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International audience; Cooling of nuclear spins in doped semiconductors via their dynamic polarization by optical pumping is a powerful method for harnessing ubiquitous fluctuations of nuclear spin, which constitute a well-known decoherence channel for the electronic spins. The idea of spin cooling is based on the hypothesis of spin temperature, which states that nuclear spin system (NSS) reaches an internal thermal equilibrium long before it comes to equilibrium with the external bath (crystal lattice). Although thermodynamic framework has been successfully employed for the description of a variety of the experimental data, a rigorous check of this concept in semiconductors was impossible until recently, in particular at low magnetic field. The reason for that is the lack of experimental techniques allowing nonperturbative optical control over adiabatic transformation of the NSS.We have recently developed such methods, based on off-resonant Faraday rotation and spin noise spectroscopy [1, 2]. Using these techniques, combined with dark-interval photoluminescence spectroscopy, we established a comprehensive picture of the nuclear spin relaxation efficiency, its magnetic field, temperature, and carrier concentration dependence in doped GaAs, a model system in the field of nuclear spin physics in semiconductors [3-5]. We also analyzed the interplay between four relevant relaxation mechanisms: hyperfine interaction, quadrupole interaction, spin diffusion and Korringa mechanisms.Understanding of field dependence of NSS dynamics allowed us to obtain a new insight into the NSS thermodynamics, and verify the spin temperature concept in GaAs bulk material and microcavities [6]. We have demonstrated, that NSS exactly follows the predictions of the spin temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalization in quantum dots [7]. Our results 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.This work is supported by a joint grant of the Russian Foundation for Basic Research (RFBR, Grant No. 16-52-041301-4 and National Center for Scientific Research (CNRS, PRC SPINCOOL No. 148362)References:[1] R. Giri et al, Physical Review Letters, 111, 087603 (2013)[2] I. I. Ryzhov et al, Applied Physics Letters, 106, 242405 (2015)[3] M. Kotur et al, Physical Review B, 94(8), 081201 (2016)[4] M. Vladimirova et al, Physical Review B, 95, 125312 (2017)[5] M. Kotur et al, eprint arXiv:1802.05013 (2018)[6] M. Vladimirova et al, Physical Review B, 97, 041301 (2018)[7] P. Maletinsky et al, Nature Physics, 5, 407 (2009)

International audience; Excitons in wide nitride Quantum Wells (QWs) are naturally indirect due to the strong internal electric field: within such excitons the electron and the hole are spatially separated, resulting in strong dipole moments and long radiative lifetimes. These properties offer the possibility to explore the collective exitonic phases with complex phase diagrams predicted theoretically in the temperature-density-dipole moment parameter space. Here we study 8 nm-wide GaN/(AlGa)N QWs presenting 1MV/cm built-in electric field in the growth direction, microsecond-long radiative lifetimes, 8 meV exciton emission linewidth and 100 µm transport lengths at cryogenic temperatures. To address the quasi-equilibrium properties of the strongly dipolar excitons (i.e. whose electric dipoles are twice as long as their Bohr radii d = 2aB), we confine them in the QW’s plane using electrostatic traps with a semi-transparent Schottly electrode. These traps consist in semitransparent electrodes deposited on the sample’s surface. We make micro-photoluminescence (µPL) measurements (less than 1µm- resolution) upon point-like optical excitation. We thus obtain the spectrally resolved information on the emission of the excitons as a function of power and density. Our experiments demonstrate an efficient trapping of excitons in the plane of the QW, with a trapping potential of an order of 50 meV. We demonstrate that it is possible to uniformly fill areas up to hundreds of microns in length with excitons and/or electron hole pairs in the QW’s plane. At T=4K, the emission from these areas exhibits a peculiar behavior. At low densities, such that there is no measurable screening of the built-in electric field, the emission from the trap is weak and spatially homogeneous. However, for weak built-in electric field screenings (as low as 10 meV, ~1011 cm-2 pair density), the PL brightens up and becomes spatially inhomogeneous – droplet like. We tentatively interpret this fragmentation as the coexistence of the gas and liquid phases of the excitons. As expected, the fragmented character of the emission disappears when either power and/or temperature is further increased, due to either exciton heating (gas phase), ionization or dissociation (Mott transition). Our results suggest that excitons in GaN QWs are promising candidates for the study of collective phenomena in cold dipolar bosonic systems. The advantages of the nitrides over arsenides include both higher critical temperatures for the onset of collective phenomena, and the ability to increase the dipolarity of systems based on their small exciton Bohr radius.

International audience; Cooling of nuclear spin system (NSS) in doped semiconductors via dynamic polarization by optical pumping is a powerful method for harnessing ubiquitous fluctuations of nuclear spin. The idea of NSS cooling is based on the hypothesis of spin temperature, which states that NSS reaches an internal thermal equilibrium long before it comes to equilibrium with the external bath (crystal lattice). Although thermodynamic framework has been successfully employed for the description of a variety of the experimental data, a rigorous check of this concept in semiconductors was impossible until recently, in particular at low magnetic field. The reason for that is the lack of experimental techniques allowing nonperturbative optical control over adiabatic transformation of the NSS. We have recently developed such methods, based on off-resonant Faraday rotation and spin noise spectroscopy [1,2]. Using these techniques, combined with photoluminescence spectroscopy, we established a comprehensive picture of the nuclear spin relaxation efficiency, its magnetic field, temperature, and carrier concentration dependence in both n- and p-doped GaAs, a model system in the field of nuclear spin physics in semiconductors [3-5]. We also analyzed the interplay between four relevant relaxation mechanisms: hyperfine interaction, quadrupole interaction, spin diffusion towards paramagnetic impurities, and Korringa mechanisms. Figure 1 illustrates these processes in a nuclei-electron coupled system. Understanding of field dependence of NSS dynamics allowed us to obtain a new insight into the NSS thermodynamics, and verify the spin temperature concept in GaAs bulk material and microcavities [6]. We have demonstrated that NSS exactly follows the predictions of the spin temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalization in quantum dots [7]. These results 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. References[1] R. Giri et al, Physical Review Letters, 111, 087603 (2013)[2] I. I. Ryzhov et al, Applied Physics Letters, 106, 242405 (2015)[3] M. Kotur et al, Physical Review B, 94, 081201(R) (2016)[4] M. Vladimirova et al, Physical Review B, 95, 125312 (2017)[5] M. Kotur et al, Physical Review B, 97, 165206 (2018)[6] M. Vladimirova et al, Physical Review B, 97, 041301 (2018)[7] P. Maletinsky et al, Nature Physics, 5, 407 (2009)

International audience; 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. 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 the indirect excitons. The excitonic transport was first observed at low temperature in the case of hetero-epitaxy of the GaN QWs on sapphire substrates [1]. The improvement of the sample quality through homo-epitaxy on GaN substrates allowed observing 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 (deduced from the local blue shift of the QW transition), with a length scale of 100µm at 10K and 30µm at T=300K in our most recent homo-epitaxial samples. 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 exciton collective states. The realization of an excitonic trap is a first step forward, and we recently investigated the exciton transport in electrostatic traps formed when a semi-transparent gate is deposited on top of the QW sample. Trapping of the excitons is observed, 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.

International audience; 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.

International audience; Dynamic Nuclear Polarization and Nuclear Fields

International audience; We present spatially- and time-resolved photoluminescence experiments on polar GaN/AlGaN quantum well structures,showing exciton propagation distance of tens of micrometers from generation position and exciton radiative lifetimes oftens of microseconds. We study in detail the dynamics and range of their relaxation/diffusion along the growth plane, asa function of temperature.

Mn spin dynamics is measured in very diluted (Cd,Mn)Te crystals by time-resolved Kerr rotation. Spin beats due to the hyperfine interaction between the 3d electrons of the Mn ions and their own nuclei are detected. It is shown that the effect of the crystal field can be strongly suppressed for "magic" orientations of the magnetic field. This particular orientation of the field permits the optical read-out of the Mn nuclear spin state. Manganese ions trapped on a semiconductor lattice have uniform properties and relatively long spin lifetimes, which make them promising for optical manipulation. In particular, Mn2+ ions embedded in a II-VI semiconductor are S-state ions, weakly coupled to the lattice. For this reason quite long Mn electronic spin relaxation times are expected and have been observed [1]. Here we show that the spin coherence time is mainly limited by dipole-dipole interactions at a Mn concentration x=0.001, and reaches up to 15 ns. At this low concentration, fine and hyperfine structures of Mn2+ are resolved in electron spin resonance experiments. In the time-domain it corresponds to low-frequency beats as shown in Fig. 1.

International audience; We propose a new experimental technique for the optical study of dark and bright spatially indirect excitons in coupled quantum wells. We demonstrate that indirect excitons strongly contribute to nonlinear optical effects: photoinduced reflectivity and Kerr rotation, detected at the direct exciton transition frequency. In contrast to conventional photoluminescence measurements which are sensitive only to optically active |±1> states inside the light cone, the proposed technique allows to access yet hidden dark components of excitonic systems.

International audience

International audience; Nuclear spin dynamics in semiconductors has been studied extensively by optical means for more than forty years. Yet generally the existing techniques require the injection of out of equilibrium carriers in the semiconductor in order to probe the nuclear polarization, which in turn disturbs the nuclei to be measured. This is the case for photoluminescence experiments, but also for time resolved Faraday/Kerr rotation, in which a shift in the electron Larmor frequency is induced by the nuclear field.1 In this work, we demonstrate a non-destructive detection of nuclear magnetization in n-GaAs via cavity enhanced Faraday rotation of an off-resonant light beam. We use a two-step protocol, as proposed by Artemova and Merkulov2, where preparation and measurement are separated in time. The nuclear spins are first cooled down by optical pumping, then the pump beam is switched off and the subsequent evolution of nuclear polarization is detected by the Faraday rotation of a below band gap probe beam. This experimental protocol applied to metallic GaAs just above the metal-insulator transition reveals a non- monotonous evolution of the nuclear induced Faraday rotation. We interpret this behavior as a consequence of the existence of two contributions in the Faraday rotation, which are related to optical transitions involving either localized or delocalized states. Therefore, this detection scheme turns out to be sensitive to nuclear spins situated within the localization radius of bound electron states, as well as to all other nuclear spins in the sample. The contributions to the Faraday rotation of different nuclear species with different localizations in the sample are identified in nuclear magnetic resonance experiments. Our results suggest that the nuclear relaxation in metallic n-GaAs is dominated by diffusion towards fast relaxing centers, rather than by the Korringa mechanism.

International audience; We report time-resolved Kerr rotation experiments on bulk CdMnTe with very low Mn concentration. We observe spin beatings which are the result of a complex spin dynamics of the d-electrons in the hyperfine field of their nucleus and the tetragonal crystal field. We find the transverse Mn spin relaxation time to increase with the increase of the effective temperature of Mn subsystem. At higher temperatures we measure T2 up to 12 ns, which is much longer than any previously measured values.

Place branding and country branding are evolving concepts, which continue to engage attention. This paper reviews the conceptual foundation of place branding and its application to public relations and corporate communication practices in the island state of Malta, a popular tourist destination in the centre of the Mediterranean Sea. The Maltese authorities use communication practices to develop Brand Malta and the study shows how the image of Malta is used to position its offer in the valuable UK market. Tourism is vital to the Maltese economy and the authorities need to create a favourable image and a strong brand. A number of interviews were carried out with representatives from the Malta brand promoter, Malta Tourism Authority (MTA) to establish the brand strategies and communications approaches adopted for rebranding and repositioning in the market. In addition two tour operators were contacted, who specialize in promoting Malta, to illustrate the practices of the private sector and their coordination with the Maltese authorities. The findings are analysed in accordance with the theoretical frameworks featured in the literature. The studies show that Malta has achieved some success in developing and sustaining the brand in a competitive tourism market. Niche propositions and added value have been important. A coherent approach to public relations and a corporate communication rationale do provide a strategic framework which helps competitiveness and sustains overall value.

Faraday rotation is a well-known magneto-optical phenomenon: polarization plane of the light wave is rotated upon transmission through a magnetized medium. It is believed that most efficient Faraday rotators are transparent magnetic materials, such as rare-earth doped glasses and garnets, as well as diluted magnetic semiconductors, where magnetic ions create strong magnetization. In contrast, in conventional non-magnetic semiconductors the rotation due to magnetization of electron gas in the presence of equivalent magnetic field is much smaller. We show, that using optical orientation of electron gas in GaAs confined in a microcavity (Q=50000) it is possible to reach the rotation angles larger than 10° at only 1% spin polarization of the electron gas and in the absence of magnetic field ( Fig. 1 ). This strong rotation exceeds by orders of magnitude the rotation ever measured in bulk n-GaAs samples of similar concentration () [1] [2] . We deduce from these experiments the Verdet constant associated with the electron spin polarization density. This also allows for the direct detection of the extremely slow electron spin polarization decay (~250 ns) in the absence of the pump ( Fig. 2 ). It suggests that spin relaxation time in the dark can be much longer than the relaxation time extracted from Hanle depolarization in either photoluminescence [3] or Faraday rotation ( Fig. 1 ) experiments.

International audience; The injection of nonequilibrium carriers in a paramagnetic diluted magnetic semiconductor in the presence of magnetic field may induce a spontaneous magnetization patterning. This phenomenon is studied in CdMnTe quantum wells using time resolved Kerr rotation experiments and the model based on the coupled rate equations for interacting electron and magnetic ion spins is developed.

International audience; Time-domain spin resonances in p-doped CdMnTe quantum wells have been studied via time-resolved magneto-optical Kerr rotation. The resonances related to quantum well electrons Mn and electrons excited in the substrate are identified. An additional spin resonance which shows an unusual temperature dependence is detected in one of the samples. The corresponding frequency tends to zero when approaching the Curie temperature. This "soft" spin precession mode has been studied as a function of magnetic field, temperature, magnetic field orientation, and optical excitation density. Its origin is discussed in the framework of the existing collective spin excitation model.