It’s relatively straightforward to describe the electronic structure of crystalline solids, but noncrystalline materials, such as liquids and glassy solids, are extremely difficult to treat in the same way. In the 1960s, renowned theorists P.W. Anderson, W.L. McMillan, N.F. Mott, S.F. Edwards, and J.M. Ziman formulated theoretical models for the electronic structure of liquid metals. For more than half a century, however, a key feature of those models—a backward-bending band—remained experimentally unobserved.
Recently, a team led by Keun Su Kim of Yonsei University, Korea, took a different approach to tackling this issue. They decorated the surface of a crystalline insulator (black phosphorus) with an alkali metal (e.g. sodium, potassium, rubidium, or cesium) in a random (liquid-like) spatial distribution. This caused the black phosphorus surface to become doped with electrons, which were then subject to multiple scattering by the alkali-metal ions. This modified the electronic structure of the black phosphorus to resemble that of a liquid metal. Using angle-resolved photoemission spectroscopy (ARPES) at Advanced Light Source Beamline 7.0.2 (MAESTRO), the researchers observed the characteristic features—the elusive backward-bending band and pseudogap—long predicted for liquid metals.
In fact, the researchers point out that there is a puzzling spectrum of crystalline insulators that exhibit the mysterious pseudogap (e.g., high-temperature superconductors). “Our findings reveal how the pseudogap is formed by the short-range order of dopants,” said Kim. “It would be interesting to see if this pseudogap mechanism works as well in other crystalline insulators doped by disordered dopants.”
S.H. Ryu, M. Huh, D.Y. Park, C. Jozwiak, E. Rotenberg, A. Bostwick, and K.S. Kim, “Pseudogap in a crystalline insulator doped by disordered metals,” Nature 596, 68 (2021), doi:10.1038/s41586-021-03683-0.