Physics Colloquium

Dr. Eli Rotenberg, Lawrence Berkeley National Lab

"Low energy physics in graphene: Observation of Multiparticle States in Graphene"

4:15pm, The John A. Wheeler Lecture Hall (RLM 4.102)

Coffee and cookies will be served at 4:00pm in RLM 4.102

Abstract:

How electrons interact with each other, and with other degrees of freedom in solids, determines much about the ground state of materials - whether a material is ordinary or superconducting at low temperature, for example. The emergence of such interesting and useful ground states from all the competing interactions is not straightforward, high Tc superconductivity being a prominent example. Also, whether these many-body interactions can be tailored by choosing material properties is an interesting question. Clearly, a detailed understanding of the fundamental interactions in solids is still lacking.

Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, is currently getting a lot of attention due to its special properties. Among these, the charges move at the same velocity regardless of their energy, in similar fashion to photons in vacuum, and therefore act effectively as massless fermions. This massless character has a profound effect on the many body interactions, on the one hand enhancing them but on the other hand restricting their phase space for mutual scattering.

Using angle-resolved photoemission spectroscopy, we could not only directly determine how the charges carriers refract and scatter as they pass through the crystal, we could also observe new charged, composite "quasi-particles". One of these is a charged particle bound to a plasmon called a plasmaron. The second of these is a charged particle bound to an exciton, also called a trion. The kinematics and lifetime estimates of these particles is directly observed for the first time, and reveal details about the electrodynamics that are a challenge to explain theoretically.