Researchers from University of California at Berkeley, along with additional collaborators, have developed a graphene-based technique for visualizing "Wigner crystals" - a unique sort of crystal that had so far remained impossible to visualize as it tends to “melt” when probed. The crystals are named after physicist Eugene Wigner, who predicted that at low densities and cold temperatures, electrons that usually zip through materials would freeze into place, forming an electron ice, or "Wigner crystal".
By placing a graphene sheet over the semiconductor sandwich, the team was able to probe the Wigner crystal with a scanning tunneling microscope without melting the sample and demonstrate the crystalline lattice structure, as Wigner predicted.
According to doctoral candidate Hongyuan Li and former postdoctoral fellow Shaowei Li, co-first authors of the paper, the study not only lays a solid foundation for understanding electron Wigner crystals, but also provides an approach that is generally applicable for imaging correlated electron lattices in other systems.
“The technique is non-invasive to the state you want to probe. To me, it is a very clever idea,” physicist Kin Fai Mak of Cornell University told Nature.
The team created the Wigner crystals making a sandwich of single-atom layers of two similar semiconductors: tungsten disulfide and tungsten diselenide. The researchers used an electric field to tune the density of the electrons between the two layers and then cooled the entire device to a temperature of about 5 Kelvin, near absolute zero.
Their first attempt to measure the electron density in the two-dimensional crystal sandwich with a scanning tunneling microscope (STM) destroyed the crystal. The team suggested covering the device with a graphene sheet, which acts like a sheet of photo paper to record the location of the electrons. The scientists used the STM to read the electron image on the graphene — as predicted, the electrons settled into a crystal arrangement with a separation nearly 100 times greater than the separation of atoms in the semiconductor sheets.