Educação matemática pela arte
Gusmão, Lucimar Donizete
2013-08-28
Search results
34 records were found.
The significance of nuclear spin polarisation in time-resolved optical studies of III–V semiconductors is addressed. Electron Larmor beats in pump-probe reflectivity from a GaAs/AlGaAs quantum well show Overhauser shift of 0.7 T due to accumulated nuclear polarisation I/I=0.065. This leads to precision values of electron g-factor, elucidates nuclear spin pumping and diffusion mechanisms in quantum wells and informs discussion of implications for spin-electronics and transport.
The influence of quantum confinement and built-in strain on conduction-electron g factors in lattice-matched GaAs/Al0.35Ga0.65As and strained-layer In0.11Ga0.89As/GaAs quantum wells is investigated for well widths between 3 and 20 nm. The magnitude, sign, and anisotropy of the g factors were obtained from quantum beats due to Larmor precession of electron spins in time-resolved, polarization-sensitive, pump-probe reflection at 10 K in magnetic fields applied along and at 45° to the growth axis. Slowly varying shifts of precession frequency, due to buildup of nuclear polarization in the samples over ∼1 h and equivalent to up to 0.5 T, occurred for fixed circular pump polarization and oblique applied fields. These Overhauser shifts confirmed the sign of the g factors and were eliminated by modulation of pump polarization to give precise ...
Recent and ongoing optical experiments on mechanisms and methods for control and gating of spin relaxation in semiconductor quantum wells are reviewed. We discuss work on high-mobility two-dimensional electron gases in (001)-grown GaAs/AlGaAs wells which reveals two new aspects of D'yakonov, Perel' and Kachorovskii (DPK) spin dynamics, namely oscillatory spin evolution in a quasi-collision-free regime at low temperatures and strong deviation from the standard expectation that spin-relaxation rate will be proportional to electron mobility at higher temperatures. The latter may indicate that electron–electron scattering, neglected hitherto, is important for spin relaxation. Experiments on (011)-grown GaAs/AlGaAs quantum wells confirm that this orientation leads to extension of electron spin memory by as much as two orders of magnitude at...
Ultrafast optical methods are used to study low temperature nuclear spin polarisation and electron spin precession and relaxation in an oblique magnetic field in undoped GaAs/AlGaAs quantum wells. We find that electron localisation dramatically extends electron spin relaxation and accelerates nuclear spin polarisation which is manifest as a large time-dependent Overhauser shift in electron Larmor precession frequency. Nuclear polarisation is a two-stage process, which we suggest involves contact hyperfine interaction at electron localisation centres and nuclear spin diffusion to the remaining nuclei of the sample. Special samples are used to investigate nuclear spin diffusion through the barriers, which is found to be an order of magnitude slower than within the wells, consistent with extra dipolar and quadrupolar disorder in the alloy.
Quantum structures of III–V semiconductors grown on (1 1 0)-oriented substrates are promising for spintronic applications because they allow us to engineer and control spin dynamics of electrons. We summarise the theoretical ideas, which are the basis for this claim and review experiments to investigate them.
Recent and ongoing optical experiments on mechanisms and methods for control and gating of spin relaxation in semiconductor quantum wells are reviewed. We discuss work on high-mobility two-dimensional electron gases in (001)-grown GaAs/AlGaAs wells which reveals two new aspects of D'yakonov, Perel' and Kachorovskii (DPK) spin dynamics, namely oscillatory spin evolution in a quasi-collision-free regime at low temperatures and strong deviation from the standard expectation that spin-relaxation rate will be proportional to electron mobility at higher temperatures. The latter may indicate that electron–electron scattering, neglected hitherto, is important for spin relaxation. Experiments on (011)-grown GaAs/AlGaAs quantum wells confirm that this orientation leads to extension of electron spin memory by as much as two orders of magnitude at...
Nonradiative energy transfer from a GaAs quantum well to a thin overlayer of an infrared organic semiconductor dye is unambiguously demonstrated. The dynamics of exciton transfer are studied in the time domain by using pump-probe spectroscopy at the donor site and fluorescence spectroscopy at the acceptor site. The effect is observed as simultaneous increase in the population decay rate at the donor and of the rise time of optical emission at the acceptor sites. The hybrid configuration under investigation provides an alternative nonradiative, noncontact pumping route to electrical carrier injection that overcomes the losses imposed by the associated low carrier mobility of organic emitters.
