Temperature dependent infrared spectroscopy of the Rashba spin-splitting semiconductor BiTeI

We performed temperature dependent infrared spectroscopy measurements on BiTeI single crystals, which exhibit large Rashba spin-splitting. Similar to a previous optical study, we found electronic excitations in good agreement with spin-split electronic bands. In addition, we report a low energy intraband transition with an onset energy of about 40 meV and an unexpectedly large number of vibrational modes in the far-infrared spectral region. At least some of the modes have asymmetric Fano line-shape. These new observations cannot be explained considering only the bulk band structure or crystal symmetry of BiTeI, and we proposed that the optical response is also affected by the surface topology.