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Hole spin relaxation in [001] strained asymmetric Si/Si$_{0.7}$Ge$_{0.3}$(Ge/Si$_{0.3}$Ge$_{0.7}$) quantum wells is investigated in the situation withonly the lowest hole subband being relevant. The effective Hamiltonian of thelowest hole subband is obtained by the subband L\"owdin perturbation method inthe framework of the six-band Luttinger ${\bf k}\cdot{\bf p}$ model, withsufficient basis functions included. The lowest hole subband in Si/SiGe quantumwells is light-hole like with the Rashba spin-orbit coupling term depending onmomentum both linearly and cubically, while that in Ge/SiGe quantum wells is aheavy hole state with the Rashba spin-orbit coupling term depending on momentumonly cubically. The hole spin relaxation is investigated by means of the fullymicroscopic kinetic spin Bloch equation approach, with all the relevantscatterings considered. It is found that the hole-phonon scattering is veryweak, which makes the hole-hole Coulomb scattering become very important. Thehole system in Si/SiGe quantum wells is generally in the strong scatteringlimit, while that in Ge/SiGe quantum wells can be in either the strong or theweak scattering limit. The Coulomb scattering leads to a peak in both thetemperature and hole density dependences of spin relaxation time in Si/SiGequantum wells, located around the crossover between the degenerate andnondegenerate regimes. Nevertheless, the Coulomb scattering leads to not only apeak but also a valley in the temperature dependence of spin relaxation time inGe/SiGe quantum wells.... (The remaining is omitted due to the limit of space).

Hole spin relaxation in [001] strained asymmetric Si/SiGe and Ge/SiGe quantum wells