Lorentz-boost-driven magneto-optics in Dirac matter

Optical and magneto-optical methods can determine experimentally the energy band gap with a great precision. Here we show that such conventional methods, applied with great success to many materials in the past, do not work in topological Dirac semimetals with a dispersive nodal line. There, the optically deduced band gap depends on how the magnetic field is oriented with respect to the crystal axes. Such highly unusual behaviour is explained in terms of band-gap renormalization driven by Lorentz boosts, accompanied by an interpretation in the language of condensed matter physics.