Christophe Consejo

Two-level Ising-like model for spin-crossover phenomenon including the magnetic field effect: the mean-field approximation and Monte Carlo resolutions

Abstract In this paper we analysed the magnetic field effect on the spin-crossover phenomenon using different theoretical approaches: thermodynamic and Ising-like models. This latter, including magnetic field, was treated in the mean-field approximation and—for the first time—by Monte Carlo numerical method. We propose an adaptation of the metropolis algorithm to take into account the different degeneracies of the high and low-spin levels and their Zeeman splitings due to the effect of magnetic field in both states. A comparison of both methods is given and discussed.

Dilute Aluminium Concentration in 4H-SiC: from SIMS to LTPL Measurements

From a systematic comparison of SIMS measurements with low temperature photoluminescence data, we show that one can directly perform a non destructive (optical) investigation of the residual Al content in 4H-SiC samples. This technique offers the non negligible advantage to be easily extended to Al concentrations below the SIMS detection limit.

Multidimensional characterization, Landau levels and Density of States in epitaxial graphene grown on SiC substrates

Using high-temperature annealing conditions with a graphite cap covering the C-face of, both, on axis and 8° off-axis 4H-SiC samples, large and homogeneous single epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of the almost free-standing character of these monolayer graphene sheets, which was confirmed by magneto-transport measurements. On the best samples, we find a moderate p-type doping, a high-carrier mobility and resolve the half-integer quantum Hall effect typical of high-quality graphene samples. A rough estimation of the density of states is given from temperature measurements.

Non-trivial Berry phase in the Cd3As2 3D Dirac semimetal

We present magnetotransport measurements performed on Cd3As2 samples. Our results confirm the existence of 3D Dirac fermions in this material with a π Berry phase. A metal-insulator transition driven by the magnetic field is also observed for B < 1 T. Finally the in situ rotation of the sample reveals that the Fermi surface is a weakly anisotropic ellipsoid.

Temperature-driven massless fermions in HgCdTe heterostructures

HgCdTe compounds can be engineered to fabricate “gapped-at-will” structures. Therefore, 1D, 2D and even 3D massless particles can be observed in topological phase transitions driven by intrinsic and external physical parameters. In this work, we report on our experimental results, obtained by temperature-dependent Terahertz and Mid-Infrared magneto-spectroscopy, of topological phase transitions in HgCdTe-based quantum wells and bulk samples. These transitions are accompanied with the appearance of 2D and 3D massless particles called Dirac and Kane fermions, respectively.

Magnetospectroscopy of double HgTe/CdHgTe quantum wells

The magnetoabsorption spectra in double HgTe/CdHgTe quantum wells (QWs) with normal and inverted band structures are investigated. The Landau levels in symmetric QWs with a rectangular potential profile are calculated based on the Kane 8 × 8 model. The presence of a tunnel-transparent barrier is shown to lead to the splitting of states and “doubling” of the main magnetoabsorption lines. At a QW width close to the critical one the presence of band inversion and the emergence of a gapless band structure, similar to bilayer graphene, are shown for a structure with a single QW. The shift of magnetoabsorption lines as the carrier concentration changes due to the persistent photoconductivity effect associated with a change in the potential profile because of trap charge exchange is detected. This opens up the possibility for controlling topological phase transitions in such structures.

Magnetospectroscopy of HgTe based topological insulators

We report on a magnetospectroscopy study of a set of four HgTe quantum wells of different thickness. In quantizing magnetic fields, intraband and interband transitions have been observed. The obtained results are compared with the allowed transition energies calculated using the 8 × 8 Kane model.

Terahertz detection by field effect transistors security imaging

In this work we review the most important results concerning the physics and applications of FETs as Terahertz detectors. We present two experiments showing: i) Terahertz detection based on low cost 130 nm silicon technology Field Effect Transistors in the sub-THz range (0.2 THz up to 1.1 THz) and ii) first results on detection by FETs of emission from 3.1 THz Quantum Cascade Lasers.