A non-invasive method for direct imaging of Wigner crystals

Figure 1: STM measurement of the dual-gated WSe2/WS2 moiré superlattice Wigner crystal state. Source: DOI: 10.1038/s41586 - 021 - 03874 - 9

A team of researchers at the University of California, Berkeley, in collaboration with a team at Lawrence Berkeley National Laboratory, developed a non-invasive method for direct imaging of Wigner crystals. In a paper published in the journal Nature, the team describes their approach and explains how it can be used to advance research on Vigna's crystal state. Carmen Rubio-Verdú of Columbia University published a news and opinion article in the same journal outlining the nature of Wigner crystals and describing the team's work.

Wigner crystals have a lattice structure that forms when electrons are sparsely distributed in certain two-dimensional materials. They are observed in materials such as two-dimensional semiconductors and liquid helium, but are known to be difficult to observe or image because they are very fragile. In this new effort, the researchers have developed a method that does not interfere with Wigner crystals, which makes imaging more precise.

The researchers placed a thin layer of tungsten disulfide on a thin layer of tungsten diselenide to create a tiny heterogeneous structure. It is worth noting that both are transition metal dihalogroups, and in this case, only 1 nanometer thick. The team then added electrons to both layers, which naturally formed a 2D structure, although the spacing of electrons in one of the layers was slightly smaller. The mismatch of the electronic patterns led to the creation of the moiré pattern, which is also a Vigner crystal. The researchers then placed a layer of graphene on top of the heterostructure to protect the crystal structure below. They then used a scanning tunneling microscope to generate crystal images without interfering with the crystal. Later, the team added a layer of hexagonal boron nitride to the heterostructure to better protect it, allowing detection with a microscope.

The researchers also tried to add or remove electrons from the structure before adding a protective barrier, finding that doing so caused the crystal structure to form shapes such as triangles or hexagons. Rubio-Verdú suggests that the new technology could lead to new ways to image other tiny, fragile structures.

Further information: Li Hongyuan et al., 2D Generalized Wigner Crystal Imaging, Nature (2021). DOI: 10.1038 / s41586 - 021 - 03874 - 9

Rubio-Verdú, Electron Crystals Under the Microscope, Nature (2021). DOI: 10.1038 / d41586 - 021 - 02573 - 9