Ultrafast dynamics of a photo-doped narrow-gap semiconductor of 3D Kane fermion characteristics*

Kane fermions are intriguing ultrarelativistic quasiparticles found in semimetals and narrow-gap semiconductors. While they share a linear energy-momentum band structure with Dirac and Weyl semimetals, Kane fermions differ in their wavefunctions as they do not satisfy the Dirac equation. Despite having a narrow gap between the conduction and valence bands, they exhibit a linear band dispersion away from the band edges. Experimental verification of Kane fermions often necessitates resource-intensive methods, such as comprehensive spectroscopic studies at low temperatures or in strong magnetic fields. In this study, we focus on investigating the 3D Kane fermions in the narrow-gap semiconductor Hg_1-xCd_xTe. This material offers the advantage of a direct bandgap that can be continuously adjusted by manipulating the intrinsic cadmium concentration or external temperature. To explore the Kane dispersion, we employ ultrafast infrared-pump-terahertz probe spectroscopy to monitor the ultrafast free-carrier response. Through a detailed analysis of the free-carrier dynamics in Hg_1-xCd_xTe under varying excitation intensities, we observe a scaling behavior that deviates from trivial narrow-gap semiconductors but aligns with the characteristics of Kane fermions. This approach holds promise for identifying and studying other material systems governed by Kane fermions in future research.