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)

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

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

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

Etude des propriétés ferromagnétiques de (Ga, Mn) As au moyen de l'hydrogénation

Le (Ga,Mn)As est un matériau de choix pour l'électronique de spin, étant à la fois semiconducteur et ferromagnétique, à la suite de l'interaction d'échange entre les spins des atomes de manganèse (3d54s2), et ceux des trous qu'ils apportent. Sa température de Curie (Tc) se situe typiquement entre 50 et 150 K. A_n de découpler les dopages magnétique (xMn) et électrique (p), une technique de passivation par hydrogène a été développée, pour former des complexes (Mn,H) électriquement neutres. Ses e_ets magnétiques (apparition d'une phase paramagnétique), électriques (diminution de p) et structuraux sur la couche ont tout d'abord été étudiés. Puis, des échantillons de dopage croissant ont été fabriqués grâce à ce procédé, pour des couches en tension ou en compression. Un bon accord avec les prévisions de la théorie de champ moyen a été obtenu pour l'évolution de leur anisotropie magnétique, et de leurs Tc en fonction de la contrainte épitaxiale, et de la densité de porteurs. Une dernière étude s'est penchée sur les domaines magnétiques et les mécanismes de renversement d'aimantation de couches continues, puis de microstructures monodomaines aimantées perpendiculairement au plan. Un procédé de passivation locale par hydrogénation a été développé pour structurer magnétiquement des couches de (Ga,Mn)As (taille minimale des motifs _ 200 nm). Il a entre autres permis d'abaisser fortement les champs de retournement d'aimantation de réseaux de plots microniques, par rapport à ceux obtenus dans des structures fabriquées par gravure. En_n, des résultats préliminaires de déplacement de parois de domaines par un champ magn étique ou un courant polarisé ont été obtenus dans des pistes structurées par hydrogénation.(Ga,Mn)As is a model material for spintronics, being both semiconducting and ferromagnetic, as the result of the exchange interaction between the spins of the manganese atoms (3d54s2), and those of the holes they bring to the matrix. Its Curie temperature (Tc) is typically situated between 50 and 150 K. In order to decouple magnetic (xMn) and electrical doping (p), we used a hydrogen passivation technique to form electrically inactive (Mn,H) complexes. The di_erent consequences of the incorporation of hydrogen in the layer were _rst established, considering magnetic properties (appearance of a paramagnetic phase), electronic structure (reduction of p), and structural e_ects on the lattice. A series of samples of increasing doping was then fabricated using this process, for layers in tensile or compressive strain. A good agreement with mean _eld theory predictions was obtained concerning the evolution of the magnetic anisotropy and of Tc as a function of the carrier density and epitaxial strain. The last study focused on magnetic domains, and the magnetization reversal mechanisms of continuous layers and single-domain structures with a perpendicularto- plane easy axis. We developped a local passivation process using hydrogenation to pattern magnetically (Ga,Mn)As (minimum structure size _ 200 nm). Among other results, this technique reduces the mean switching _elds in arrays of micron-sized dots, compared to structures fabricated by the more traditionnal etching technique. Finally, preliminary results of domain wall propagation induced by a magnetic _eld or a spin-polarized current were obtained in stripes patterned by hydrogenationPARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF