22 research outputs found
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
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)
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
Barrier breakdown in a multiple quantum well structure
We explore a regime of unipolar electronic transport in a multiple quantum
well structure with very large current discontinuities - up to five orders of
magnitude. Magneto-transport experiments reveal different transport regimes.
Quantum well impact ionization shifts the structure from a resistive down
state, where the current flows through inter-well quantum tunneling, to a
highly conductive up state. In the latter regime, the current leaks through a
barrier suddenly broken down because of an efficient ionization of the first
quantum well.Comment: 16 pages, 5 figure
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
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 Ï 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 ~3000âcm2·Vâ1·sâ1 at 4âK
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
3D Topological Semimetal Phases of Strained -Sn on Insulating Substrate
-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 -Sn allotrope, suffer from parallel conduction,
restricting transport investigations and potential applications. Here, the
successful MBE growth of high-quality -Sn layers on insulating, hybrid
CdTe/GaAs(001) substrates, with bulk electron mobility approaching 20000
cmVs 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 Berry phase in Shubnikov-de Haas oscillations agrees
with the topologically non-trivial nature of the investigated samples. Our
findings establish -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
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