Imaging the freezing and melting of electrons in 2D Wigner crystals

In most condensed matter systems we think of electrons as delocalized particles that roam about the energy terrain of a material much like waves in the ocean. But what happens when Coulomb repulsion between electrons becomes the dominant energy in a material? In this regime you might expect electrons to freeze in place, like water turning into ice, since it costs too much energy for them to move. Such behavior is, in fact, a 92-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 for dimensions > 1. 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 how it is possible to image the freezing and melting of Wigner crystals in single-layer and bilayer sandwiches of 2D semiconductors using STM techniques.

Speaker: Mike Crommie, UC Berkeley