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Direct visualization of exotic electron crystals in graphene


Direct visualization of exotic electron crystals in graphene

In conventional solids, atoms or molecules "freeze" at well defined positions, thus forming a lattice. However, electrons, one of the smallest constituents of atoms, may also solidify and form a so-called Wigner crystal. The magnetic field can enhance this tendency of crystallization, and even more exotic solids may be found, with bubbles containing two or more electrons per lattice site.

 

The recent discovery of graphene, a two-dimensional material where the electrons live at the surface, has raised the hope to "see" electrons at work. In contrast to conventional two-dimensional electron gases, that are buried in a semiconductor, the electrons in graphene are directly accessible, e.g., by a scanning tunneling microscope. Thus graphene yields the promising prospect to directly observe exotic electronic-solid phases.

 

In a recent publication [1], O. Poplavskyy (LPS, now University of Cambridge), M. O. Goerbig (LPS), and C. Morais Smith (University of Utrecht, The Netherlands) studied in detail the local density of states – a density map at a fixed energy – of high-field electron crystals in graphene. They calculated the density patterns for the Wigner and bubble crystals and found that the local density of states exhibits a scaling relation: it is possible to infer the behavior of a complex bubble-crystal, by knowing the behavior of a Wigner crystal. These density patterns may find and experimental verification in future spectroscopic measurements.

 

Figure: Examples of patterns of the local density of states for the Wigner crystal at characteristic energies.

 

[1] O. Poplavskyy, M. O. Goerbig, and C. Morais Smith, Phys. Rev. B 80, 195414 (2009); Editor’s suggestion.