Researchers control electronic properties of moiré crystals
If you make a material thinner and thinner, there comes a point when it undergoes a seemingly miraculous transformation: A two-dimensional material that consists of only one or two layers of a crystalline solid sometimes takes on completely different properties than the same material with greater thickness. A research team led by physicist Prof Ursula Wurstbauer from the University of Münster is investigating how the properties of two-dimensional crystals stacked on top of each other can be controlled to exhibit different behaviours, e.g. as an insulator, an electrical conductor, a superconductor and a ferromagnet. To do this, the scientists utilised the interactions between the charge carriers (electrons) and the so-called ‘energy landscape’ of the crystals. Now, for the first time, the team has generated and quantitatively demonstrated collective excitations of the charge carriers within different energy landscapes. The study, which has been published in the journal Physical Review Letters, is pioneering in that it helps us understand the electronic characteristics of materials and how to specifically influence them.
To obtain the different properties, the scientists placed two layers of a two-dimensional crystal on top of each other and twisted them slightly against each other. This twisting creates geometric patterns, so-called moiré patterns – similar to two layers of thin curtain fabric laid on top of each other. These patterns characterise the energy landscape and force the electrons to move much more slowly. These changes result in the electrons interacting intensively with each other, which can lead to so-called ‘strongly correlated behaviour’.
“The electrons ‘feel and see’ each other, and so in the neighbourhood of an electron, it turns out that a moiré lattice site cannot be occupied or can only be occupied with a high energy input due to repulsion according to Coulomb's law,” explains Ursula Wurstbauer. “The correlations are formed depending on the pattern and the number of electrons.” She compares the behaviour of the electrons to ‘wild’ dancing in a disco, in contrast to the more orderly standard ballroom dancing in moiré patterns. “The way in which the electrons ‘dance’ or can move in the moiré patterns depends strongly on the pattern, the number of charge carriers and the resulting energy landscape.”
The properties of these material systems are not only interesting in basic research, points out Ursula Wurstbauer. They might also offer innovative application possibilities in quantum technology or for the realisation of so-called neuromorphic components and circuits.
The team, which included scientists from the University of Hamburg, RWTH Aachen University and the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, as well as Ursula Wurstbauer’s research group, prepared different two-dimensional crystals (graphene, molybdenum diselenide and tungsten diselenide) and analysed the samples using optical spectroscopy methods at cryogenic temperatures (“resonant inelastic light scattering spectroscopy”). The researchers combined the experimental work with theoretical analyses.
Funding
The German Research Foundation (DFG) supported the work financially as part of Priority Programme 2244.
Original publication
Nihit Saigal et al. (2024): Collective charge excitations between moiré-minibands in twisted WSe2 bilayers from resonant inelastic light scattering. Physical Review Letters; DOI: https://doi.org/10.1103/PhysRevLett.133.046902