In the particle world, sometimes two is better than one. Take, for example, electron pairs. When two electrons bond together, they can slide through a material without friction, giving the material superconducting properties. These double electrons, or Cooper pairs, are a type of hybrid particle – a compound of two particles that behave as a single particle, with properties greater than the sum of its parts.
Now physicists at the Massachusetts Institute of Technology have discovered another type of hybrid particle in an unusual two-dimensional magnetic material. They determined that a hybrid particle is a mixture of an electron and a phonon (a quasi-particle produced from atoms of a vibrating material). When they measured the force between the electron and the phonon, they found that the gum – or bond – was 10 times stronger than any other electron-phonon hybrid known to date.
The particle’s exceptional bonding indicates that the particle’s electron and phonon can be tuned side by side; For example, any change in the electron should affect the phonon, and vice versa. In principle, electronic excitation, such as a voltage or light, applied to a hybrid particle can excite the electron as it normally would, and also affects the phonon, affecting the structural or magnetic properties of the material. Such dual control could enable scientists to apply voltage or light to a material to tune not only its electrical properties but also its magnetism.
Particularly relevant were the results, as the team identified a nickel-phosphorous trisulfide (NiPS) hybrid particle.3), a two-dimensional material that has recently attracted attention for its magnetic properties. If these properties can be manipulated, for example through newly discovered hybrid particles, scientists believe the material could one day be useful as a new type of magnetic semiconductor, which can be made into smaller, faster and more energy-efficient electronics.
“Imagine if we could excite an electron, and the magnetism response,” says Noh Gedik, professor of physics at MIT. “Then you can make the devices completely different from how they work today.”
Jedek and his colleagues published their findings today in the journal Nature Communications. Co-authors include Emre Ergesen, Patir Elias, Dan Mao, Hui Chun-bo, Mehmet Burak Yilmaz and Senthil Todadri from MIT, along with Junghyun Kim and Je-Geun Park from Seoul National University in Korea.
The field of modern condensed matter physics is focused, in part, on researching interactions in matter at the nanoscale. Such interactions between atoms of matter, electrons and other subatomic particles can lead to surprising results, such as superconductivity and other strange phenomena. Physicists look for these interactions by condensing chemicals on surfaces to form sheets of two-dimensional materials, which can be as thin as a single atomic layer.
In 2018, a research group in Korea discovered some unexpected interactions in NiPS composite panels3, a two-dimensional material that becomes an antiferromagnet at very low temperatures of about 150 K, or -123 degrees Celsius. The microstructure of an antimagnet resembles a honeycomb network of atoms spinning anti-spin their jars. In contrast, a ferromagnetic material consists of atoms that rotate aligned in the same direction.
In NiPS فحص assay3, that group discovered that the strange excitation became visible as the material cooled down its antimagnetic transition, although the exact nature of the interactions responsible was not clear. Another group found signs of a hybrid particle, but its exact components and relationship to this strange excitation were not clear either.
Gidick and his colleagues wondered if they could detect the hybrid particle, and elicit the two particles that make up the whole, by capturing their signature motions with an ultrafast laser.
The movement of electrons and other subatomic particles is usually very fast to photograph, even with the world’s fastest camera. The challenge is like taking a picture of someone running, says Gedek. The resulting image is blurry because the shutter, which allows light to capture the image, is not fast enough, and the person is still working in the frame before the shutter can take a clear picture.
To get around this problem, the team used an ultrafast laser that emits pulses of light lasting just 25 femtoseconds (one femtosecond is a millionth of a billionth of a second). They split the laser pulse into two separate pulses and direct them to a NiPS . sample3. The two pulses are set with a slight delay apart so that the first stimulates, or “kicks” the sample, and the second captures the sample’s response, with a time resolution of 25 femtoseconds. In this way, they were able to create ultrafast “movies” from which the interactions of various particles within matter could be inferred.
In particular, they measured the exact amount of light reflected from the sample as a function of the time between the two pulses. This reflection must change in a certain way in the case of hybrid molecules. This turned out to be the case when the sample was cooled below 150 degrees Kelvin, when the material becomes antimagnetic.
“We found that this hybrid particle was only visible under a certain temperature, when the magnetism was turned on,” says Ergeçen.
To determine the specific components of the particle, the team changed 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 specific type of transition known to occur as an electron moving between two d orbitals. They also looked at the spacing of the visible periodic pattern within the reflected light spectrum and found that it matched the energy of a particular type of phonon. This shows that the hybrid particle is formed by the excitation of d orbital electrons and this specific phonon.
They did some additional 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 electron-phonon hybrids.
“One potential way to harness this hybrid particle is that it can allow you to pair one component and indirectly tune the other,” Elias says. “This way, you can change the properties of a material, such as the magnetic state of the system.”
New phonon-based monochromatic magnetic tunable terahertz source
Emre Ergeçen et al, states associated with the magnetically bright dark electron phonon in van der Waals antimagnets, Nature Communications (2022). DOI: 10.1038 / s41467-021-27741-3
Provided by the Massachusetts Institute of Technology
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