Optical response of relativistic-like materials using Fourier Transform Infrared spectroscopy

Optics offer a powerful lens for understanding the physical properties of crystalline materials that exhibit exotic phenomena, such as topological insulators and Weyl and Dirac semimetals. Fourier Transform Infrared (FTIR) spectroscopy detects electronic and vibrational transitions in the sample through the absorption or reflection of an incident light beam. 

Weyl and Dirac semimetals exhibit a relativistic-like linear energy-momentum relationship. When exposed to a constant magnetic field, the energy states of electrons in crystals become discretized in Landau levels (LLs). FTIR spectroscopy at various magnetic fields show electronic transitions between LLs in similarly discretized peaks. Magnetic field dependence of LL transitions in relativistic-like materials will be uniquely curved unlike linearly dependent transitions in typical materials. The peak widths of these transitions can determine the electron mobility in the sample. The data peaks can be extracted, fitted with Gaussian curves, and analyzed with a Python algorithm. The peak widths of different Dirac semimetals were found to corroborate an increasing correlation between peak widths and LLs as well as magnetic fields.

CdAs₂ is a newly discovered material proposed to be a topological insulator with an internal Weyl semimetal band structure. To examine the temperature dependence of the properties of CdAs₂, a combination of FTIR spectroscopy and ellipsometry is used. By measuring with polarizations of 0° and 90°, phonons can be seen from two axes of the sample. This data allows us to probe the evolution and formation of phonons with temperature in CdAs₂.