Nodal Semimetals: A Survey on Optical Conductivity

Among different topological and related phases of condensed matter, nodal semimetals occupy a special place - the electronic band topology in these materials is related to three-dimensional bulk, rather than to surface, states. A great variety of different realizations of electronic band crossings (the nodes) leads to a plethora of different electronic properties, ranging from the chiral anomaly to solid-state realizations of a black-hole horizon. The different nodal phases have similar low-energy band structure and quasiparticle dynamics, which both can be accessed experimentally by a number of methods. Optical measurements with their large penetration depth and high energy resolution are ideally suited as such a bulk probe; especially at low energies where other spectroscopic methods often lack the required resolution. In this contribution, we review recent optical-conductivity studies of different nodal semimetals, discuss possible limitations of such measurements, and provide a comparison between the experimental results, simple theoretical models, and band-structure-based calculations.