Scientists at ETH Zurich have discovered a new ferromagnetism in a custom-designed moiré material, challenging conventional magnetic theory. This magnetism is based on the alignment of electron spins to minimize kinetic energy and provides new insights into quantum effects and solid-state magnetism.
For a magnet to stick to a refrigerator door, several physical effects must come together perfectly. The magnetic moments of the electrons all point in the same direction, and this phenomenon occurs even in the absence of an external magnetic field.
This is due to the complex interplay of exchange interactions, electrostatic repulsion between electrons and quantum mechanical properties of the electron spin, which generate magnetic moments. This mechanism explains why materials such as iron and nickel are ferromagnetic, that is, permanently magnetic unless heated above a certain temperature.
At ETH Zurich, a research team led by Atati Imamoğlu from the Institute for Quantum Electronics and Eugene Demler from the Institute for Theoretical Physics has discovered a new type of strong magnetic moment alignment in artificially produced materials. Magnetism was detected. In a completely different sense.They recently published their results in a scientific journal Nature.
Electro-filled artificial materials
In Imamol’s lab, doctoral student Livio Ciorcharo, postdoctoral fellow Tomasz Smolenski and others are stacking atomically thin layers of two different semiconductor materials: molybdenum diselenide and tungsten disulfide. Special materials were also manufactured.
At the interface, the difference in the lattice constants of the two materials, i.e. the distance between the atoms, creates a two-dimensional periodic potential with a large lattice constant (30 times larger than that of the two materials). semiconductor), electrons can be filled by applying voltage.
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In the moiré material produced by ETH, the electron spins are perturbed when there is exactly one electron per lattice site (left). As soon as there are more electrons than lattice sites (right) and pairs of electrons can form doubloons (red), the spins align ferromagnetically because the kinetic energy is minimized .Credit: ETH Zurich
“Such Moiré materials have attracted great interest in recent years because they can be used to very well study the quantum effects of strongly interacting electrons,” Imamour says. “But so far, little is known about their magnetic properties.”
To investigate these magnetic properties, Imamol and his colleagues measured whether the magnetic moment of a given electron filling the Moiré material is randomly oriented, paramagnetic, or ferromagnetic. . They illuminated the material with laser light and measured how strongly the light was reflected for different polarizations.
Polarization indicates in which direction the electromagnetic field of laser light oscillates, and depending on the orientation of the magnetic moment, or the rotation of the electrons, a material will reflect one polarization more strongly than the other. From this difference, we can calculate whether the spins are pointing in the same direction or in different directions, and from there we can determine the magnetization.
shocking evidence
By steadily increasing the voltage, physicists filled the material with electrons and measured the corresponding magnetization. This material remained paramagnetic until it was filled with just one electron per site in the Moire lattice (also known as a Mott insulator). As the researchers continued to add electrons to the lattice, something unexpected happened. In other words, the material suddenly behaves like a ferromagnetic material.
“This was remarkable evidence of a new type of magnetism that cannot be explained by exchange interactions,” Imamoğlu said. In fact, if exchange interactions are the cause of magnetism, they should also appear in the reduction of electrons in the lattice. Therefore, the sudden onset indicates a different effect.
kinetic magnetism
Eugene Demler, working with postdoctoral researcher Ivan Morera, finally came up with an important idea. They believe they may be studying a mechanism that Japanese physicist Yosuke Nagaoka had already predicted theoretically in 1966. In the same direction, the electron minimizes its kinetic energy (kinetic energy), which is much greater than the exchange energy.I
In experiments conducted by researchers at ETH, this happens as soon as there is more than one electron per lattice site in the moiré material. As a result, pairs of electrons can come together to form so-called doubloons. If the doubloons can spread across the lattice through quantum mechanical tunneling, the kinetic energy is minimized.
However, this is only possible if the single electrons in the lattice are ferromagnetically spin-aligned, otherwise the quantum mechanical superposition effect that allows the free expansion of doubloons prevents You can
“Until now, such kinetic magnetic mechanisms have only been detected in model systems, for example four coupled quantum dots. But they have never been detected in extended solid-state systems like the one we are using. “No,” Imamoğlu said.
As a next step, he wants to change the parameters of the Moire lattice to investigate whether ferromagnetism is maintained at high temperatures. In the current experiment, the material needed to be cooled down to 1 degree for an additional 10 min. absolute temperature.
Reference: “Dynamic magnetism in triangular moire materials” L. Ciorciaro, T. Smoleński, I. Morera, N. Kiper, S. Hiestand, M. Kroner, Y. Zhang, K. Watanabe, T. Taniguchi, E. Demler and A. İmamoğlu, November 15, 2023, Nature.
DOI: 10.1038/s41586-023-06633-0