the two particles that make up the whole, they could identify the hybrid particle and tease out the two particles that make up the whole.
Magnetically visible
Even with the world’s fastest camera, the speed of electrons and other subatomic particles is normally too rapid to image. According to Gedik, the task is similar to photographing a person jogging.
Because the camera’s shutter, which lets light in to record the image, isn’t fast enough, the resulting image is fuzzy, and the person is still racing in the frame before the shutter can take a crisp picture.
The scientists employed an ultrafast laser that creates light pulses that last only 25 femtoseconds to get around this obstacle (one femtosecond is 1 millionth of 1 billionth of a second). They split the laser pulse into two parts and directed them towards a NiPS3 sample.
With a time resolution of 25 femtoseconds, the two pulses were set with a tiny delay from each other, so that the first stimulated, or “kicked,” the sample, while the second caught the sample’s reaction. Scientists were able to produce ultrafast “movies” in this way, from which they could extrapolate the interactions of different particles within the material.
They examined the amount of light reflected from the sample as a function of time between the two pulses in particular.
If hybrid particles are present, this reflection should change in some way. When the sample was cooled below 150 kelvins, the material became antiferromagnetic, which proved to be the case.
“We found this hybrid particle was only visible below a certain temperature, when magnetism is turned on,” says Ergeçen.
The researchers experimented with the color, or frequency, of the first laser and discovered that the hybrid particle was visible when the frequency of the reflected light was around a specific type of transition known to occur when an electron passes between two d-orbitals.
They also discovered that the spacing of the periodic pattern exhibited within the reflected light spectrum corresponded to the energy of a particular type of phonon. This confirmed that the hybrid particle is made up of d-orbital electron excitations and this unique phonon.
Based on their findings, they discovered that the force that binds the electron to the phonon is nearly ten times larger than what has been estimated for previous known electron-phonon hybrids.
“One potential way of harnessing this hybrid particle is, it could allow you to couple to one of the components and indirectly tune the other,” Ilyas says. “That way, you could change the properties of a material, like the magnetic state of the system.”
This research was supported, in part, by the U.S. Department of Energy and the Gordon and Betty Moore Foundation.
