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In recent years, narrow gap heterostructures with Hg1-xCdxTe/CdyHg1-yTe quantum wells have been actively studied both in connection with the topic of topological insulators and from the point of view of their application in optoelectronics in the terahertz domain. In this work, we study the photoconductivity and photoluminescence spectra of structure with a band gap of 40 meV. In addition to interband transitions, the features of the spectra associated with the resonance states of acceptors have been identified. The possibilities of using the structures under study to develop interband emitters at a wavelength of ~30 μm, which is inaccessible to existing quantum-cascade lasers, are discussed. Keywords: narrow-gap semiconductors, HgCdTe, resonance states, acceptors, photoluminescence.

We calculate Fermi level position in bulk HgCdTe with cadmium fraction from 19 to 22% as a function of temperature for different concentrations of mercury vacancies forming double-charged acceptors with ionization energies of 11 and 21 meV for neutral and singly charged states respectively. The concentration of free carriers in the bands at different temperatures and the proportion of acceptor centres in different charge states are calculated as well. The results explain the fast temperature quenching of photoconductivity involving the vacancies states. It is also shown that in a p-type material conductivity dependence on temperature includes an exponential growth region with a characteristic energy much greater than a half of the bandgap at zero temperature expected for an intrinsic semiconductor. Keywords: narrow-gap semiconductors, HgCdTe, Fermi level, doubly charged acceptors.

The capture times of holes to the ground level of the neutral mercury vacancy in a solid solution of Hg1-xCdxTe during the emission of an optical phonon are calculated. The calculation showed that this process can compete with the capture of holes into small excited states of vacancy centers with the emission of acoustic phonons. Such competition can lead to quenching of lines in the spectrum of vacancy photoluminescence of Hg1-xCdxTe layers with increasing temperature. Keywords: mercury vacancy, optical phonon, non - radiative transition.

We report on the observation of symmetry breaking and the circular photogalvanic effect in Cd$_x$Hg$_{1-x}$Te alloys. We demonstrate that irradiation of bulk epitaxial films with circularly polarized terahertz radiation leads to the circular photogalvanic effect (CPGE) yielding a photocurrent whose direction reverses upon switching the photon helicity. This effect is forbidden in bulk zinc-blende crystals by symmetry arguments, therefore, its observation indicates either the symmetry reduction of bulk material or that the photocurrent is excited in the topological surface states formed in a material with low Cadmium concentration. We show that the bulk states play a crucial role because the CPGE was also clearly detected in samples with non-inverted band structure. We suggest that strain is a reason of the symmetry reduction. We develop a theory of the CPGE showing that the photocurrent results from the quantum interference of different pathways contributing to the free-carrier absorption (Drude-like) of monochromatic radiation., Comment: 11 pages, 7 figures

We report on transport and capacitance spectroscopy study of two kinds of quantum wells, namely Cd$_{0.02}$Hg$_{0.98}$Te and Cd$_{0.06}$Hg$_{0.94}$Te with the thicknesses of 7.4 and 11.5 nm, accordingly. The fraction of Cd was chosen in a way that the both quantum wells are expected to have gapless band structure typical for a Dirac fermions system. We have established that the first quantum well exhibits a massless Dirac fermions system with a quality slightly better then in conventional HgTe quantum wells of critical thickness. Second quantum well exhibits a high-quality two-dimensional topological insulator state with the energy gap of around 10 meV and well-defined edge transport making it as a good candidate for further study and applications of topological insulators., Comment: 5 pages, 5 figures

The optical properties of inhomogeneous cerium dioxide (CeO2) films on aluminum (Al) sublayer are investigated. The dependencies of the reflection, scattering, and absorption of coherent electromagnetic radiation as functions of the incidence angle and polarization are studied. The experimental and numerical studies show the existence of an incidence angle at which the scattering and absorption of coherent radiation with s-polarization increase simultaneously and the specular reflection becomes just about a few percents. The angle corresponds to an excitation of Fabry-Perot resonator modes. This effect opens up great prospects for manipulation (tuning) of the reflection from the inhomogeneous films., Comment: 9 pages, 11 figures

Among the most interesting predictions in two-dimensional materials with a Dirac cone is the existence of the zeroth Landau level (LL), equally filled by electrons and holes with opposite chirality. The gapless edge states with helical spin structure emerge from Zeeman splitting at the LL filling factor $��=0$ gapped quantum Hall state. We present observations of a giant nonlocal four-terminal transport in zero-gap HgTe quantum wells at the $��=0$ quantum Hall state. Our experiment clearly demonstrates the existence of the robust helical edge state in a system with single valley Dirac cone materials., 9 pages, 8 figures

The potential of a non linear spectroscopic technique – degenerate four wave mixing (DFWM) spectroscopy – for photo dissociation studies is investigated. By applying a carefully considered control of the input beam polarizations the technique is used to measure the line shapes of specific rovibrational transitions in the A?2S+?-?X?2? (00) band of nascent OH radicals upon pulsed photo dissociation of H2O2. The photofragmentation is performed by linearly polarized radiation at 266?nm and DFWM spectra are observed at various time delays. In contrast to linear spectroscopic methods isotropic and anisotropic components of the non linear susceptibility tensor are separately accessible by using appropriate polarization geometries. In particular four wave mixing signals can be generated that are exclusively due to the occurrence of the transient anisotropy of the recoil velocity and angular momentum vector distributions of the fragments emerging from the dissociation reaction. The observed line shapes in isotropic and anisotropic DFWM spectra are governed by the Doppler effect and vectorial correlations between the parent H2O2 molecule's transition dipole moment the OH fragments’ recoil velocity and angular momentum as well as by the processes of collision induced rotational and translational relaxation

We report on the observation of cyclotron resonance induced photocurrents, excited by continuous wave terahertz radiation, in a 3D topological insulator (TI) based on an 80 nm strained HgTe film. The analysis of the photocurrent formation is supported by complimentary measurements of magneto-transport and radiation transmission. We demonstrate that the photocurrent is generated in the topologically protected surface states. Studying the resonance response in a gated sample we examined the behavior of the photocurrent, which enables us to extract the mobility and the cyclotron mass as a function of the Fermi energy. For high gate voltages we also detected cyclotron resonance (CR) of bulk carriers, with a mass about two times larger than that obtained for the surface states. The origin of the CR assisted photocurrent is discussed in terms of asymmetric scattering of TI surface carriers in the momentum space. Furthermore, we show that studying the photocurrent in gated samples provides a sensitive method to probe the effective masses and the mobility of 2D Dirac surface states, when the Fermi level lies in the bulk energy gap or even in the conduction band.

One of the main challenges for detectors at future high-energy collider experiments is the high precision measurement of hadron and jet energy and momentum. One possibility to achieve this is the dual-readout technique, which allows recording simultaneously scintillation and Cherenkov light in an active medium in order to extract the electromagnetic fraction of the total shower energy on an event-by-event basis. Making use of this approach in the high luminosity LHC, however, puts stringent requirements on the active materials in terms of radiation hardness. Consequently, the R&D carried out on suitable scintillating materials focuses on the detector performance as well as on radiation tolerance. Among the different scintillating materials under study, scintillating glasses can be a suitable solution due to their relatively simple and cost effective production. Recently a new type of inorganic scintillating glass: Cerium doped DSB has been developed by Radiation Instruments and New Components LLC in Minsk for oil logging industry. This material can be produced either in form of bulk or fiber shape with diameter 0.3-2mm and length up to 2000 mm. It is obtained by standard glass production technology at temperature 1400 degrees C with successive thermal annealing treatment at relatively low temperature. The production of large quantities is relatively easy and the production costs are significantly lower compared to crystal fibers. Therefore, this material is considered as an alternative and complementary solution to crystal fibers in view of a production at industrial scale, as required for a large dual readout calorimeter. In this paper, the first results on optical, scintillation properties as well as the radiation damage behaviour obtained on different samples made with different raw materials and various cerium concentrations will be presented.