F. Bernardot

Hole–Nuclear Spin Interaction in Quantum Dots

We have measured the carrier spin dynamics in p-doped InAs/GaAs quantum dots by pump-probe and time-resolved photoluminescence experiments. We obtained experimental evidence of the hyperfine interaction between hole and nuclear spins. In the absence of an external magnetic field, our calculations based on dipole-dipole coupling between the hole and the quantum dot nuclei lead to a hole-spin dephasing time for an ensemble of dots of 14 ns, in close agreement with experiments.

Domain structure and magnetic anisotropy fluctuations in (Ga,Mn)As: Effect of annealing

We investigate the effect of post-growth annealing on the magnetic domain structure and magnetization reversal process of (Ga,Mn)As epilayers grown with tensile strain on a (Ga,In)As buffer. In the case of perpendicular magnetic easy-axis, annealing drastically changes the domain structure observed at magnetization reversal. In as-grown samples, strongly anisotropic domain growth is observed. Dendritic-like domain expansion with guided branching along the directions results in a grid-like pattern. This is tentatively attributed to spatial fluctuations of the uniaxial anisotropy constant, correlated with the cross-hatch pattern. In annealed samples, domain wall motion is much more isotropic, which likely results from a decrease of the relative amplitude of the uniaxial anisotropy fluctuations with increasing carrier density. However domain wall motion is impeded by linear or slightly curved defects, hundreds of micrometers long, and point-like pinning centers. The density of nucleation centers for magnetization reversal strongly decreases upon annealing.

Spin decoherence and relaxation processes in zero-dimensional semiconductor nanostructures

In recent years, interest in spin physics has been renewed due to its potential application in spintronics and quantum information. In these frameworks, the main required property is the presence of long spin memory. We present a short review of recent results concerning spin decoherence and relaxation processes in zero-dimensional (0D) nanostructures, especially in quantum dots.

Electron and hole spin cooling efficiency in InAs quantum dots: The role of nuclear field

The spin dynamics of a resident carrier, hole or electron, in singly charged InAs/GaAs quantum dots has been measured by pump-probe experiments. The relative strength of the hole to the electron hyperfine couplings with nuclei is obtained by studying the magnetic-field dependence of the resident-carrier spin polarization. We find, in good agreement with recent theoretical studies, that the hole hyperfine coupling is ten times smaller than the electron one.

Enhancement of the electron spin memory by localization on donors in a CdTe quantum well

We present easily reproducible experimental conditions giving rise to a long electron spin memory at low temperature. The proposed system consists of an electron localized by a donor potential, and immerged in a quantum well. We have measured, by using photoinduced Faraday rotation technique, the spin relaxation time of electrons localized on iodine donors placed at the middle of a 80 A CdTe quantum well, and we have obtained 20 ns; this spin relaxation time is two orders of magnitude longer than for free electrons in a similar CdTe quantum well [J. Tribollet , Phys. Rev. B 68, 235316 (2003)].

Signature of the Overhauser field on the coherent spin dynamics of donor-bound electrons in a single CdTe quantum well

We have studied the coherent spin dynamics in an oblique magnetic field of electrons localized on donors and placed in the middle of a single CdTe quantum well, by using a time-resolved optical technique: the photo-induced Faraday rotation. We showed that this dynamics is affected by a weak Overhauser field created via the hyperfine interaction of optically spin-polarized donor-bound electrons with the surrounding nuclear isotopes carrying non-zero spins. We have measured this nuclear field, which is on the order of a few mT and can reach a maximum experimental value of 9.4 mT. This value represents 13 % of the maximal nuclear polarization, and corresponds also to 13 % of maximal electronic polarization.

Tuning the ferromagnetic properties of hydrogenated GaMnAs

The neutralization of Mn acceptors by hydrogenation is used as a tool to investigate the dependence of the magnetic properties of ferromagnetic GaMnAs upon the hole concentration. The suppression of ferromagnetism and the emergence of a paramagnetic phase are observed after hydrogenation. The ferromagnetic phase is progressively restored after subsequent annealings, with Curie temperature increasing with hole density.

Exciton spin coherence in InGaAs/GaAs quantum dots revisited by heterodyne pump-probe experiment (Withdrawal Notice)

Publisher’s Note, 1 December 2016: This paper, originally published on 11/9/2016, was withdrawn at request of the authors.

Hole and trion spin dynamics in quantum dots under excitation by a train of circularly polarized pulses

We have performed pump-probe experiments in p -doped InAs/GaAs quantum dots leading to the all-optical initialization and readout of hole spins. In order to describe these experiments, we have modelized the interconnected dynamics of the photoelectron spin and the resident hole spin, triggered through the optical excitation by a train of short pulses. A complete description of this spin dynamics is obtained by including the hyperfine coupling as the common decoherence mechanism for the electron and hole spins. Periodic excitation conditions for arbitrary values of the pump power and the external magnetic field are also included in the model. In particular, a good agreement concerning the temporal behavior of the photoinduced circular dichroism is obtained for zero or low magnetic fields. When the applied magnetic field screens the hole-hyperfine interaction, we show that the agreement between the experimental and calculated time-dependent curves requires an additional relaxation mechanism for holes with a characteristic time in the microsecond range.

Hole spin initialization in quantum dots by a periodic train of short pulses

We model pump-probe experiments leading to the all-optical initialization of the hole spin of a p-doped InAs/GaAs quantum dots ensemble. We consider selection rules of mixed hole states and include periodic excitation conditions. Hyperfine interaction is taken into account as the common decoherence mechanism for the spins of electrons and holes. We show that the degree of hole spin polarization can be maximized by quenching the action of the hole hyperfine interaction with a small applied magnetic field. However additional hole spin relaxation mechanisms, in the microsecond time range, determine the absolute value of this maximum.