Dipolar, or indirect excitons (IXs) offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of such excitons. Indeed, compared to their counterparts in GaAs-based heterostructures, IXs in nitrides possess two key advantages. They have higher binding energies, and they are “naturally” indirect. This means, that they can be engineered and manipulated in as-grown heterostructures, even without application of an external electric bias. In this work we demonstrate (i) the propagation of IXs over hundreds of micrometers in the plane of GaN/(AlGa)N quantum wells, (ii) confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface (iii) the electrical control of the IX fluxes via an applied gate voltage. Once confined, the exciton fluid accumulates in uniform zones. By modeling the emission spectra of these zones, we get insight into the thermodynamic properties of the thermalized exciton fluid. These results constitute a prerequisite for realization of the GaN-based excitonic devices, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

Dipolar, or indirect excitons (IXs) offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of such excitons. Indeed, compared to their counterparts in GaAs-based heterostructures, IXs in nitrides possess two key advantages. They have higher binding energies, and they are “naturally” indirect. This means, that they can be engineered and manipulated in as-grown heterostructures UP TO ROOM TEMPERATURES, even without application of an external electric bias. In this work we demonstrate (i) the propagation of IXs over hundreds of micrometers in the plane of GaN/(AlGa)N quantum wells, (ii) confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface (iii) the electrical control of the IX fluxes via an applied gate voltage. These results constitute a prerequisite for realization of the GaN-based excitonic devices, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

Graphene grown epitaxially on SiC, close to the charge neutrality point (CNP), in an orthogonal magnetic field shows an ambipolar behavior of the transverse resistance accompanied by a puzzling longitudinal magnetoresistance. When injecting a transverse current at one end of the Hall bar, asizeable non local transverse magnetoresistance is measured at low temperature. While Zeeman spin effect seems not to be able to justify these phenomena, some dissipation involving edge states at the boundaries could explain the order of magnitude of the non local transverse magnetoresistance, but not the asymmetry when the orientation of the orthogonal magnetic field is reversed. As a possible contribution to the explanation of the measured non local magnetoresistance which is odd in the magnetic field, we derive a hydrodynamic approach to transport in this system, which involves particle and hole Dirac carriers, in the form of charge and energy currents. We find that thermal diffusion can take place on a large distance scale, thanks to long recombination times, provided a non insulating bulk of the Hall bar is assumed, as recent models seem to suggest in order to explain the appearance of the longitudinal resistance. In presence of the local source, some leakage of carriers from the edges generates an imbalance of carriers of opposite sign, which are separated in space by the magnetic field and diffuse along the Hall bar generating a non local transverse voltage.

HgTe quantum wells (QWs) are two-dimensional semiconductor systems that change their properties at the critical thickness dc, corresponding to the band inversion and topological phase transition. The motivation of this work was to study magnetotransport properties of HgTe QWs with thickness approaching dc, and examine them as potential candidates for quantum Hall effect (QHE) resistance standards. We show that in the case of d > dc (inverted QWs), the quantization is influenced by coexistence of topological helical edge states and QHE chiral states. However, at d ≈ dc, where QW states exhibit a graphene-like band structure, an accurate Hall resistance quantization in low magnetic fields (B ≤ 1.4 T) and at relatively high temperatures (T ≥ 1.3 K) may be achieved. We observe wider and more robust quantized QHE plateaus for holes, which suggests—in accordance with the “charge reservoir” model—a pinning of the Fermi level in the valence band region. Our analysis exhibits advantages and drawbacks of HgTe QWs for quantum metrology applications, as compared to graphene and GaAs counterparts.

Today, in the presence of global warming, understanding how plants respond to drought stress is essential to meet the challenge of developing new cultivars and new irrigation strategies, consistent with the maintenance of crop productivity. In this context, the study of the relation between plants and water is of central interest for modeling their responses to biotic and abiotic constraints. Paradoxically, there are very few direct and noninvasive methods to quantify and measure the level and the flow of water in plants. The present work aims to develop a noninvasive methodology for living plant based on nuclear magnetic resonance (NMR) at low magnetic field and imaging (MRI) to tackle the issue of water quantity in plants. For this purpose, a portable NMR device measuring the signal level at 8 mT was built. This instrument addresses specific challenges such as miniaturization, accessibility, and overheating in order to maintain the plant intact of time over long period. Time dependence of the water content in sorghum plants is reported under abiotic stress as well as the fraction of transpirable soil water and the photosynthesis activity through the leaves. At high magnetic field (9.4 T), T2 maps were acquired on the same sorghum plants at two time points. The combination of these approaches allows us to identify ecophysiological biomarkers of drought stress. One particular interesting result concerns the spatial distribution of water in two anatomically contrasted sorghum genotypes.