Nuclear magnetism inn-doped semiconductors with a positive hyperfine constant is revisited. Two kindsof nuclear magnetic ordering can be induced by resident electrons in a deeply cooled nuclear spin system.At positive nuclear spin temperature below a critical value, randomly oriented nuclear spin polarons similarto that predicted by Merkulov [Phys. Solid State40, 930 (1998)] should emerge. These polarons are orientedrandomly, and within each polaron, nuclear and electron spins are aligned antiferromagnetically. At negativenuclear spin temperature below a critical value, we predict another type of magnetic ordering—a dynamicallyinduced nuclear ferromagnet. This is a long-range ferromagnetically ordered state involving both electrons andnuclei. It can form if electron spin relaxation is dominated by the hyperfine coupling, rather than by the spin-orbitinteraction. Application of the theory to then-doped GaAs suggests that ferromagnetic order may be reached atexperimentally achievable nuclear spin temperatureN≈−0.5μK and lattice temperatureTL≈5K.

Spatiotemporal spin noise spectroscopy is combined with dynamic light scattering in order to reach spatial resolutions down to ∼λ/10. Applied to a system of localized electron spins, an insulating n-doped CdTe layer, this allows us to reveal long spin jump distances ∼ 2.7 μm. Spin noise spectra at large wave vectors q (q 1) provide a snapshot of the spin dynamics before jump (therefore not affected by spin motion), while at smaller q, spin motion sets in. This allows us to unravel the contributions of spin-orbit and hyperfine fields in the electron spin relaxation and to determine self-consistently all parameters relevant to the spin dynamics. We propose a phenomenological equation inspired by studies of atomic jump diffusion by neutron scattering, which includes the relevant spin relaxation mechanisms and the effect of time of flight of the spin fluctuation across the laser spot. This modeling reproduces all experimental results.

Planar UV-C LEDs still suffer from low efficiency, mainly due to substrate crystalline quality, p doped conductivity and extraction efficiency. One possible way to overcome partly these issues is to realize the whole UV structure on AlGaN pyramids by selective area growth in order to benefit from the advantages of such structures, i.e. the dislocation filtering and the semi polar planes. We present here a detailed study about the epitaxy of AlGaN nano-sized pyramids by metal organic vapor phase epitaxy (MOVPE) on patterned templates presenting different holes apertures and pitches as 1.5 µm and 4 µm or 100 nm and 250 nm respectively. While increasing the Al content, their height decreases while the thickness of the deposition on the mask increases whatever the design of the mask. Those changes of the pyramid shapes and deposition are directly linked to the properties of Al adatoms, i.e. low Al diffusion length. Using the conventional growth mode for the epitaxy of those pyramids did not permit the incorporation of Al from the base of the pyramids to their truncated apex. Its presence was concentrated on the edges and top of the pyramids. On the contrary, a pulsed growth mode, coupled with a strongly reduced pitch, allowed an incorporation of Al since the base of the nanopyramid, and a decrease of the deposition height on the mask. These results can be explained by the desorption of Ga species, due to the presence of H2 in the reactor chamber during the step without the metal precursors, leading to a higher Al/Ga ratio. It is even enhanced inside the holes by the reduced pitch.

The energy and its entire related infrastructure are the main drivers for a economic development andforensuringagoodlevelofemployment.Aspartofaglobalstudyaboutinternationalenergysector,weevaluate here the impact of technological changes on the state of the energy infrastructure. This studyincludes a detailed analysis of the global challenges facing the energy industry. We propose scenariosfor the development of a modernized energy infrastructure with an assessment of the entire energysystem. Our evaluation indicators are chosen in terms of the reliability of the energy infrastructure, ofits quality, of the accidents that may happen and of the consequent environmental risks. This study isparticularly adapted for forecasting the necessary measures (of technical nature, of governance kind)that have to be implemented to reduce the acceleration of the infrastructure deterioration rate. Ourresults reveal that the use of digital and information technologies has many positive impacts on thedevelopment and on the control of an efficient consumption of energy. In addition, our predictions,due to the further modernization of the energy sector, can contribute and help in the creation of preconditions that will be highly stimulating and profitable to the growth of investments in the energy infrastructure.

Twisted two-dimensional bilayer anisotropy materials exhibit many exotic physical phenomena. Manipulating the “twist angle” between the two layers enables the hybridization phenomenon of polaritons, resulting in fine control of the dispersion engineering of the polaritons in these structures. Here, combined with the hybridization phenomenon of anisotropy polaritons, we study theoretically the near-field radiative heat transfer (NFRHT) between two twisted hyperbolic systems. These two twisted hyperbolic systems are mirror images of each other. Each twisted hyperbolic system is composed of two graphene gratings, where there is an angle φ between these two graphene gratings. By analyzing the photonic transmission coefficient as well as the plasmon dispersion relation of the twisted hyperbolic system, we prove the enhancement effect of the topological transitions of the surface state at a special angle [from open (hyperbolic) to closed (elliptical) contours] on radiative heat transfer. Meanwhile the role of the thickness of dielectric spacer and vacuum gap on the manipulating the topological transitions of the surface state and the NFRHT are also discussed. We predict the hysteresis effect of topological transitions at a larger vacuum gap, and demonstrate that as the thickness of the dielectric spacer increases, the transition from the enhancement effect of heat transfer caused by the twisted hyperbolic system to a suppression.

Antiferromagnetic materials are promising platforms for next-generation spintronics owing to their fast dynamics and high robustness against parasitic magnetic fields. However, nanoscale imaging of the magnetic order in such materials with zero net magnetization remains a major experimental challenge. Here we show that non-collinear antiferromagnetic spin textures can be imaged by probing the magnetic noise they locally produce via thermal populations of magnons. To this end, we perform nanoscale, all-optical relaxometry with a scanning quantum sensor based on a single nitrogen-vacancy (NV) defect in diamond. Magnetic noise is detected through an increase of the spin relaxation rate of the NV defect, which results in an overall reduction of its photoluminescence signal under continuous laser illumination. As a proof-of-concept, the efficiency of the method is demonstrated by imaging various spin textures in synthetic antiferromagnets, including domain walls, spin spirals and antiferromagnetic skyrmions. This imaging procedure could be extended to a large class of intrinsic antiferromagnets and opens up new opportunities for studying the physics of localized spin wave modes for magnonics.