Figure 2. Relative change of absorbance with magnetic field in sample A.

False colour maps present inter-LL transitions as a function of magnetic field. White lines represent the fits using the simplified Kane model, allowing for determination of and . Solid lines correspond to inter-band transitions, whereas dashed lines account for intra-band transitions. The bandgap values at different temperatures, determined by the inter-band transition energies at zero magnetic field, are depicted with arrows. Shaded areas represent the Reststrahlen bands (between 15–20 meV for HgTe/Cd_xHg_1−xTe band and 32–37 meV for GaAs substrate band). (a) At 2 K, only inter-band transitions are seen and a -like behaviour of inter-LL resonances and spin splitting of LLs is observed. (b) At 57 K, intra-band transitions become visible in addition to previous lines. (c,d) At higher temperatures the energy difference between inter-band and intra-band lines at zero field clearly increases, corresponding to a gap opening as a function of temperature. The horizontal line observed at 21 meV in c,d corresponds to the energy of TO CdTe-like phonons, arising in magnetoabsorption due to the electron–phonon interaction. As discussed in refs ²⁷, ²⁸, the frequency of such phonon mode in HgCdTe alloys is almost independent on temperature. In addition, the horizontal line observed at 9 meV in c,d is attributed to optical transitions from impurities as discussed in refs ²⁹, ³⁰. Indeed, because of weak Hg–Te bonds, mercury vacancies are always present in HgCdTe alloys, even in high-quality n-type materials. The amplitude of these lines rises as temperature increases. It has to be noted that thermal energy at 120 K is 10 meV; thus, any features visible below that energy should not be considered as relevant.