Title: Optical and low-momentum excitations in 2D systems from momentum resolved electron energy loss spectroscopy
Abstract: "Momentum-dependent electron energy-loss spectroscopy (q-EELS) has recently proven to be a pivotal tool for investigating charge excitations in suspended low-dimensional (2D) materials using a transmission electron microscope (TEM). Unlike optical techniques, q-EELS provides access to the energy dispersion of phonons, excitons, and plasmons. Comparing instead with other finite-momentum scattering techniques, such as neutron and X-ray spectroscopies, q-EELS with TEM does not require substrates or large crystals, making it an ideal tool to study freestanding 2D materials. With sufficiently high momentum resolution, q-EELS can probe the optical-limit regime ($q \approx 0$) as well as low-momentum dispersions, thus enabling an unprecedented characterization of monolayer materials. In this talk, I will discuss recent advances both in the interpretation of low-momentum measures, along with advances in the calculation of electron-energy losses with many-body perturbation theory, required as electron interaction is enhanced in freestanding 2D materials. Then, I will present recent experimental observations of phonon, exciton, and plasmon dispersions in prototypical 2D materials such as graphene and monolayer h-BN, and show how their peculiar 2D dispersions can be interpreted with the help of simulations."