Probing the effective mass and fermi velocity of charges by momentum-resolved electron energy loss spectroscopy

The effective mass (m*) and Fermi velocity (vF) are two fundamental gauges of the electronic properties of materials and conventionally measured by magnetotransport characterizations. In this Review, we introduce momentum(q)-resolved electron energy loss spectroscopy (q-EELS) as an alternative method for probing m* and vF, and demonstrate its applications in semiconductor Si and semimetal FeGe. The q-EELS methodology is based on the q-dependent plasmon dispersion in the context of the random-phase approximation (RPA) for a free-electron gas (FEG), featuring a quantitative dependence on m* and vF and thus providing the route for retrieving these parameters. We outline the experimental principles for characterizing plasmon dispersions from the optical light line (the order of 10-3 Å-1) to Brillouin-zone boundaries (the order of Å-1), and elucidate the theoretical framework for pertinent elaborations on m* and vF. This work provides both the conceptual and practical guidelines for employing the q-EELS to extract m* and vF of fundamental significances to electronic characteristics of matters.