Direct surface cyclotron resonance terahertz emission from a quantum cascade structure

A strong magnetic field applied along the growth direction of a semiconductor quantum well gives rise to a spectrum of discrete energy states, the Landau levels. By combining quantum engineering of a quantum cascade structure with a static magnetic field, we can selectively inject electrons into the excited Landau level of a quantum well and realize a tunable surface emitting device based on cyclotron emission. By applying the appropriate magnetic field between 0 and 12 T, we demonstrate emission from a single device over a wide range of frequencies (1-2 THz and 3-5 THz)

Disorder-perturbed Landau levels in high electron mobility epitaxial graphene

We show that the Landau levels in epitaxial graphene in presence of localized defects are significantly modified compared to those of an ideal system. We report on magneto-spectroscopy experiments performed on high quality samples. Besides typical interband magneto-optical transitions, we clearly observe additional transitions that involve perturbed states associated to short-range impurities such as vacancies. Their intensity is found to decrease with an annealing process and a partial self-healing over time is observed. Calculations of the perturbed Landau levels by using a delta-like potential show electronic states both between and at the same energies of the Laudau levels of ideal graphene. The calculated absorption spectra involving all perturbed and unperturbed states are in very good agreement with the experiments

Photocurrent measurements in a Quantum Cascade Detector under strong magnetic field

International audienceIn the present work, we performed photocurrent measurement on a quantum cascade detector structure under strong magnetic field applied parallel to the growth axis. The photocurrent shows strong oscillations as a function of B. We develop a model in order to describe current as a function of magnetic field. The excellent agreement with the experimental data supports the idea that an elastic scattering process plays a central role in the behavior of those structures. Thanks to zero magnetic field consideration, we establish that dominant process is impurities scattering process. These experiments lead to the key parameters to understand and optimize those structure further

Magnetooptical determination of a topological index

When a Dirac fermion system acquires an energy-gap, it is said to have either trivial (positive energy-gap) or non-trivial (negative energy-gap) topology, depending on the parity ordering of its conduction and valence bands. The non-trivial regime is identified by the presence of topological surface or edge-state dispersing in the energy gap of the bulk and is attributed a non-zero topological index. In this work, we show that such topological indices can be determined experimentally via an accurate measurement of the effective velocity of bulk massive Dirac fermions. We demonstrate this analytically starting from the Bernevig-Hughes-Zhang Hamiltonian (BHZ) to show how the topological index depends on this velocity. We then experimentally extract the topological index in Pb1-xSnxSe and Pb1-xSnxTe using infrared magnetooptical Landau level spectroscopy. This approach is argued to be universal to all material classes that can be described by a BHZ-like model and that host a topological phase transition.Comment: Accepted for publication in Nature Partner Journal Quantum Material

Dark current analysis of Quantum Cascade Detectors by Magneto-Resistance measurements

International audienceMagneto-transport experiments have been performed on Quantum Cascade Detectors. These experiments lead to the identification of the different electronic transitions from subbands in one cascade period to subbands in the following one. These transitions contribute to the total current flowing through the structure in the absence of illumination. This dark current is well described within a simple model based on the sum of diffusion events from one cascade to the next one through optical phonon mediated transitions. For the first time, the optical and electronic properties of such a complex heterostructure can be fully predicted without any other adjustable parameter than the doping density. This opens the way to a full quantum design of an infrared detector, in contrast with the phenomenological optimization of structures usually performed in this field

High Electron Mobility in Epitaxial Trilayer Graphene on Off-axis SiC(0001)

International audienceThe van de Waals heterostructure formed by an epitaxial trilayer graphene is of particular interest due to its unique tunable electronic band structure and stacking sequence. However, to date, there has been a lack in the fundamental understanding of the electronic properties of epitaxial trilayer graphene. Here, we investigate the electronic properties of large-area epitaxial trilayer graphene on a 4° off-axis SiC(0001) substrate. Micro-Raman mappings and atomic force microscopy (AFM) confirmed predominantly trilayer on the sample obtained under optimized conditions. We used angle-resolved photoemission spectroscopy (ARPES) and Density Functional Theory (DFT) calculations to study in detail the structure of valence electronic states, in particular the dispersion of π bands in reciprocal space and the exact determination of the number of graphene layers. Using far-infrared magneto-transmission (FIR-MT), we demonstrate, that the electron cyclotron resonance (CR) occurs between Landau levels with a (B)1/2 dependence. The CR line-width is consistent with a high Dirac fermions mobility of ~3000 cm2·V−1·s−1 at 4 K

Photocurrent analysis of quantum cascade detectors by magnetotransport

to be published in Phys. Rev. BInternational audiencePhotocurrent measurements have been performed on a quantum cascade detector structure under strong magnetic field B applied parallel to the growth axis. The photocurrent shows oscillations as a function of B. In order to describe this behavior, we have developed a rate equation model. The interpretation of the experimental data supports the idea that an elastic scattering contribution plays a central role in the behavior of these structures. We present a calculation of the electron lifetime versus magnetic field which suggests that impurities scattering in the active region is the limiting factor. These experiments lead to a better understanding of these complex structures and identify key parameters to optimize them further

Magnetotransport in quantum cascade detectors: analyzing the current under illumination

Photocurrent measurements have been performed on a quantum cascade detector structure under strong magnetic field applied parallel to the growth axis. The photocurrent shows oscillations as a function of B. In order to describe that behavior, we have developed a rate equation model. The interpretation of the experimental data supports the idea that an elastic scattering contribution plays a central role in the behavior of those structures. We present a calculation of electron lifetime versus magnetic field which suggests that impurities scattering in the active region is the limiting factor. These experiments lead to a better understanding of these complex structures and give key parameters to optimize them further

3D Topological Semimetal Phases of Strained α\alpha-Sn on Insulating Substrate

$\alpha$-Sn is an elemental topological material, whose topological phases can be tuned by strain and magnetic field. Such tunability offers a substantial potential for topological electronics. However, InSb substrates, commonly used to stabilize $\alpha$-Sn allotrope, suffer from parallel conduction, restricting transport investigations and potential applications. Here, the successful MBE growth of high-quality $\alpha$-Sn layers on insulating, hybrid CdTe/GaAs(001) substrates, with bulk electron mobility approaching 20000 cm$^2$V$^{-1}$s$^{-1}$ is reported. The electronic properties of the samples are systematically investigated by independent complementary techniques, enabling thorough characterization of the 3D Dirac (DSM) and Weyl (WSM) semimetal phases induced by the strains and magnetic field, respectively. Magneto-optical experiments, corroborated with band structure modeling, provide an exhaustive description of the bulk states in the DSM phase. The modeled electronic structure is directly observed in angle-resolved photoemission spectroscopy, which reveals linearly dispersing bands near the Fermi level. The first detailed study of negative longitudinal magnetoresistance relates this effect to the chiral anomaly and, consequently, to the presence of WSM. Observation of the $\pi$ Berry phase in Shubnikov-de Haas oscillations agrees with the topologically non-trivial nature of the investigated samples. Our findings establish $\alpha$-Sn as an attractive topological material for exploring relativistic physics and future applications.Comment: Main text: 35 pages, 7 figures; Supplementary Materials: 22 pages, 12 figure

SYSTEMES D'ELECTRONS DANS LES NANOSTRUCTURES SEMI-CONDUCTRICES A CONFINEMENT QUANTIQUE DANS 2 OU 3 DIRECTIONS

CETTE THESE CONSTITUE UNE ETUDE, PAR MAGNETOSPECTROSCOPIE AUX ONDES MILLIMETRIQUES, SUBMILLIMETRIQUES ET DANS L'INFRAROUGE LOINTAIN, DE LA STRUCTURE ELECTRONIQUE DES SYSTEMES CONFINES DE SEMI-CONDUCTEURS TELS QUE LES FILS (SYSTEMES 1D) OU LES BOITES QUANTIQUES (SYSTEMES 0D). UNE FACON DE REALISER DES STRUCTURES DE DIMENSIONS SUBMICRONIQUES ET NANOMETRIQUES EST DE MODULER LATERALEMENT LA DENSITE ELECTRONIQUE D'UN GAZ BIDIMENSIONNEL D'ELECTRONS (GE2D) CONTENU DANS UNE HETEROSTRUCTURE A DOPAGE SELECTIF. CETTE MODULATION EST OBTENUE PAR LITHOGRAPHIE ELECTRONIQUE SUIVIE D'UNE GRAVURE IONIQUE. NOUS AVONS ETUDIE, D'UNE PART, LE CONFINEMENT ET LA POPULATION ELECTRONIQUE DANS DES FILS GRAVES GAAS/A1GAAS. D'AUTRE PART, NOUS AVONS ETUDIE L'INFLUENCE DE LA PROFONDEUR DE GRAVURE SUR LA DENSITE D'UN GE2D CONTENU DANS UNE HETEROSTRUCTURE SI/SIGE. CETTE ETUDE THEORIQUE ET EXPERIMENTALE A MONTRE QU'IL ETAIT POSSIBLE, SELON LA PROFONDEUR DE BARRIERE DOPEE GRAVEE, DE REALISER TOUTES LES ETAPES DE MODULATION DU GAZ BIDIMENSIONNEL, A SAVOIR UN GAZ BIDIMENSIONNEL FAIBLEMENT MODULE, PUIS UN RESEAU D'ANTIPLOTS (GE2D COMPORTANT UN RESEAU PERIODIQUE DE ZONES VIDES D'ELECTRONS) ET ENFIN UN RESEAU DE BOITES PARFAITEMENT ISOLEES LES UNES DES AUTRES DONT LA TAILLE EFFECTIVE DIMINUE FORTEMENT AVEC LA GRAVURE. UNE AUTRE MANIERE D'OBTENIR DES SYSTEMES CONFINES D'ELECTRONS EST LA CROISSANCE AUTO-ORGANISEE UTILISANT LE GRAND DESACCORD DE MAILLE ENTRE DEUX MATERIAUX. NOUS AVONS ETUDIE LE COUPLAGE ENTRE LES ELECTRONS ET LES PHONONS OPTIQUES DANS DES BOITES QUANTIQUES INAS, NE CONTENANT QU'UN SEUL ELECTRON, OBTENUES SUR UN SUBSTRAT GAAS. DANS LES SEMI-CONDUCTEURS MASSIFS, LES GAZ BIDIMENSIONNELS OU LES FILS QUANTIQUES, LES PHONONS OPTIQUES JOUENT UN ROLE FONDAMENTAL. EN EFFET, L'EMISSION DE PHONONS OPTIQUES EST LE MECANISME DE LOIN LE PLUS EFFICACE POUR LA RELAXATION DES ELECTRONS PLACES DANS DES NIVEAUX EXCITES. NOUS AVONS MONTRE L'EXISTENCE D'UN COUPLAGE FORT ENTRE LES ELECTRONS ET LES PHONONS AU SEIN DES BOITES QUANTIQUES CONDUISANT A LA FORMATION D'ETATS HYBRIDES ELECTRON-PHONON(S) : DES POLARONS A LONGUE VIE. LA RELAXATION DES PORTEURS PAR EMISSION DE PHONONS OPTIQUES EST ALORS IMPOSSIBLE DANS LES BOITES QUANTIQUES. CES POLARONS CONSTITUENT UN ASPECT NOUVEAU DE L'INTERACTION ELECTRON-PHONON DANS LES SEMI-CONDUCTEURS QUI POURRAIT AVOIR DES IMPLICATIONS IMPORTANTES SUR LA RELAXATION DES PORTEURS DANS LES NANOSTRUCTURES.PARIS-BIUSJ-Thèses (751052125) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF