“The reason we have magnetism in our daily lives is because of the strength of electron exchange interactions,” said study co-authors. Atachi Imamogluis also a physicist at the Institute of Quantum Electronics.
However, as Nagaoka theorized in the 1960s, exchange interactions may not be the only way to make materials magnetic. Nagaoka envisioned a square, two-dimensional lattice with only one electron at each position on the lattice. He then calculated what would happen if one of the electrons was removed under certain conditions. When the remaining electrons in the lattice interacted, the hole containing the missing electron would slide around the lattice.
In Nagaoka’s vision, when the electron spins are all aligned, the overall energy of the lattice will be at its lowest.Every electron configuration looks the same, as if the electrons are identical tiles in the world’s most boring tiles Sliding tile puzzleThese parallel spins, in turn, make the material appear ferromagnetic.
When two twisted meshes form a pattern
Imamoglu and his colleagues had a hunch that they could create Nagaoka magnetism by experimenting with single layers of atomic sheets that could be stacked together to form complex moiré patterns (pronounced mware). In atomically thin layered materials, moire patterns can fundamentally change the way electrons and therefore the material behave.For example, in 2018, physicist Pablo Jarillo-Herrero and colleagues proved A two-layer graphene stack acquires superconducting capabilities when the two layers of graphene cancel each other out through twisting.
Moiré materials have since emerged as a compelling new system for studying magnetism, alongside complex materials such as supercooled atomic clouds and cuprates. “Moiré materials give us a playground to essentially synthesize and study many-body states of electrons,” Imamoglu said.
The researchers first synthesized a material composed of a single layer of semiconductor molybdenum diselenide and tungsten disulfide, which belong to a class of materials past simulation It is suggested that Nagaoka-style magnetism can be exhibited. They then applied weak magnetic fields of varying strengths to the moiré material while tracking how much of the material’s electron spins aligned with the magnetic field.
The researchers then repeated these measurements while applying different voltages to the material, which changed the number of electrons in the Moiré lattice. They discovered something strange. The material aligns more easily with an external magnetic field – that is, exhibits stronger ferromagnetism – only if it has 50% more electrons than lattice sites. When the lattice has fewer electrons than lattice sites, the researchers see no sign of ferromagnetism. This is contrary to the standards they expected to see. – Question Nagaoka Ferromagnetism has always been at work.
However, the material is magnetizing and the exchange interaction does not appear to be driving it. But even the simplest version of Nagaoka’s theory doesn’t fully explain its magnetism.
When your stuff gets magnetized and you’re a little surprised
Ultimately, it comes down to movement. Electrons reduce their kinetic energy by spreading through space, which can cause the wave function describing one electron’s quantum state to overlap with that of its neighbours, binding their fates together. In the team’s material, once there are more electrons than lattice sites in the Moiré lattice, the material’s energy drops when the excess electrons delocalize like fog on a Broadway stage. They then pair briefly with electrons in the crystal lattice, forming two-electron combinations called diclones.
These flowing extra electrons, and the double Browns they continually form, cannot delocalize and diffuse within the lattice unless the electrons in the surrounding lattice sites all have aligned spins. As the material relentlessly pursues its lowest energy state, the end result is that diatomic atoms tend to create small, localized ferromagnetic regions. At a certain threshold, the more diatoms that pass through the lattice, the more detectably ferromagnetic the material becomes.