Low temperature terahertz spectroscopy of n-InSb through a magnetic field driven metal-insulator transition

The authors use fiber-coupled photoconductive emitters and detectors to perform terahertz spectroscopy of lightly doped n-InSb directly in the cryogenic (1.5K) bore of a high-field superconducting magnet. They measure transmission spectra from 0.1to1.1THz as the sample is driven through a metal-to-insulator transition (MIT) by applied magnetic field. In the low-field metallic state, the data directly reveal the plasma edge and magnetoplasmon modes. With increasing field, a surprisingly broad band (0.3–0.8THz) of low transmission appears at the onset of the MIT. This band subsequently collapses and evolves into the sharp 1s→2p− transition of electrons “frozen” onto isolated donors in the insulating state.