Ultrafast spin dynamics in optically excited bulk GaAs at low temperatures

This paper presents a study of electron spin dynamics in bulk GaAs at low temperatures for elevated optical excitation conditions. Our time-resolved Faraday rotation measurements yield subnanosecond electron spin-dephasing times over a wide range of $n$-doping concentrations in quantitative agreement with a microscopic treatment of electron spin dynamics. The calculation shows the occurrence and breakdown of motional narrowing for spin dephasing under elevated excitation conditions. We also find a peak of the spin-dephasing time around a doping density for which, under lower excitation conditions, a metal-insulator transition occurs. However, the experimental results for high excitation can be explained without a metal-insulator transition. We therefore attribute the peak in spin-dephasing times to the influence of screening and scattering on the spin dynamics of the excited electrons.

Figure 1

Time-resolved Faraday rotation signal for $N_{\text{dop}}=3.6\times {10}^{16}{\text{cm}}^{-3}$, $B=4\text{T}$, and $T=4\text{K}$ as obtained by experiment (top), and numerical calculation (bottom). For the calculation $N_{\text{exc}}=1\times {10}^{16}{\text{cm}}^{-3}$ was assumed. The spin-dephasing time $T_2^{\ast }$ is determined by the exponential decay time of the envelope.

Figure 2

Spin-dephasing times vs photoexcited carrier density for doped (upper curves) and undoped (lower curves) bulk GaAs at $T=4\text{K}$. The experimental result (symbols) and the microscopic calculation (black lines) were obtained with an external magnetic field of $B=4\text{T}$. The doping density is $3.6\times {10}^{16}{\text{cm}}^{-3}$ (calculation) and $\left(3.6\pm 0.3\right)\times {10}^{16}{\text{cm}}^{-3}$ (experiment). For reference, the gray dash-dotted line shows the analytic result for the Dyakonov-Perel spin-dephasing time in relaxation-time approximation.

Figure 3

Spin-dephasing time vs doping density at 4 K and with 4 T external magnetic field. Shown together with the calculated result including all scattering mechanisms (solid line) are curves obtained by including only impurity scattering (dashed gray line) and without impurity scattering (dash-dotted gray line). The triangles are experimental values. The excited carrier density for calculation and experiment is always $N_{\text{exc}}=2\times {10}^{16}{\text{cm}}^{-3}$.