Title:   Imaging freezing and melting electrons in a 2D Wigner crystal

Abstract:       In most condensed matter systems we think of electrons as delocalized particles that roam about the energy landscape of a material. This is especially true when the number of electrons in a crystal is less than the number of lattice sites, in which case most materials are metallic. But what if Coulomb repulsion between electrons is the dominant energy in a material? In this case you might expect electrons to freeze in place, like water turning into ice, since it costs too much energy for them to move around. Such behavior is, in fact, a 90-year-old prediction of quantum mechanics, but achieving such “electron-ice” (also called a “Wigner crystal”) is surprisingly difficult in practice. So far Wigner crystals have only been seen in a few experimental systems. The first experimental platform for studying Wigner crystals involved floating electrons on the surface of liquid helium back in the 1970s, and the next utilized electrons trapped at buried semiconductor interfaces in the 1980s and 1990s. These systems, however, are not compatible with high-resolution microscopy techniques, and so for almost 90 years it was impossible to glimpse the inner structure of theoretically predicted electron-ice. This situation has changed recently due to the development of new 2D materials only a few atoms thick. The electrons contained in these materials are very close to the surface and can, in principle, be imaged using scanned probe techniques such as scanning tunneling microscopy (STM). Here I will describe our recent efforts to image the freezing and melting of Wigner crystals in single-layer and bilayer sandwiches of 2D semiconductors using STM techniques.

Bio:     Mike Crommie received his B.S. degree in physics from UCLA in 1984 and his Ph.D. from UC Berkeley in 1991. He was a post-doc at IBM Almaden with Don Eigler for two years before becoming an assistant professor in the Physics Dept. at Boston University in 1994. He moved his laboratory to the UC Berkeley Physics Dept. in 6/99 as an associate professor and has been a full professor there since 2003.  He is also an LBNL Faculty Senior Scientist and co-director of the Kavli Energy NanoScience Institute. Awards and honors include the Vannevar-Bush faculty fellowship (2023), the Davisson-Germer prize (2021), APS Fellowship (2007), a National Science Foundation Young Investigator Award (1994), the AAAS Newcomb Cleveland Prize for 1993-94, and a Sloan Foundation Fellowship (1997).