New type of magnetism appears in a layered semiconductor

The magnetic properties of materials usually originate from exchange interactions between their electrons, but researchers at ETH Zurich in Switzerland have now discovered a new type of magnetism that disobeys this rule. Known as kinetic magnetism, and previously only predicted theoretically, the new mechanism occurs in a regime where the strength of electron exchange interactions vanishes. While the discovery is unlikely to lead directly to new devices, it could advance our understanding of materials like Mott insulators and other systems featuring strongly correlated electrons.

The magnetic properties of a material arise from the quantum mechanical spins of its electrons. In a ferromagnetic material, for example, exchange interactions between electrons cause all the spins to align in the same direction, even in the absence of an external applied magnetic field. With the new mechanism, however, alignment occurs even without exchange interactions. Instead, it arises because the electrons’ kinetic energy – which is much larger than their exchange energy when electrons are strongly correlated – is minimized when the spins are aligned. This effect was first predicted by the Japanese physicist Yosuke Nagaoka in 1966.

In the new work, which is detailed in Nature, researchers led by Atac Imamoglu at ETH Zurich’s Institute for Quantum Electronics and Eugene Demler at the Institute of Theoretical Physics studied materials known van der Waals heterostructures. They fabricated these in their lab by placing atomically thin layers of two different semiconductor materials, molybdenum diselenide (MoS₂) and tungsten disulphide (WS₂), atop each other. At the plane of contact between the two, the materials’ different lattice constants (that is, the separation between their atoms) produces a two-dimensional periodic potential with a lattice constant 30 times bigger than those of the two semiconductors by themselves. This moiré lattice, as it is known, can be “filled” with electrons by applying a voltage.

Electron filling effects

Imamoglu and colleagues exposed this material to polarized laser light and measured how strongly the incident light was reflected for different polarizations. Because the amount of each polarization that gets reflected depends on the orientation of the material’s magnetic moments (and therefore its electron spins), these “polarization-resolved attractive polaron oscillator strength” measurements enabled them to determine whether the material’s spins tend to point in the same direction (ferromagnetism) or in random directions (paramagnetism).

As they increased the voltage, the researchers explain that the moiré lattice sites become filled with electrons. Up to a filling of exactly one electron per site of the moiré lattice (an arrangement that produces a system known as a Mott insulator), the material is paramagnetic. As the number of electrons are further increased, however, the material begins to behave like a ferromagnet.

This effect, explains Imamoglu, is “striking evidence” for a new type of magnetism that cannot be explained by exchange interactions, which arise from quantum mechanical effects that occur when two identical particles are swapped. In fact, if the exchange interaction were responsible, the effect the team observed should have also appeared with a smaller number of electrons in the lattice.

Doublons in the strongly interacting regime

According to the researchers, electronic band gap theory predicts that when each site of an electron lattice is occupied by a single electron, the system should be metallic. In the strongly interacting regime, however, the material becomes an insulator. As the number of electrons increases even more, sites with two electrons, termed “doublons”, form.

“In principle, doublons, which have a total spin of zero and are thus non-magnetic, could freely hop from site to site, making the material go from being a Mott insulator to being electrically conductive,” explains Imamoglu. “The energy of these doublons would be minimized if the doublon hopping were subject to constructive quantum interference between different pathways for hopping from one site to another: this is only possible if the spins of the electrons in the singly occupied sites are aligned, thus forming a ferromagnetic state.”

The researchers admit that there are features in their experiment that they still do not understand. One example is the abrupt disappearance of ferromagnetic correlations when the electron filling factor of the lattice is 3/2.

Looking forward, they hope to use the effect they observed to uncover new physics. As a next step, Imamoglu says they would like to design new structures that exhibit ferromagnetic order at higher temperatures. At the moment, the material they studied had to be cooled down to between a few degrees and a fraction of a degree of absolute zero.