Optical response of relativistic electrons in the polar BiTeI semiconductor

The transitions between the spin-split bands by spin-orbit interaction are relevant to many novel phenomena such as the resonant dynamical magneto-electric effect and spin Hall effect. Here, we present a combined experimental and theoretical study of the dynamics of relativistic electrons in the recently discovered giant bulk Rashba spin splitting system BiTeI. Several novel features are observed in the optical spectra including sharp edge singularity due to the reduced-dimensionality of joint density of states and a systematic doping dependence of the intraband transitions between the Rashba-split branches. These confirm the bulk nature of the Rashba-type splitting in BiTeI and manifest relativistic nature of the electron dynamics in a solid.