The Ferromagnetism in the Vicinity of Lifshitz Topological Transitions

We show that the critical temperature of a ferromagnetic phase transition in a quasi-two-dimensional hole gas confined in a diluted magnetic semiconductor quantum well strongly depends on the hole chemical potential and hole density. The significant variations of the the Curie temperature occur close to the Lifshitz topological transition points where the hole Fermi surface acquires additional components of topological connectivity due to the filling of excited size-quantization subbands. The model calculations demonstrate that the Curie temperature can be doubled by a small variation of the gate voltage for the CdMnTe/CdMgTe quantum well based device

Magnetic field effects on spin relaxation in heterostructures

Effect of magnetic field on electron spin relaxation in quantum wells is studied theoretically. We have shown that Larmor effect and cyclotron motion of carriers can either jointly suppress D'yakonov-Perel' spin relaxation or compensate each other. The spin relaxation rates tensor is derived for any given direction of the external field and arbitrary ratio of bulk and structural contributions to spin splitting. Our results are applied to the experiments on electron spin resonance in SiGe heterostructures, and enable us to extract spin splitting value for such quantum wells.Comment: 6 pages, 4 figure

Spin noise in quantum dot ensembles

We study theoretically spin fluctuations of resident electrons or holes in singly charged quantum dots. The effects of external magnetic field and effective fields caused by the interaction of electron and nuclei spins are analyzed. The fluctuations of spin Faraday, Kerr and ellipticity signals revealing the spin noise of resident charge carriers are calculated for the continuous wave probing at the singlet trion resonance.Comment: 8 pages, 4 figure

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

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 with only the lowest hole subband being relevant. The effective Hamiltonian of the lowest hole subband is obtained by the subband L\"owdin perturbation method in the framework of the six-band Luttinger ${\bf k}\cdot{\bf p}$ model, with sufficient basis functions included. The lowest hole subband in Si/SiGe quantum wells is light-hole like with the Rashba spin-orbit coupling term depending on momentum both linearly and cubically, while that in Ge/SiGe quantum wells is a heavy hole state with the Rashba spin-orbit coupling term depending on momentum only cubically. The hole spin relaxation is investigated by means of the fully microscopic kinetic spin Bloch equation approach, with all the relevant scatterings considered. It is found that the hole-phonon scattering is very weak, which makes the hole-hole Coulomb scattering become very important. The hole system in Si/SiGe quantum wells is generally in the strong scattering limit, while that in Ge/SiGe quantum wells can be in either the strong or the weak scattering limit. The Coulomb scattering leads to a peak in both the temperature and hole density dependences of spin relaxation time in Si/SiGe quantum wells, located around the crossover between the degenerate and nondegenerate regimes. Nevertheless, the Coulomb scattering leads to not only a peak but also a valley in the temperature dependence of spin relaxation time in Ge/SiGe quantum wells.... (The remaining is omitted due to the limit of space).Comment: 12 pages, 11 figures, PRB in pres