Time-resolved Kerr reflectivity of two-dimensional electron gases in II-VI semiconductors provides a direct measure of electron spin precession and relaxation over a temperature range from 4 to 300 kelvin. The introduction of n-type dopants increases the electronic spin lifetimes several orders of magnitude relative to insulating counterparts, a trend that is also observed in doped bulk semiconductors. Because the electronic spin polarization in these systems survives for nanoseconds, far longer than the electron-hole recombination lifetime, this technique reveals thousands of spin precession cycles of 15 gigahertz per tesla within an electron gas. Remarkably, these spin beats are only weakly temperature dependent and persist to room temperature.
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