A research team at the University of California, Berkeley achieved the first imaging of a two-dimensional generalized Wigner crystal by developing a new non-invasive scanning tunneling microscopy (STM) measurement method.

The research results were published in Nature, a leading multidisciplinary science journal. Li Hongyuan, an alumnus of Xi'an Jiaotong University (XJTU), is the first author of the paper.

An image of a two-dimensional generalized Wigner crystal showing a honeycomb lattice

Researchers built a two-dimensional Moiré superlattice by stacking WS2 and WSe2. In the superlattice, the kinetic energy of electrons is greatly suppressed, causing electronic crystals to appear. Under different electron densities, the electronic lattice can have different lattice structures.

Under certain conditions, electronic crystals can even possess an anisotropic lattice structure, breaking the triple rotational symmetry possessed by the two-dimensional Moiré superlattice as an electronic "container".

The imaging of Wigner crystals places strict requirements on observation technology. It requires the observation technology to have high spatial resolution, high single-electron detection sensitivity, and sufficiently small interference to the detection system.

Traditional STM measurement can meet the first two requirements, but it has strong interference to the observed system. In order to overcome this problem, the researchers innovatively adopted a sensing layer to assist them while measuring.

A single-layer graphene sensor was placed on the surface not far from the Wigner crystal. This allowed the researcher to reduce the interference of STM measurement, as well as sense the image of the Wigner crystal like a "film".

This non-invasive measurement method can also be applied to real-space measurement of many other novel correlated quantum states.

Li was admitted to XJTU in 2013 and enrolled in 2018 in the University of California, Berkeley as a doctoral student in applied physics. During his days in XJTU, Li won many scholarships, such as the National Scholarship and the Mount Everest Scholarship. Thanks to the international training policy of XJTU, Li went to the University of Glasgow in the UK and Texas A&M University in the US during his undergraduate period.

In UC Berkeley, Li has mainly focused on strongly related electronic systems. He has published papers in journals such as Nature Physics and Nature Materials as first author.

Link to the paper: https://doi.org/10.1038/s41586-021-03874-9