Electron charge and spin delocalization revealed in the optically probed longitudinal and transverse spin dynamics in n -GaAs

The spin dynamics of localized electrons in solids is drastically different from that of mobile electrons. One of the main features characteristic for localized spins is the prominent difference between the longitudinal and transverse spin relaxation times ${T}₁$ and ${T}₂$${}^{\ensuremath{\star}}$ in magnetic fields. While up till now these times were believed to be of different nature, here the authors find the surprising relation ${T}₁$ $\ifmmode\times\else\texttimes\fi{}$ ${T}₂$${}^{\ensuremath{\star}}$ \ensuremath{\approx} constant for electrons localized on donors in $n$-GaAs. This is explained by the impact of the magnetic field on spin diffusion. On the other hand, mobile electrons are characterized by a single spin lifetime in the motional-narrowing regime. Upon rising temperature or donor concentration, the authors observe a striking 100-fold increase of ${T}₂$${}^*$ due to the onset of motional narrowing.