Electron spin lifetimes in long-wavelength Hg_1-xCd_xTe and InSb at elevated temperature

We have made direct pump-probe measurements of spin lifetimes in long-wavelength narrow-gap semiconductors at wavelengths between 4 and 10 µm and from 4 to 300 K. In particular, we measure remarkably long spin lifetimes t_s~300 ps even at 300 K for epilayers of degenerate n-type InSb. In this material the mobility is approximately constant between 77 and 300 K, and we find that t_s is approximately constant in this temperature range. In order to determine the dominant spin relaxation mechanism we have investigated the temperature dependence of t_s in nondegenerate lightly n-type Hg_0.78Cd_0.22Te of approximately the same band-gap as InSb and find that t_s varies from 356 ps at 150 K to 24 ps at 300 K. In this material lattice scattering dominates giving a T^-3/2 dependence for the mobility, and we expect a strong temperature dependence of t_s. There are two main models that have been invoked for describing spin relaxation in narrow-gap semiconductors: the Elliott-Yafet (EY) model which gives a T^-7/2 dependence of t_s in this limit and the D'yakonov-Perel model which gives a T^-3/2 dependence. Our results, both in magnitude and temperature dependence of t_s, imply that the EY model dominates in these materials.