Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells

We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of $\sim 4$ meV to zero, at a field of $\sim$35 kV/cm. This decrease, due to the field-induced deformation of the exciton wavefunction, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separation. However, a new theory that considers the modification of screening with that separation is needed to account for the observed dependence on excitation power of the individual energies of the two exciton components.Comment: 5 pages, 4 eps figures, RevTeX, Physical Review Letters (in press

Spin splitting in a polarized quasi-two-dimensional exciton gas

We have observed a large spin splitting between "spin" $+1$ and $-1$ heavy-hole excitons, having unbalanced populations, in undoped GaAs/AlAs quantum wells in the absence of any external magnetic field. Time-resolved photoluminescence spectroscopy, under excitation with circularly polarized light, reveals that, for high excitonic density and short times after the pulsed excitation, the emission from majority excitons lies above that of minority ones. The amount of the splitting, which can be as large as 50% of the binding energy, increases with excitonic density and presents a time evolution closely connected with the degree of polarization of the luminescence. Our results are interpreted on the light of a recently developed model, which shows that, while intra-excitonic exchange interaction is responsible for the spin relaxation processes, exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors.Comment: Revtex, four pages; four figures, postscript file Accepted for publication in Physical Review B (Rapid Commun.

Spin dynamics in high-mobility two-dimensional electron systems

Understanding the spin dynamics in semiconductor heterostructures is highly important for future semiconductor spintronic devices. In high-mobility two-dimensional electron systems (2DES), the spin lifetime strongly depends on the initial degree of spin polarization due to the electron-electron interaction. The Hartree-Fock (HF) term of the Coulomb interaction acts like an effective out-of-plane magnetic field and thus reduces the spin-flip rate. By time-resolved Faraday rotation (TRFR) techniques, we demonstrate that the spin lifetime is increased by an order of magnitude as the initial spin polarization degree is raised from the low-polarization limit to several percent. We perform control experiments to decouple the excitation density in the sample from the spin polarization degree and investigate the interplay of the internal HF field and an external perpendicular magnetic field. The lifetime of spins oriented in the plane of a [001]-grown 2DES is strongly anisotropic if the Rashba and Dresselhaus spin-orbit fields are of the same order of magnitude. This anisotropy, which stems from the interference of the Rashba and the Dresselhaus spin-orbit fields, is highly density-dependent: as the electron density is increased, the kubic Dresselhaus term becomes dominant and reduces the anisotropy.Comment: 13 pages, 6 figure

Exciton Spin Dynamics in Semiconductor Quantum Wells

In this paper we will review Exciton Spin Dynamics in Semiconductor Quantum Wells. The spin properties of excitons in nanostructures are determined by their fine structure. We will mainly focus in this review on GaAs and InGaAs quantum wells which are model systems.Comment: 55 pages, 27 figure

Spin relaxation in low-dimensional systems

We review some of the newest findings on the spin dynamics of carriers and excitons in GaAs/GaAlAs quantum wells. In intrinsic wells, where the optical properties are dominated by excitonic effects, we show that exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors. In doped wells, the two spin components of an optically created two-dimensional electron gas are well described by Fermi-Dirac distributions with a common temperature but different chemical potentials. The rate of the spin depolarization of the electron gas is found to be independent of the mean electron kinetic energy but accelerated by thermal spreading of the carriers.Comment: 1 PDF file, 13 eps figures, Proceedings of the 1998 International Workshop on Nanophysics and Electronics (NPE-98)- Lecce (Italy

Spin relaxation of excitons in strained InGaAs/GaAs quantum wells

We have investigated the exciton spin dynamics in strained InxGa1-x As/GaAs (x ≤ 0.2) quantum wells. The variation of the initial polarization with the excitation energy is well correlated with the increasing mixing of the valence band states. The time behavior of the depolarization curves clearly demonstrates the prime importance of the hole and exciton spin relaxation processes