Electron spin coherence in Si/SiGe quantum wells

The mechanisms limiting the spin coherence time of electrons are of great importance for spintronics. We present electron spin resonance (ESR) and transport measurements of six different two dimensional electron gases in silicon/silicon-germanium (Si/SiGe 2DEGs). The spin decoherence time $T_2^*$ is presented in conjunction with the 2DEG density $n_e$ and momentum scattering time $\tau_p$ as measured from transport experiments. A pronounced dependence of $T_2^*$ on the orientation of the applied magnetic field with respect to 2DEG layer is found which is not consistent with that expected from any mechanism described in the literature.