Quantum secrets revealed – Scientists have discovered a new type of magnetism

Quantum secrets revealed – Scientists have discovered a new type of magnetism

Abstract concept of magnetic energy

ETH Zurich scientists have identified a new ferromagnetism in a specially designed moiré material, challenging traditional magnetic theories. This magnetism, based on electron spin alignment to minimize kinetic energy, provides new insights into quantum effects and solid-state magnetism.

For a magnet to stick to your refrigerator door, several physical influences must align perfectly. The magnetic moments of their electrons all point in the same direction, a phenomenon that occurs even without an external magnetic field.

This is due to the exchange interaction, a complex interplay of electrostatic repulsion between electrons and the quantum mechanical properties of electron spin, which generates magnetic moments. This mechanism explains why materials such as iron and nickel are ferromagnetic, meaning they are permanently magnetic unless heated above a certain temperature.

At ETH Zurich, a team of researchers led by Atak Imamoglu of the Institute for Quantum Electronics and Eugene Dimmler of the Institute for Theoretical Physics has discovered a new type of ferromagnetism in an artificially produced material, where the magnetic moments are aligned around in a completely different way. They recently published their results in the scientific journal nature.

Synthetic material with electron filling

In Imamoglu’s lab, doctoral student Livio Ciurciaro, postdoc Tomasz Smolenski, and colleagues produced a special material by layering atomically thin layers of two different semiconductor materials (molybdenum diselenide and tungsten disulfide) on top of each other.

At the contact level, the different lattice constants of the two materials – the separation between their atoms – leads to the formation of a two-dimensional periodic potential with a large lattice constant (thirty times larger than the constants of the two materials Semiconductors), which can be filled with electrons by applying an electric voltage.

A new type of magnetic drawing

In the wavy material produced at ETH, electron spins are disordered if there is exactly one electron at each lattice site (left). Once there are more electrons than lattice sites (right) and the electron pairs can form a doubloon (red), the spins align magnetically, reducing kinetic energy. Credit: ETH Zurich

“Such wavy materials have attracted great interest in recent years, as they can be used to study the quantum effects of strongly interacting electrons very well,” Imamoglu says. “However, very little has been known so far about their magnetic properties.”

To investigate these magnetic properties, Imamoglu and his colleagues measured whether, for a given electron filling the material, a moiré material is paramagnetic, with its magnetic moments randomly oriented, or ferromagnetic. They illuminated the material with laser light and measured how strongly the light was reflected at different polarizations.

Polarization refers to the direction in which the electromagnetic field of laser light oscillates, and depending on the direction of the magnetic moments – and thus the electron spin – the material will reflect one polarization more strongly than the other. From this difference, one can then calculate whether the spins are pointing in the same direction or in different directions, from which the magnetization can be determined.

Conclusive evidence

By steadily increasing the voltage, physicists filled the material with electrons and measured the corresponding magnetization. Until exactly one electron filled each site of the moiré lattice (also known as the Mott insulator), the material remained magnetized. As the researchers continued to add electrons to the lattice, something unexpected happened: The material suddenly began to behave like a ferromagnet.

“It was striking evidence of a new type of magnetism that cannot be explained by an exchange reaction,” Imamoglu says. In fact, if an exchange reaction is responsible for the magnetism, this should also appear with fewer electrons in the lattice. The sudden onset therefore indicates a different effect.

Kinetic magnetism

Finally, Eugene Demmler, in collaboration with postdoc Ivan Moreira, came up with a crucial idea: they could investigate a mechanism that Japanese physicist Yusuke Nagaoka had theoretically predicted as early as 1966. At that mechanism, by making their spins point In the same direction, electrons reduce their kinetic energy (energy of motion), which is much greater than the exchange energy. I

In the experiment conducted by the ETH researchers, this happens once there is more than one electron per lattice site within the moiré material. As a result, pairs of electrons can combine to form so-called doubloons. Kinetic energy is reduced to a minimum when the diodes propagate over the entire network through quantum mechanical tunneling.

However, this is only possible if the single electrons in the lattice align their spin magnetically, otherwise the effects of quantum mechanical superposition that enable the free expansion of doubloons are disturbed.

“So far, such mechanisms for kinetic magnetism have only been discovered in model systems, for example in four coupled quantum dots, but never in extended solid-state systems like the ones we use,” Imamoglu says.

As a next step, he wants to change the parameters of the moiré network in order to check whether ferromagnetism is preserved for higher temperatures; In the current experiment, the material must still be cooled to a tenth of a degree above Absolute zero.

Reference: “Kinematic magnetism in triangular moiré materials” by 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

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