We report on the emission of Terahertz radiation from Landau quantized Dirac electrons in HgTe/CdHgTe based quantum wells. We show that this cyclotron emission is continuously tunable with magnetic field from 500 GHz up to 2.5 THz. Moreover, given the relativistic nature of charge carriers, the cyclotron mass and therefore the emitted frequency are also tunable with the electron density. These results pave the way for the long-awaited Landau laser which could additionally be tunable by a gate voltage with a fixed magnetic field.

We report on the effects of visible light on the low temperature electronic properties of the spinpolarized two dimensional electron system (2DES) formed at the interfaces between LaAlO 3 , EuTiO 3 and (001) SrTiO 3. A strong, persistent modulation of both longitudinal and transverse conductivity was obtained using light emitting diodes (LEDs) with emissions at different wavelengths in the visible spectrum range. In particular, Hall effect data show that visible light induces a non-volatile electron filling of bands with mainly 3d xz,yz character, and at the same time an enhancement of the anomalous Hall effect associated to the magnetic properties of the system. Accordingly, a suppression of the weak-anti localization corrections to the magneto-conductance is found, which correlates with an enhancement of the spin-polarization and of the ferromagnetic character of 2DES. The results establish the LED-induced photo-doping as a viable route for the control of the ground state properties of artificial spin-polarized oxide 2DES.

By using the self-consistent Born approximation, we investigate disorder effect induced by shortrange impurities on the band-gap of a seminal two-dimensional (2D) system, whose phase diagram contains trivial, single-band-inverted and double-band-inverted states. Following the density-ofstates (DOS) evolution, we demonstrate multiple closings and openings of the band-gap with the increase of the disorder strength. Calculations of the spectral function describing the quasiparticles at the Γ point of the Brillouin zone evidence that the observed band-gap behavior is unambiguously caused by the topological phase transitions due to the mutual inversions between the first and second electron-like and hole-like subbands. We also find that an increase in the disorder strength in the double-inverted state always leads to the band-gap closing due to the overlap of the tails of DOS from conduction and valence subbands.

In the case of molecular beam epitaxy (MBE), the Mg acceptors are electrically active in the as‐grown material and a priori no additional annealing procedure is necessary. However, there are still some peculiarities in the electrical properties of ammonia‐process grown GaN:Mg and some annealing effect can be observed. Additionally, the character of weak temperature dependence in the vicinity of room temperature suggests that to describe the conduction process an additional conduction channel not related to the free carriers in the valence band must be taken into account. For these reasons, this article presents the results of low‐temperature resistivity and Hall Effect studies of Mg‐doped, ammonia‐process‐grown GaN. The studied samples are grown on low‐temperature buffers of GaN deposited on a sapphire substrate. High‐temperature annealing process (≈800 K) is carried out for all of them. The temperature dependences of the electrical transport properties before and after the annealing procedure are especially investigated at temperatures ranging from 10 up to 300 K. It is found that the low temperatures transport properties are sensitive to the annealing procedure and to describe the observed effects the hopping phenomena must be taken into account.

Abstract Controlling the electrical properties of SiC requires knowledge of the nature and properties of extended defects. We have employed orthodox defect-selective etching and photo-etching methods to reveal typical and new structural defects in commercial SiC wafers. For photo-etching, the etch rate increases as the free carrier concentration decreases. The etch rate can be used to estimate the free carrier concentration with higher precision, and over a larger lateral and depth range than that accessed by Raman scattering. The logarithmic dependence of the etch rate on the free carrier concentration has been characterized.