In group-III nitride quantum wells, indirect excitons (IXs) are naturally formed because the electron-hole pair is separated along the growth (0001) axis by strong internal electric fields. These IXs therefore exhibit strong permanent dipole moments and extremely long radiative lifetimes (> 10µs). Previous extensive studies of IXs in GaAs-based, biased double quantum wells, at low temperatures, have emphasized promising properties: IXs can propagate over large distances, be controlled in-situ by light and external gate voltage, form cold and dense gases of interacting bosons, and may form collective quantum states of matter. Compared to 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 a range of exciton densities larger by two orders of magnitude. By using spatially- and time-resolved photoluminescence experiments we investigate the transport of dipolar excitons in GaN-(Al,Ga)N QWs, up to room-temperature, over distances of several tens of micrometers. Variations of conditions and comparison with models allow us to discuss the relative impacts on IX transport of important factors, such as disorder and exciton-exciton interaction.

Excitons in nitride quantum wells (QWs) are naturally indirect due to the strong built-in electric field: electron and hole within such excitons are spatially separated, leading to strong dipole moments and long radiative lifetimes. The physics of indirect excitons (IXs) has been extensively studied in GaAs-based heterostructures: they can propagate over large distances, be trapped in gate-controlled electrostatic traps, and form a cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostructures, 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.The length scale of the excitonic transport is an important figure of merit of IXs. We have first investigated the excitonic transport in the case of hetero-epitaxy of the GaN/AlGaN QWs on sapphire substrates, at low temperature [1]. The sample quality has then been improved through homo-epitaxy on GaN substrates, and we have evidenced an efficient transport up to room temperature [2]. Under a focused non-resonant excitation of the quantum well, we observe a spatial decay of the exciton density. The transport length now reaches 100µm at 10K and 30µm at T=300K.Such long propagation distances are a pre-requisite to the development of IX-based optoelectronic devices operating on a large temperature range, and to the exploration of excitonic collective states. The realization of an excitonic trap is a first step towards this goals, and we recently investigated the exciton confinement in electrostatic traps of various shapes. The traps are realized via deposition of semi-transparent gates on top of the QW sample. Trapping of the excitons is demonstrated, with a typical trap depth of 60-90 meV. The excitons can also be generated outside the trap and relax into the trap, so to spatially separate the trap from the generation spot, which is expected to be a source of heat and decoherence for the excitonic cloud.These results are promising for the investigation of trapped collective states of IXs, and their cooling into exotic phases. Acknowledgement: This work is supported by the French National Research Agency (ANR-15-CE30-0020-02 OBELIX andANR-11-LABX-0014 GANEX).[1]F. Fedichkin et al, Phys. Rev. B 91, 205424 (2015).[2]F. Fedichkin et al, Phys. Rev. Appl. 6, 014011 (2016).

Spin-lattice relaxation of the nuclear spin system in p-type GaAs is studied using a three-stage experimental protocol including optical pumping and measuring the difference of the nuclear spin polarization before and after a dark interval of variable length. This method allows us to measure the spin-lattice relaxation time T1 of optically pumped nuclei “in the dark,” that is, in the absence of illumination. The measured T1 values fall into the subsecond time range, being three orders of magnitude shorter than in earlier studied n-type GaAs. The drastic difference is further emphasized by magnetic-field and temperature dependencies of T1 in p-GaAs, showing no similarity to those in n-GaAs. This unexpected behavior finds its explanation in the spatial selectivity of the optical pumping in p-GaAs, that is only efficient in the vicinity of shallow donors, together with the quadrupole relaxation of nuclear spins, which is induced by electric fields within closely spaced donor-acceptor pairs. The developed theoretical model explains the whole set of experimental results.

We develop a method of nonperturbative optical control over adiabatic remagnetization of the nuclear spin system and apply it to verify the spin temperature concept in GaAs microcavities. The nuclear spin system is shown to exactly follow the predictions of the spin temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalization. These findings open a way for the deep cooling of nuclear spins in semiconductor structures, with the prospect of realizing nuclear spin-ordered states for high-fidelity spin-photon interfaces.

The concept of nuclear spin temperature is one of the cornerstones of the nuclear magnetism in solids. It has made possible realisation of the cryogenic cooling into the microKelvin range and observation of nuclear spin ordering in metals and insulators [1]. However, proving its validity for semiconductor nano- and microstructures is challenging due to the lack of techniques capable of precise sensing of weak nuclear magnetisation in a small volume. We develop a method of non-perturbative optical control over adiabatic remagneti- sation of the nuclear spin system in semiconductors and apply it to study nuclear spin thermodynamics in GaAs microcavities. The nuclear spin system is found to exactly follow the predictions of the spin-temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalisation [2]. These ndings open a way to deep cooling of nuclear spins in semiconductor structures, with a prospect of realisation of nuclear spin-ordered states for high delity spin-photon interfaces. References [1] A. S. Oja, O. V. Lounasmaa, Rev. Mod. Phys 69, 1 (1997). [2] P. Maletinsky, M. Kroner, A. Imamoglu, Nat. Phys 5, 407 (2009).

This special issue contains 9 original review papers, research papers and discussion papers on indirect excitons. An exciton is a Coulomb-correlated electron-hole pair. Frenkel excitons dominate optical properties of organic semiconductors, while Wannier-Mott excitons are responsible for the hydrogen-like absorption spectra of inorganic semiconductors at low temperatures. The interest to the physics of excitons has strongly increased in the new century. This interest is motivated by unique bosonic properties of excitons that lead to the phenomena of exciton-polariton lasing and stimulated scattering, build-up of the spontaneous coherence and polarisation in cold exciton gases. In addition to the rich fundamental physics, excitons offer the perspective of applications in opto-electronic devices such as exciton transistors, switches, optical integrated circuits, etc.The indirect excitons form an important research sub-field of the exciton physics. The reduction of electron-hole overlap in the real space leads to longer exciton radiative lifetimes, which is crucial for the studies of thermolyzed exciton gases. Large spatial dipole moments of indirect excitons are responsible for the strong exciton-exciton repulsion. This prevents formation of bi-excitons and excitonic molecules and facilitates formation of coherent collective excitonic states.The papers contained in this volume are devoted to different aspects of the modern physics of indirect excitons. In the review paper by Leonid Butov the intriguing perspective for implementation of excitonic device concepts is discussed. This review is nicely complemented by the work by Anton Nalitov et al. who propose using indirect excitons for realisation of bosonic cascade lasers emitting the coherent terahertz light. The review of Joe Wilkes and Egor Muljarov discusses the effect of electric and magnetic fields on indirect exciton and microcavity exciton-polaritons. Ursula Wurstbauer et al. present the experimental study of laterally trapped dipolar exciton ensembles at high magnetic fields in the Faraday configuration. Colin Hubert et al. study experimentally the dynamics of indirect excitons in GaAs/AlGaAs double quantum wells grown along the (111) direction. Tao Liu et al. study theoretically the spin dynamics of spatially trapped Bose-Einstein condensates of indirect excitons. Carsten Leding Vendelbjerg and Massimo Rontani present the theoretical research of the spin transfer torque induced onto a nano-magnet in hybrid magnetic nanostructures. Andrea Bertoni et al. address the time-dependent quantum scattering of a spatially indirect exciton by an external potential. Finally, Yotam Mazuz-Harpaz et al. analyse and discuss the recent experimental results on cold dipolar exciton gases in electrostatic traps.The nine groups that contributed to this special issue are the world-leaders in the rapidly growing research field of the physics of indirect excitons. The papers that composed this volume represent the state-of-the-art in this research domain. We hope that this volume may be used by young researchers and PhD students as well as the leading scientists as a reference book for further studies of indirect excitons.

Light emission in polar group-III nitride quantum wells optically pumped at densities significantly below the lasing threshold is usually interpreted in terms of excitons. Such excitons are characterized by a non-zero dipole moment and long radiative lifetimes, because their constituent electron and hole are spatially separated by the built-in electric field in the growth direction. These features result in many interesting properties of dipolar excitons: they can propagate over large distances and thus cool down to the lattice temperature before recombination, offering the possibility for studies of cold and dense gas of interacting bosons. In this context, two important issues must be addressed: the strength of the exciton-exciton interaction, and the excitonic Mott transition, that sets the maximum density of excitons. Combining spatially and time-resolved photoluminescence in different excitation geometries, we provide a set of experimental data showing that the emission linewidth is proportional to the emission energy in a wide range of carrier densities and temperatures. The proportionality coefficient is almost temperature independent. These results are compared to the existing data in various material systems and discussed in terms of the Mott transition and its criteria, exciton-exciton interaction and its possible contribution to the linewidth, as well as the role of disorder in the density-dependent emission spectra.