We report on magnetospectroscopy of HgTe quantum wells in magnetic fields up to 45 T in a temperature range from 4.2 up to 185 K. We observe intra- and inter-band transitions from zero-mode Landau levels, which split from the bottom conduction and upper valence sub-bands and merge under the applied magnetic field. To describe experimental results, realistic temperature-dependent calculations of Landau levels have been performed. We show that although our samples are topological insulators at low temperatures only, the signature of such a phase persists in optical transitions at high temperatures and high magnetic fields. Our results demonstrate that temperature-dependent magnetospectroscopy is a powerful tool to discriminate trivial and topological insulator phases in HgTe quantum wells.

Miniaturized total analysis systems are becoming a powerful tool for analytical and bioanalytical applications. Biological and chemical sensors for health and environment demand an adaptable technology [1]. A flexible and low-cost process using good quality material in terms of mechanical and thermal properties is necessary. Hybrid materials mixing organic and inorganic parts, offer the advantages of polymer-like materials and glasses. In this work, microfluidic sensor has been fabricated on a new hybrid organic-inorganic photosensitive material using an accurate and flexible fabrication process based on a pulsed UV laser lithography. The photosensitive material is based on, poly (amic acid) PAA polymer with lower molecular weight, 1,3,5-tris[2-vinyloxy)ethoxy]benzene TVEB as a crosslinking dissolution inhibitor, a VEPTES pre-hydrolysed as an organic-inorganic material, and a PAG as a photoacid generator. A fine positive pattern was fabricated in a 3 μm thick film with … mJ/cm2 of i-line exposure.This work presents the material synthesis, and the characterization of the mechanical and thermal properties. The nanoindentation technique was used to analyze the force required to indent the coating with a diamond tip. At low forces, this technique, based on indentation depth, predicts the hardness and the elastic modulus of the coating, to study the effect of the addition of the mineral part on the mechanical properties of the layer. On the other hand, thermogravimetric analysis (TGA) technique was used to study the thermal properties of the new hybrid material as function of inorganic part. These results indicate that the newly prepared photosensitive hybrid materials may have potential applications for microfluidic devices.

Nuclear Magnetic Resonance spectroscopy and imaging can be classified as inductive techniques working in the near- to far-field regimes. We investigate an alternative capacitive detection with the use of micrometer sized probes positioned at sub wavelength distances of the sample in order to characterize and model evanescent electromagnetic fields originating from NMR phenomenon. We report that in this experimental configuration the available NMR signal is one order of magnitude larger and follows an exponential decay inversely proportional to the size of the emitters. Those investigations open a new road to a better understanding of the evanescent waves component in NMR with the opportunity to perform localized spectroscopy and imaging.

Ce document présente une séance de travaux pratiques de caractérisation d'un magnétomètre à induction haute performance destinée aux étudiants de M2 OSAE de l'Observatoire de Meudon. Ce TP se déroule sur une journée et nécessite du matériel spécifique (bobines Helmholtz, analyseur de spectre).Le TP a pour but de comparer deux modes de fonctionnement du magnétomètre inductif permettant de s'affranchir de la résonance: la contre-réaction de flux et l'amplification en courant. Le TP permet de réaliser des caractérisations en champ magnétique et donner un cadre concret à des caractérisations de bruit.

It has recently been shown that electronic states in bulk gapless HgCdTe offer another realization of pseudo-relativistic three-dimensional particles in condensed matter systems. These single valley relativistic states, massless Kane fermions, cannot be described by any other relativistic particles. Furthermore, the HgCdTe band structure can be continuously tailored by modifying cadmium content or temperature. At critical concentration or temperature, the bandgap collapses as the system undergoes a semimetal-to-semiconductor topological phase transition between the inverted and normal alignments. Here, using far-infrared magneto-spectroscopy we explore the continuous evolution of band structure of bulk HgCdTe as temperature is tuned across the topological phase transition. We demonstrate that the rest mass of Kane fermions changes sign at critical temperature, whereas their velocity remains constant. The velocity universal value of (1.07±0.05)x1E6 m/s remains valid in a broad range of temperatures and Cd concentrations, indicating a striking universality of the pseudo-relativistic description of the Kane fermions in HgCdTe.

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.