“The very reason magnetism exists in our daily life is due to the strength of electron exchange interactions,” study co-authors said. Atachi Imamoralso a physicist at the Institute for Quantum Electronics.
But exchange interactions may not be the only way to make materials magnetic, as Nagaoka theorized in the 1960s. Nagaoka envisioned his two-dimensional lattice of squares in which there was only one electron at every site on the lattice. He then calculated what would happen if he removed one of these electrons under certain conditions. As the remaining electrons in the lattice interact, the holes where the missing electrons were will fly around the lattice.
In Nagaoka’s scenario, the energy of the entire lattice is at its lowest when all electron spins are aligned. All electron configurations look the same. It’s as if the electrons were identical tiles in the world’s most boring space. sliding tile puzzle. These parallel spins make the material ferromagnetic.
If two grids are twisted and a pattern exists
Imamol and his colleagues had a hunch that they could create Nagaoka magnetism by experimenting with single-layer sheets of atoms that could form complex moiré patterns (pronounced moiré patterns). Ta. Muwa line). In atomically thin layered materials, moiré patterns can fundamentally change the behavior of electrons and thus the material. For example, in 2018 physicist Pablo Jarillo-Herrero and his colleagues proven A two-layer stack of graphene gained superconducting ability when the two layers were twisted and offset.
Moiré materials have since emerged as a fascinating new system for studying magnetism, inserted alongside supercooled atoms and clouds of complex materials like cuprates. “Moiré materials essentially provide a playground for synthesizing and studying many-body states of electrons,” Imamour said.
The researchers began by synthesizing the material from a single layer of the semiconductors molybdenum diselenide and tungsten disulfide. past simulation It was implied that it might exhibit Nagaoka-style magnetism. They then applied weak magnetic fields of varying strengths to the Moiré material and tracked the number of electron spins in the material that matched the field.
The researchers then repeated these measurements while applying different voltages to the material, varying the number of electrons present within the Moiré lattice. They found something strange. The material tended to align better with external magnetic fields, meaning it behaved more ferromagnetically, only when it had up to 50% more electrons than lattice sites. And when the lattice had fewer electrons than the lattice sites, the researchers observed no signs of ferromagnetism. This was the opposite of what would be expected if standard Nagaoka ferromagnetism were at work.
However, although the material was magnetized, exchange interactions did not appear to be driving it. However, even the simplest version of Nagaoka’s theory could not fully explain its magnetic properties.
When something becomes magnetized and you’re a little surprised
In the end, it came down to movement. Because electrons reduce their kinetic energy as they spread out in space, the wavefunction representing the quantum state of one electron can overlap with the quantum state of its neighbor, potentially linking the fates of the electrons. In the team’s material, when there were more electrons in the Moiré lattice than there were lattice sites, the extra electrons delocalized, like fog on a Broadway stage, reducing the material’s energy. They then momentarily paired with electrons in the lattice, forming two-electron combinations called doubloons.
These circulating extra electrons, and the doubloons they continue to form, cannot delocalize and spread into the lattice unless the electrons at surrounding lattice sites all have aligned spins. Eventually, the doubloons tended to generate small localized ferromagnetic regions as the material relentlessly pursued its lowest energy state. Up to a certain threshold, the more doubloons that pass through the lattice, the more detectable the ferromagnetism of the material becomes.
