Cyclotron Resonance Assisted Photocurrents in Surface States of a 3D Topological Insulator Based on a Strained High Mobility HgTe Film

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

Impact ionization induced by terahertz radiation in HgTe quantum wells of critical thickness

We report on the observation of terahertz (THz) radiation induced band-to-band impact ionization in \HgTe quantum well (QW) structures of critical thickness, which are characterized by a nearly linear energy dispersion. The THz electric field drives the carriers initializing electron-hole pair generation. The carrier multiplication is observed for photon energies less than the energy gap under the condition that the product of the radiation angular frequency $\omega$ and momentum relaxation time $\tau_{\text l}$ larger than unity. In this case, the charge carriers acquire high energies solely because of collisions in the presence of a high-frequency electric field. The developed microscopic theory shows that the probability of the light impact ionization is proportional to $\exp(-E_0^2/E^2)$, with the radiation electric field amplitude $E$ and the characteristic field parameter $E_0$. As observed in experiment, it exhibits a strong frequency dependence for $\omega \tau \gg 1$ characterized by the characteristic field $E_0$ linearly increasing with the radiation frequency $\omega$

Magnetic quantum ratchet effect in (Cd,Mn)Te- and CdTe-based quantum well structures with a lateral asymmetric superlattice

We report on the observation of magnetic quantum ratchet effect in (Cd, Mn) Te-and CdTe-based quantum well structures with an asymmetric lateral dual grating gate superlattice subjected to an external magnetic field applied normal to the quantum well plane. A dc electric current excited by cw terahertz laser radiation shows 1/B oscillations with an amplitude much larger as compared to the photocurrent at zero magnetic field. We show that the photocurrent is caused by the combined action of a spatially periodic in-plane potential and the spatially modulated radiation due to the near-field effects of light diffraction. Magnitude and direction of the photocurrent are determined by the degree of the lateral asymmetry controlled by the variation of voltages applied to the individual gates. The observed magneto-oscillations with enhanced photocurrent amplitude result from Landau quantization and, for (Cd, Mn) Te at low temperatures, from the exchange enhanced Zeeman splitting in diluted magnetic heterostructures. Theoretical analysis, considering the magnetic quantum ratchet effect in the framework of semiclassical approach, describes quite well the experimental results

Symmetry breaking and circular photogalvanic effect in epitaxial CdxHg1-xTe films

We report on the observation of symmetry breaking and the circular photogalvanic effect in CdxHg1-xTe 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 noninverted band structure. We suggest that strain is a reason for 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

Development of the Market of Themed Hotels with Historical Interior Style in Russia

Рассматриваются примеры тематических отелей с историческим стилем интерьера в Европе и России, изучается возможность развития рынка тематических отелей в историческом стиле в России. На современном этапе развития гостиничной индустрии одним из трендов становится стремление предложить потребителю оригинальный гостиничный продукт. Выявление перспектив развития тематических отелей на территории России является задачей исследования, в рамках которого был проведен опрос, показавший, что развитие рынка тематических отелей — один из факторов повышения туристской привлекательности того или иного региона.This article explores the concepts of image and corporate identity, considers the most striking examples of themed hotels with a historical interior style in Europe and Russia, and explores the possibility of developing a market for themed hotels in a historical style in Russia. At the present stage of development of the hotel industry, one of the trends is the desire to offer the consumer an original hotel product.Identifying the prospects for the development of themed hotels in Russia is the task of the study, in which a survey was conducted, which showed that the development of the market of themed hotels is one of the factors of increasing the tourist attractiveness of a particular region