The surface preparation of topological insulators (TIs) is a critical task in order to realize their efficient applications. A chemical treatment in an anhydrous solution of hydrogen chloride in isopropanol (HCl-iPA) and a subsequent annealing at relatively low temperature in ultrahigh vacuum (UHV) was successfully used for the surface preparation of bulk 3D (0 0 0 1) TIs Bi2Te3, Sb2Te3, Bi2Se3 and an MBE - grown Bi2-xSbxTe3-ySey (BSTS) thin films. The surface treatment showed a significant modification of the initial TI surfaces, which was free from the structural disorder, oxidation and chemical impurities determined by X-ray photoelectron spectroscopy and low-energy electron diffraction. The insulating nontrivial bulk gap and well resolved gapless surface states with a linear dispersion of a massless Dirac cone were observed by angle-resolved photoelectron spectroscopy (ARPES). In the BSTS film, the Fermi level is located within the bulk band gap. The negative magnetoconductance corresponding to weak antilocalization demonstrated the contribution of the surface states of BSTS, that promise to be protected from backscattering. The surface treatment method proposed in this work is highly efficient for both bulk and thin TI films that can be useful for the deposition of an insulators/metals to fabricate transistor and spin valve systems.

The magnetoresistance and the Hall effect in transistor structures fabricated on films of the three-dimensional topological insulator (Bi,Sb)2(Te,Se)3 are studied. It is shown that the negative magnetoresistance at low magnetic field is described in terms of quantum corrections to the conductivity. The magnitude of these corrections depends on the gate voltage and increases when approaching the charge neutrality point. The Hall coefficient RH is nonlinear at low magnetic fields for any gate voltage, and the RH nonlinearity is the most pronounced at high negative gate voltages. At high fields, the slope of the magnetic field dependence of the Hall coefficient changes its sign at some gate voltage.

Уважаемые коллеги!Благодарим Вас за проявленный интерес к Четвертой российской конференции «Графен: молекула и 2D кристалл» и желание принять участие в её работе.Конференция проходит в научно-образовательном центре города Новосибирска -Академгородке.Мероприятие посвящено актуальным направлениям исследований и разработок в области углеродных и низкоразмерных материалов.Проведение конференции поможет координации усилий ученых в решении современных проблем материаловедения и привлечению молодых исследователей для решения актуальных научных задач.Оргкомитет выражает особую благодарность НГУ, Центру компетенций НТИ «Моделирование и разработка новых функциональных материалов с заданными свойствами», компаниям «Диаэм», «НТ-МДТ Спектрум Инструментс» и корпорации "Графеновая Долина" за финансовую поддержку и журналам Аналитика, Наноиндустрия и РЭНСИТ за информационную поддержку.Искренне надеемся, что пребывание в Новосибирском Академгородке и в стенах Новосибирского государственного университета оставит множество положительных эмоций и

Electron states with the spin-momentum-locked Dirac dispersion at the surface of a three-dimensional (3D) topological insulator are known to lead to weak antilocalization (WAL), i.e. low temperature and low-magnetic field quantum interference-induced positive magnetoresistance (MR). In this work we report on the MR measurements in (Bi,Sb)$_2$(Te,Se)$_3$ 3D topological insulator thin films epitaxially grown on Si(111), demonstrating an anomalous WAL amplitude. This anomalously high amplitude of WAL can not be explained by parabolic or linear MR and indicates the existence of an additional, MR mechanism. Another supporting observation is not linear in the classically weak magnetic field Hall effect in the same films. The increase of the low-field Hall coefficient, with respect to the higher-field value, reaches 10$\%$. We consistently explain both transport features within a two-liquid model, where the mobility of one of the components drops strongly in a weak magnetic field. We argue that this dependence may arise from the Zeeman field induced gap opening mechanism.

The article presents the results of the analysis of the indicators of the work of the family health and reproduction service in the Astrakhan region, namely the epidemiology, structure, dynamics and caus-es of female and male infertility. The effectiveness of the center's laboratory in relation to such meth-ods as in vitro fertilization and intracytoplasmic sperm injection has been demonstrated. It has been shown that the pregnancy rate after artificial insemination over the past 8 years exceeds the average level in Russia, and the pregnancy rate for transfer corresponds to the all-Russian data.

Electron states with the spin-momentum-locked Dirac dispersion at the surface of a three-dimensional (3D) topological insulator are known to lead to weak antilocalization (WAL), i.e., low temperature and low -magnetic-field quantum interference-induced positive magnetoresistance (MR). In this work, we report on the MR measurements in (Bi, Sb)2(Te, Se)3 3D topological insulator thin films epitaxially grown on Si(111), demonstrating an anomalous WAL amplitude. This anomalously high amplitude of WAL cannot be explained by parabolic or linear MR and indicates the existence of an additional MR mechanism. Another supporting observation is not linear in the classically weak magnetic field Hall effect in the same films. The increase of the low-field Hall coefficient, with respect to the higher-field value, reaches 10%. We consistently explain both transport features within a two-liquid model, where the mobility of one of the components strongly drops in a weak magnetic field. We argue that this dependence may arise from the Zeeman-field-induced gap opening mechanism.

Magneto-transport properties were studied on thin films of a 3D topological insulator (TI) Bi2Se3 grown on graphene (Gr) by physical vapor deposition. It was shown that the main contribution to the conductance is from the bulk states, whereas magnetoresistance is determined by both surface and bulk channels. The input of the charge transport over the surface states in the Si/SiO2/Gr/Bi2Se3 structure reveals itself in the weak antilocalization effect. The transition from a weak antilocalization to a weak localization is observed with decreasing the film thickness. The band bending on both interfaces makes it possible to explain the contribution to a weak antilocalization from different surfaces at different TI film thicknesses.

Successful applications of a topological insulator (TI) in spintronics require its bandgap to be wider then in a typical TI and the energy position of the Dirac point in the dispersion relations to be away from the valence and conduction bands. In this study we grew Bi1.1Sb0.9Te2S crystals and examined their elemental composition, structural, optical and electronic properties as well as the electronic band structure. The high structural quality of the grown crystals was established by X-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy demonstrated a near parabolic character of the valence and conduction bands and a direct bandgap of 0.36 eV. The dispersion relations also revealed a Dirac cone, confirming the topological insulator nature of this material, with the position of the Dirac point being 100 meV above the valence band maximum. Far infrared reflectivity spectra revealed a plasma edge and two phonon dips. Fitting these spectra with theoretical functions based on the Drude-Lorentz model allows determination of the high frequency dielectric constant (41.3), plasma frequency (936 cm(-1)) and the frequencies of two infrared phonons (177.7 cm(-1) and 77.4 cm(-1)). (C) 2021 Elsevier B.V. All rights reserved.

BiySb2-yTe3-xSex (BSTS) topological insulator (TI) thin films were grown by physical vapor deposition (PVD) and molecular-beam epitaxy (MBE) and compared by growth parameters, substrate selection, preparation, and resulting film properties. For the MBE-grown BSTS on Si(111)-(7 x 7), preliminary deposition of the Bi2Te3 buffer layer at the Te-rich growth condition was found to improve the film structural properties. Using a BSTS crystal as the main source for deposition allows us to significantly decrease the molecular Te flux. For the PVD growth, optimal conditions (gas flux, source, and substrate temperatures) were found for a morphologically smooth BSTS film on mica. In order to grow the epitaxial BSTS film on a Si/SiO2 substrate, graphene was successfully used as the pre-epitaxial layer. A newly developed chemical preparation method of atomically clean and structurally ordered epi-layer surfaces allowed us to study the electronic structure of the grown TI films without using capping layers. The surface states with Dirac-like dispersion at the Gamma-point in the surface Brillouin zone were detected by angle-resolved photoelectron spectroscopy. Photoemission measurements showed a change in the surface Fermi level position depending on the composition of BSTS films. The magnetoconductivity data demonstrated a strong dependence of the parameters describing a weak antilocalization on the substrate used.

Heterostructures of Bi2Se3 topological insulators were epitaxially grown on graphene by means of the physical vapor deposition at 500 °C. Micrometer-sized flakes with thickness 1 QL (quintuple layer ~ 1 nm) and films of millimeter-scale with thicknesses 2–6 QL had been grown on CVD graphene. The minimum thickness of large-scaled continuous Bi2Se3 films was found to be ~ 8 QL for the regime used. The heterostructures with a Bi2Se3 film thickness of > 10 QL had resistivity as low as 200–500 Ω/sq and a high room temperature carrier mobility ~ 1000–3400 cm2/Vs in the Bi2Se3/graphene interface channel. Moreover, the coexistence of a p-type graphene-related conductive channel, simultaneously with the n-type conductive surface channel of Bi2Se3, was observed. The improvement of the bottom Bi2Se3/graphene interface with the increase in the growth time clearly manifested itself in the increase of conductivity and carrier mobility in the grown layer. The grown Bi2Se3/G structures have lower resistivities and more than one order of magnitude higher carrier mobilities in comparison with the van der Waals Bi2Se3/graphene heterostructures created employing exfoliation of thin Bi2Se3 layers. The grown heterostructures demonstrated the properties that are perspective for new functional devices, for a variety of signal processing and logic applications.

We study the tunneling magnetoresistance in the ensembles of ferromagnetic granules with random easy axes of magnetic anisotropy taking into account the exchange interaction between granules. It is shown that due to the exchange interaction magnetoresistance is effectively decoupled from magnetization, i.e. the strongest negative magnetoresistance can be observed at the field where magnetization is almost saturated. Under some conditions, the sign of magnetoresistance can be reversed and tunneling magnetoresistance can become positive at certain magnetic fields. Our theory agrees with measurements of magnetoresistance in ensembles of Fe granules in SiC x N y matrix.

The effect of magnetic field on the long-term photoconductance relaxation in two-dimensional arrays of Ge tunnel-coupled quantum dots grown on Si by molecularbeam epitaxy is studied. It was shown that the relaxation process can be slowed down as well as accelerated by magnetic field. The sign of changing the relaxation rate depends on the localization radius and the quantum dot occupancy. To explain an unusual acceleration effect we proposed a model based on the difference in probabilities of carrier transitions to single- or double-occupied quantum dots due to Zeeman effect.

Thin Bi2Se3 films were deposited on mica substrates by physical vapor deposition without the use of the carrier gas. It was found that the films with high structural quality and high conductivity are grown at a source temperature of approximately 500 degrees C. The resistance of 20-300 nm thick films is in the range of 10(2)-10(4) Omega/sq as compared with similar to 10 Omega/sq for thicker films. Bi2O2Se crystals with a similar resistivity are revealed to grow at higher temperatures (600-700 degrees C). It was suggested that the decrease of the thin film resistance is due to the contribution of the surface channels. Low resistivity of the Bi2Se3 films expands the scope of their possible applications as infra-red transparent electrodes.

We report the modification of magnetic properties due to variation of the structure of Fe-contain granules in SiCxNy:Fe films grown by chemical vapor deposition at different temperatures from various gaseous mixtures. It was shown that formation of large composite Fe-granules in the case of structures created using hexamethyldisilazane as source of Si, C, and N atoms results in the hysteresis loop in the magnetization curve appeared only at low temperature (5 K). The ESR spectra are characterized by many ESR lines with sharp asymmetrical shapes and different line widths. The samples with a dense arrays of small homogeneously distributed alpha-Fe granules, obtained with hexamethylcyclotrisilazane, demonstrate the large saturation magnetization, the hysteresis loop (similar to 135 Oe at 300 K and similar to 680 Oe at 5 K) and the collective ferromagnetic resonance signal. The pronounced magnetic properties are explained by the formation of the single magnetic domain due to quantum-mechanical tunneling between small coupled alpha-Fe clusters. Conductivity measurements confirm the existence of the tunneling coupling between Fe-granules even in case of lowest Fe concentration in the samples under study.

Ge/Si quantum dot (QD) structures doped with Mn have been tested by the EPR method to find the optimal conditions for formation of the diluted magnetic semiconductor (DMS) phase inside QDs. The effect of Mn doping has been studied for two series of samples: series A with QDs grown at 450°C and varied Mn concentration and series B with QDs grown at different temperature with Mn concentration x = 0.02. Several effects of modification of the EPR spectra due to Mn presence in the samples have been obtained. These effects are related to (i) strain reduction due to Ge—Si intermixing, (ii) QD enlargement and change in QD shape, (iii) presence of an additional magnetic field produced by Mn atoms incorporated in QDs. The data obtained allow us to understand the reasons for irreproducibility of the results available in the literature on the creation of magnetic Ge 1 − x Mn x quantum dots.

Buckypapers (BPs) with carbon nanotubes (CNTs) are very promising for a lot of applications, in which their high conductance, strength and small weight are required. In this work, isotropic BPs were prepared using the solution-based deposition that includes the single walled carbon nanotubes (SWCNTs) dispersion and the dispersion filtration from a solvent. To increase the BP conductivity, the orientation of the SWCNT bundles composing BPs and a following iodine doping were applied. The method of extrusion through the narrow (300 µm) gap was used for the SWCNT orientation. The temperature dependences of conductance for isotropic, oriented and doped BPs were studied to understand the effect of CNT alignment and the mechanism of transport through SWCNT BPs. It was shown that bundle orientation increases the BP conductivity from ~103 S × cm-1 to ~104 S × cm-1, and iodine doping of oriented samples additionally increase the conductivity by an order. The fluctuation – assisted tunneling between CNT bundles was used to describe the mechanism of low temperature conductivity.

We reconsider the theory of Hall effect in the systems with hopping conduction. The purpose of this study is to compare the percolation approach based on the optimal triad model with numerical simulations and recent experimental results. We show that, in the nearest-neighbor hopping regime, the results of the percolation theory agree to the simulation. However, in the variable range hopping (VRH) regime, the optimal triad model fails to describe the numerical results. It is related to the extremely small probability to find the optimal triad of sites in the percolation cluster in the VRH regime. The contribution of these triads to the Hall effect appears to be small. We describe the Hall mobility in the VRH regime with the empirical law obtained from the numerical results. The law is in agreement with our recent experimental data in two-dimensional quantum dot arrays with the hopping transport.

The Hall effect in heterostructures with a two-dimensional array of tunneling-coupled Ge quantum dots grown by molecular-beam epitaxy on Si is investigated. The conductivity of these structures in zero magnetic field at 4.2 K varies in the range of 10 −12 −10 −4 Ω −1 , which includes both the diffusive transport under weak localization conditions and hopping conduction. It is shown that the Hall effect can be discerned against the magnetoresistance-related background in both high- and low-conductivity structures. The Hall coefficient in the hopping regime exhibits a nonmonotonic dependence on the occupancy of quantum dots by holes. This behavior correlates with that of the localization length of the hole wavefunctions.