Center for Functional Nanomaterials Seminar

Low energy electronic excitations of graphene and strong topological insulators (TI) are two manifestations of massless Dirac fermions in condensed matter physics. The relativistic dispersion leads to several unusual properties such as resistance to confinement and anomalous Landau level properties. However, these shared properties arise from very different physical origins, and ultimately differ in important ways. While graphene has weak spin-orbit coupling and a pseudo spin degree of freedom locked parallel to the electron’s momentum, topological insulators have strong spin-orbit coupling, with the electron’s spin locked perpendicular to its momentum.

I will discuss two aspects of the electronic structure of these systems which we have studied. The first is a phenomenon unique to graphene - the creation of pseudo-electromagnetic fields as a result of strain. It has been predicted that certain special strain distributions will result in a uniform pseudo-magnetic field, forming Landau levels in the electronic density of states. Scanning tunneling spectroscopy (STS) measurements of highly strained nanobubbles formed in graphene films grown on a Pt(111) surface found such Landau levels, corresponding to pseudo-magnetic fields of approximately 300T. This demonstration of enormous pseudo–magnetic fields opens the door both to the study of charge carriers in previously inaccessible high magnetic field regimes, and to deliberate mechanical control over electronic structure in graphene or so-called “strain engineering.”