Creating and visualizing symmetry-broken phases of matter in 2D materials

Synopsis

Abstract: Gas. Liquid. Solid. These are three phases of matter that we experience in our everyday lives. However, for electrons that live within solids, geometry and quantum mechanical effects can conspire to allow for the possibility of many more phases of matter. For instance, graphene — a single two-dimensional sheet of carbon atoms in a honeycomb lattice — has been predicted to host a plethora of new and interesting electronic phases. One such phase is called a density-wave, whereby electrons and the atomic lattice become unstable and together form a brand new crystal with a new symmetry. But amongst the many possible electronic phases, how does the system “choose” which one to display? And can one possibly control which macroscopic phase “wins”?

In this talk, I will describe our recent observations [Science Advances 8, eabm5180 (2022)] that an extremely small amount of surface defects can be used to purposely initiate the onset of a symmetry-breaking density wave phase in graphene. We detect the presence of this new phase using complementary photo-emission and diffraction techniques that measure the momentum of graphene’s electrons. Like an archaeologist re-assembling a dinosaur from its fossils, by measuring the kinetic energy and momentum of photo-emitted electrons, we can likewise piece together and visualize the symmetry and spectroscopic signature of this density wave phase in several graphene systems. Our results suggest a surprising sensitivity of the graphene lattice to even dilute surface disorder and open the door to harnessing adsorbed atoms for tailoring the properties of two-dimensional materials.