We measure the spin-resolved transport of dipolar excitons in a biased GaAs double quantum well structure. From these measurements we extract both spin lifetime and mobility of the excitons. We find that below a temperature of 4.8K there is a sharp increase in the spin lifetime of the excitons, together with a sharp reduction in their mobility. Below a critical power the spin lifetime increases with increasing mobility and density, while above the critical power the opposite trend is observed. We interpret this transition as evidence of the interplay between two different spin dephasing mechanisms: at low mobility the dephasing is dominated by the hyperfine interaction with the lattice nuclei spins, while at higher mobility the spin-orbit interaction dominates and a Dyakonov-Perel spin relaxation takes over. The excitation power and temperature regime where the hyperfine interaction induced spin dephasing is observed correlates with the regime where a dark dipolar quantum liquid was reported recently on a similar sample.

Nuclear spin relaxation is studied in n-GaAs thick layers and microcavity samples with different electron densities.We reveal that both in metallic samples where electrons are free and mobile, and in insulating samples where electrons are localized, nuclear spin relaxation is strongly enhanced at low magnetic fields. The origin of this effect could reside in the quadrupole interaction between nuclei and fluctuating electron charges, that has been proposed to govern nuclear spin dynamics at low magnetic fields in the insulating samples. The characteristic values of these magnetic fields are given by dipole-dipole interaction between nuclei in bulk samples, and are greatly enhanced in microcavities, presumably due to additional strain, inherent to microstructures and nanostructures.

Dynamic Nuclear Polarization and Nuclear Fields

Excitons in quantum wells (QWs) based on group-III nitrides or ZnO are naturally indirect in real space, due to the strong internal electric field that exists along the growth axis. Within such excitons, the electron and hole are spatially separated, leading to (1) strong dipole moments and (2) long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown that a combination of these two features results in many interesting properties of IXs : they can propagate over large distances, they can be controlled in-situ by light or by external gate voltage, they can cool down to the lattice temperature before recombination, and form cold and dense gases of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in GaN- or ZnO-based QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over a much larger density range. Using spatially- and time-resolved photoluminescence experiments, we investigate the exciton transport in several QW samples based on GaN and ZnO. The IX emission is imaged, with spectral and temporal resolutions, along the sample plane. Therefore the corresponding spatial density profiles are obtained and monitored as a function of (i) temperature, (ii) the exciton density at the excitation spot, (iii) the substrate material and (iv) the excitation regime (continuous vs pulsed excitation). We provide a comprehensive analysis of the data combined with numerical modeling, and we show that the efficient propagation of IXs takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole-dipole interaction. Under these latter conditions, exciton mobility is almost temperature-independent from 10K up to room temperature. This suggests that exciton-exciton interaction is by far the dominant scattering mechanism, compared to scattering by the interface disorder. However, exciton transport is maintained up to 300K only in the GaN/AlGaN QW that was epitaxially grown on a GaN substrate, since nonradiative processes dominate the transport in the case of a sapphire substrate, for which high densities of threading dislocations are present. We provide a detailed understanding of the physical mechanisms of IX transport and its temperature and density dependence, and we show that IXs in wide band-gap QWs constitute a promising system for the formation of collective bosonic states in semiconductors. Financial support : projects "INDEX" (FP7 PITN-GA-2011-289968) and "OBELIX" (ANR-15-CE30-0020-02).

Excitons in nitride quantum wells (QWs) are naturally indirect due to the strong internal electric field: electron and hole within such excitons are spatially separated, leading to strong dipole moments and long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown that a combination of these two features results in many interesting properties of IXs: they can propagate over large distances, can be controlled in-situ by light and external gate voltage, cool down to the lattice temperature before recombination, and form cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in GaN QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over a much larger density range. Using spatially- and time-resolved photoluminescence experiments, we have investigated the exciton transport in a 7 nm-wide GaN single QW sandwiched between Al0.19Ga0.81N barriers. The IX emission is imaged, with spectral and temporal resolutions, along the sample plane. Therefore the corresponding spatial density profiles are obtained and monitored as a function of (i) temperature, (ii) the exciton density at the excitation spot, (iii) the substrate material (GaN vs Sapphire) and (iv) the excitation regime (continuous vs pulsed excitation) [F. Fedichkin et al., Phys. Rev. B 91, 205424 (2015)]. We provide a comprehensive analysis of the data combined with numerical modeling, and we show that the efficient propagation of IXs takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole-dipole interaction. Under these latter conditions, exciton mobility is almost temperature-independent from 10K up to room temperature. This suggests that exciton-exciton interaction is by far the dominant scattering mechanism, compared to scattering by the interface disorder. However, the exciton transport is maintained up to 300K only in the QW that was epitaxially grown on a GaN substrate, since nonradiative processes dominate the transport in the case of a sapphire substrate, for which high densities of threading dislocations are present.We provide a detailed understanding of the physical mechanisms of IX transport and its temperature and density dependence, and we show that nitride-hosted IXs constitute a promising system for the formation of collective bosonic states in semiconductors. Financial support: projects INDEX (FP7 PITN-GA-2011-289968) and OBELIX (ANR-15-CE30-0020-02).

Excitons in polar group-III nitride quantum wells (QWs) are naturally indirect, because electron and hole within such excitons are spatially separated. As a consequence, they have strong dipole moments and long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown, that a combination of these two features results in many interesting properties of IXs: they can propagate over large distances, can be controlled in-situ by light and external gate voltage, cool down to the lattice temperature before recombination, and form cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in nitride QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over two orders of magnitude higher density range. Using spatially- and time-resolved photoluminescence experiments we examined temperature, density, substrate material (GaN vs Sapphire) and excitation regime (cw vs pulsed excitation) dependence of the exciton transport in 7 nm wide GaN QW sandwiched between Al0.19Ga0.81N barriers, the structure is characterized by internal electric field of order of 1 MV/cm. Comprehensive analysis of the data combined with numerical modeling show that in GaN QWs efficient propagation of IX takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole - dipole interaction. Under these conditions, exciton mobility is almost temperature-independent from 10 K up to room temperature. This suggests that exciton-exciton interaction is by far dominant scattering mechanism, compared to scattering on the interface disorder. In this context, we argue that nitride-hosted IXs constitute a promising system for formation of collective bosonic states in semiconductors.