content_image/410718.jpg content_image/410711.jpg
0.4-10 K have been measured using the $^{3}$He insert, and in the temperature range 1.6-80 K in a $^{4}$He insert. The two temperature ranges were chosen to have overlap, in order to confirm consistency of the data. 

<img src='content_image/410718.jpg'>

The temperature dependencies of spin relaxation times for electrons and holes are qualitatively similar. The times almost do not change at very low temperatures, which shows that the spin dynamics in these regimes are controlled by temperature independent mechanisms. At elevated temperatures the times drop by more than an order of magnitude. As can be seen from the solid lines in Fig. 8 these behaviors can be well fitted by the following function describing thermal activation from the ground state with a long relaxation time to an excited state with a shorter relaxation time: 

Here T e ( h ) 0 are the spin relaxation times in the electron (hole) ground states, T e ( h ) exc are constants characterizing the transitions between the ground and excited states which depend on the electron-phonon interaction, Δ E$_{e}$$_{(}$$_{h}$$_{)}$ are the characteristic activation energies, and k$_{B}$ is the Boltzmann constant. One should note that this approach is rather simplified and gives physically feasible values of 

<img src='content_image/410711.jpg'>

the fitting parameters for a relatively narrow tempera- ture range. Therefore, it should be valid for holes in a temperature range not exceeding 8 K, while for electrons, for which experimental data have been recorded up to 80 K, additional mechanisms may cause the temperature dependencies of T e exc and Δ E$_{e}$ so that more elaborated approach may be required. 

The electron spin relaxation time, T e $_{s}$, is constant in the temperature range from 0.43 to 4.5 K at an extremely large value of 45 ns, see insert in Fig. 8(a). At these tem- peratures the resident electrons are localized and their spin relaxation is provided by the hyperfine interaction with the nuclei spins, which is almost temperature inde- pendent. At temperatures above 5 K the resident elec- trons are thermally activated and the Dyakonov-Perel re- laxation mechanism 1 which is very efficient for free elec- trons starts to act. As a result the relaxation time drops to 1.1 ns at 40 K and further down to 110 ps at 80 K. Such a behavior is typical for n -doped QWs with a di- luted concentration of the resident electrons $^{8,9,15}$. 

Let us turn now to the temperature dependence of the hole spin relaxation time, T h s , in Fig. 8(b). Below 2.2 K T h s saturates at a value of 2 ns. At the moment it is not clear to us what relaxation mechanism is controlling the hole spin dynamics in the range from 0.43-2.2 K. Most probably it is due to the admixture of the light-hole to the heavy-hole states, enhancing strongly the possibility for spin-flip scattering. 

The hyperfine interaction with the nuclei is signifi- cantly weaker for the holes than for the electrons. This has been confirmed by the recent report on ultralong hole spin relaxation with about 70 ns relaxation time, mea- sured in the range from 0.4-1.2 K for a p -doped 4-nm GaAs/Al$_{0}$$_{.}$$_{3}$Ga$_{0}$$_{.}$$_{7}$As QW $^{14}$. Our experimental situation differs from the one in p -doped samples with resident holes, as in n -doped samples we detect the spin dynam- ics of photogenerated holes bound in negatively charged trions. However, in the trion ground state, which is a singlet state, the two electrons have antiparallel spin ori- entations and flip-flop electron-hole process are not pos- sible without exciting the trion complex into a triplet state, which requires an energy similar to the trion bind- ing energy of 1.4 meV. In fact this energy is in very good agreement with the activation energy Δ E$_{h}$ = 1 . 4 meV, which has been obtained from fitting the experimental data in Fig. 8(b) by Eq. (6). Therefore we suggest that the strong decrease of the hole spin relaxation time at temperatures above 2.2 K is either due to trion thermal dissociation, which excites the hole into the continuum of free states with a strong spin-orbit interaction leading to a fast spin relaxation, or due to flip-flop process involving the trion triplet state. 

In conclusion, the carrier spin dynamics in an n -doped (In,Ga)As/GaAs QW have been studied by the resonant spin amplification technique at very low lattice tempera- tures down to 0.43 K. Carrier localization leads to a sat- uration of the spin relaxation times at 45 ns for electrons below 4.5 K and at 2 ns for holes below 2.3 K. Also the 

