Date of Award
Doctor of Philosophy (PhD)
Physics & Astronomy
Eugene J. Mele
Electromagnetic fields can be used to perturb the electronic charge distribution inside a material giving rise to dispersive charge currents. These currents generate many unique optical phenomena from circular dichroism to spatially dispersive photogalvanic effects. In this thesis, we study the prediction and consequence of these spatially dispersive charge currents.
In order for a system to manifest a spatially dispersive charge current, the translation symmetry of the system must be broken. In the field of hard condensed matter physics this can occur in two fundamental ways: either by spatial inhomogeneities in the crystalline structure of the material one is studying or by application of a perturbing field that itself is modulated in space. Here we study examples from both categories.
Bilayer graphene samples contain domain walls that consist of one dimensional line defects in the crystalline structure of the material. These defects themselves break the translation symmetry of the crystal and can give rise to spatially dispersive currents even under spatially homogenous light illumination. Here we study the charge currents around these defect regions and describe their measurement using near field scanning optical microscopy.
We then study circular dichroism in twisted bilayer graphene. Here the dispersion of external light in the propagation direction breaks the translation symmetry of the system and leads to induced charge currents different in the top and bottom layers of the bilayer system. This mirror symmetry breaking leads to the asymmetric absorption of right and left handed circular polarized light directed at normal incidence upon the bilayer.
Lastly, we study a spatially dispersive current quadratically proportional to the electric field. These spatially dispersive currents are proportional to the spatially gradients of an external electric field. The spatial dispersion of the external field breaks the translation symmetry of the system and allows these spatially dispersive currents to manifest. We study an inversion broken Weyl semimetal and describe the measurement of the spatially dispersive photogalvanic effect, a consequence of a particular type of these spatially dispersive currents.
Addison, Zachariah, "Spatially Dispersive Electrodynamics And Quantum Geometry In Semimetals" (2020). Publicly Accessible Penn Dissertations. 3975.