Aiming to provide new parameters to use in the electron g-factor engineering and spintronic applications, we investigate the interplay between quantum tunneling and structure inversion asymmetry (SIA) effects on the electron g-factor anisotropy in tunnel-coupled semiconductor quantum wells (QWs). Since the g-factor anisotropy sign change in narrow wells is due to SIA and determined by the electron average position, in double QW structures the quantum tunneling effect plays a pivotal role established by the central barrier length, which stands for the coupling parameter between the wells. Due to the quantum confinement, the anisotropic g-factor renormalization is take into account within envelope-function theory based on the Kane model for the bulk, deriving the effective-mass Hamiltonian for the electronic states in III-V QWs in the presence of an external magnetic field in the both fundamental configurations, i.e., in the QW plane and along the growth direction. Using first order perturbation theory, the corresponding components of the electron g factor tensor are analytically calculated, as a function of the QW width and central barrier length, for symmetric and asymmetric tunnel-coupled InP/InGaAs QWs. Results for single and noninteracting QWs are exactly reproduced as limit cases.
In this work, we investigate the overgrowth behavior of a virtual substrate based on a completely released, wrinkled and in-place bonded GaAs/InGaAs/GaAs membrane. The virtual substrate was obtained by molecular beam epitaxy (MBE) growth of the heterostructure on an AlAs sacrificial layer over a GaAs (001) substrate. In a second fabrication step, the GaAs/InGaAs/GaAs structure is release by selectively removing the AlAs sacrificial layer, cleaned and reintroduced into the MBE, where it serves as template for growth. After atomic hydrogen cleaning, we deposited 10-nm thick InxGa1-xAs layers varying the Indium content from x=0.05 to x=1. Samples are characterized using atomic force microscopy, scanning electron microscopy, 3D reciprocal space mapping at gracing incident x-ray diffraction and photoluminescence measurements. Results from microscopy shows a flat InGaAs layer growth (up to x=0.4) on the membrane, whereas layers on GaAs already show island and dislocation formation at x > 0.3. Furthermore, we observe the formation of bubbles in the membrane for higher In content as well as preferred material migration and accumulation on top of wrinkles. The shift in the critical thickness for island formation is associate to the change in the lattice parameter between virtual substrate and GaAs. This assumption is strongly supported by the x-ray diffraction experiments. Defect free growth is confirmed by transmission electron microscopy of a x=0.1 sample. In order to demonstrate the ability to grow active structures on membranes, we deposited a nominally unstrained InAlGaAs/InGaAs/InAlGaAs quantum well on top of a released wrinkled membrane. As a consequence of a red shift photoluminescence signal from this quantum well compared to a reference grown on GaAs (001) wafers, we have a strain release of the quantum compared to structures grown on GaAs. This results indicates an optical structure with great technological importance. Acknowledgments: CAPES, CNPQ and FAPESP.
The zero-bias peak (ZBP) is understood as the definite signature of a Majorana bound state (MBS) when attached to a semi-infinite Kitaev nanowire (KNW) nearby zero temperature. However, such characteristics concerning the realization of the KNW constitute a profound experimental challenge. We explore theoretically a QD coupled to metallic leads and connected laterally to a topological KNW of finite size at a non-zero temperature and show that overlapped MBSs of the wire edges can become effectively decoupled from each other and the characteristic ZBP can be fully recovered if one tunes the system into the leaked Majorana fermion fixed point. At very low temperatures, the MBSs become strongly coupled similarly to what happens in the Kondo effect with QDs. We present the universal features of the conductance as a function of the temperature and the relevant crossover temperatures. Our findings could offer additional guides to better identify the presence of the Majorana fermion in the system.
The theory of spin drift and diffusion in two-dimensional electron gases is developed in terms of a random walk model incorporating Rashba, linear and cubic Dresselhaus, and intersubband spin-orbit couplings. The additional subband degree of freedom introduces new characteristics to the persistent spin helix (PSH) dynamics. As has been described before, for negligible intersubband scattering rates, the sum of the magnetization of independent subbands leads to a checkerboard pattern of crossed PSHs with long spin lifetime. For strong intersubband scattering we model the fast subband dynamics as a new random variable, yielding a dynamics set by averaged spin-orbit couplings of both subbands. In this case the crossed PSH becomes isotropic, rendering circular (Bessel) patterns with short spin lifetime. Additionally, a finite drift velocity breaks the symmetry between parallel and transverse directions, distorting and dragging the patterns. We find that the maximum spin lifetime shifts away from the PSH regime with increasing drift velocity. We present approximate analytical solutions for these cases and define their domain of validity. Effects of magnetic fields and initial package broadening are also discussed. We acknowledge support from CNPq, CAPES, FAPEMIG, FAPESP, and the Swiss National Science Foundation.