The Fe1 + ySexTe1 – x single crystals grown by the KCl flux and Bridgman methods have been compared using the same investigation techniques. It is shown that the KCl flux growth yields high-quality Fe1 + ySexTe1 – x superconducting single crystals. The oxygen diffusion coefficient in Fe1 + ySexTe1 – x has been estimated. During the Bridgman crystal growth, a stable nonsuperconducting phase can be formed, which suggests a need for refining the phase diagram of this compound.

Despite a large number of publications proving that a number of different materials can be classified as topological insulators, unambiguous proofs of the existence of promising topological superconductors have not been obtained as yet. The implementation of such systems is important not only for solid state physics, but also for applications, because topological superconductors can be used in quantum computers. We have made an attempt to confirm experimentally the assumption put forth in a number of theoretical publications that FeSe x Te 1 – x with the s -wave pairing, which is the simplest iron-containing superconductors, can become, under certain conditions, a topological superconductor with the π-wave pairing, which must have a superconducting gap in the bulk and the Majorana gapless surface states.

The Andreev reflection spectra of high-quality superconducting Sn 1– x In x Te crystals with critical temperature T c = 3.6–3.8 K were examined. The two-band nature of the superconducting state of this compound was established. The energy gaps have s-symmetry and were measured to be 2Δ 1 / kT c = 3.5 ± 0.9 and 2Δ 2 / kT c = 7 ± 1. Neither of the contacts with a resistance of 1.5–23 Ω revealed a peak at zero voltage typical of the topological superconducting state.

Исследованы спектры андреевского отражения высококачественных сверхпроводящих кристаллов Sn1-xInxTe с критической температурой Tc=3.6-3.8 K. Доказана двухзонная природа сверхпроводящего состояния данного соединения. Измеренные энергетические щели имеют s-симметрию и равны: 2Delta1/kTc=3.5±0.9 и 2Delta2/kTc=7±1. Ни на одном из контактов с сопротивлением (1.5-23) Omega не наблюдался характерный для топологического сверхпроводящего состояния пик при нулевом напряжении. Работа выполнена в рамках государственного задания N 0023-2014-0188 по теме 20. 22 22 22 4 "Оптические, транспортные, магнитные и структурные свойства наноструктур на основе полупроводниковых, магнитных и сверхпроводниковых материалов". DOI: 10.21883/FTT.2017.10.44956.409

A comprehensive analysis of the evolution of the structural and superconducting properties of FeSe crystals stored in air for several years has been performed. It is established that the structure and phase composition of the samples remained invariable, while the superconducting parameters significantly degraded. These changes may be due to the stress relaxation or redistribution of defects in the samples.

Single crystals of topological insulators—bismuth chalcogenides Bi2Te3, Bi2 − x Sn x Te3, Bi2Se3, and Bi2 − x Cu x Se3 with different charge-carrier densities—are grown by the modified Bridgman method. Their composition and structure are investigated and temperature dependences of the electric resistance and magnetic field dependences of the Hall voltage are obtained.

The thermal expansion of a high-quality 2H-NbSe 2 single crystal is precisely measured in the basal plane and the temperature range 5.7–50 K. An anomaly of the thermal expansion in the basal plane is detected, and it is found to be caused by a superconducting transition. The change in thermal expansion coefficient α ab is used to calculate the pressure derivative of the critical temperature ( dT c / dp ab ) = (19.0 ± 2.0) × 10 −9 K/Pa, and this derivative agrees well with the reported data. An additional anomaly, which indicates a phase transition, is also detected in the pretransition range. The nature of the detected phase transition is discussed.

Plate-like β-FeSe0.90 crystals have been gro-wn. The composition, structure, and some superconducting characteristics of these crystals are studied. The critical magnetic field H c2(0) and the coherence length ξ are estimated; the energy gap is measured.

The temperature dependence of the linear thermal expansion coefficient (TEC) of an InSe single crystal in the (001) plane is measured in the temperature range 7–50 K. A peak in the thermal expansion is detected near T = 10 K, after which the sample shrinks upon heating. The effect of an external magnetic field of up to 6 T, which is parallel to the (001) plane, on the TEC is investigated. The observed partial suppression of the peak and crystal compression by the field indicates the relation of these anomalies to possible electron ordering in InSe layers.

Измерена температурная зависимость линейного коэффициента теплового расширения (КТР) монокристалла InSe в плоскости (001) в области температур 7-50 К. Обнаружен максимум теплового расширения около Т = 10 К, после которого с ростом температуры образец сжимается. Исследовано влияние на КТР внешнего магнитного поля до 6 Тл, параллельного плоскости (001). Выявлено частичное подавление максимума и сжатия кристалла магнитным полем, указывающее на связь этих аномалий с возможным электронным упорядочением в слоях InSe.

The observation of Shubnikov-de Haas and Hall oscillations in high-quality Bi 2 − x Cu x Se 3 single crystals is reported. Measurements carried out upon rotating the samples with respect to the magnetic field demonstrate that the oscillations originate from two-dimensional surface states in three-dimensional single crystals and are determined only by the perpendicular component of the magnetic field.

Complex studies of the evolution of the structural and superconducting characteristics of Bi2212 and Bi2201 monocrystals depending on their storage in air for several years have been performed. It has been found that the sample structures remain perfect, whereas the superconducting parameters change due to oxygen mobility in the crystals. The character of the changes depends on the doping degree and the composition balance with respect to cations and oxygen in the grown monocrystals.

Superconducting crystals of iron selenide FeSe are grown and their phase and elemental compositions are studied. The structural analysis of individual FeSe1 − x crystals showed the coexistence of two phases in this compound. The resistivities and magnetic susceptibilities of the crystals are investigated.

A complex study of the elemental composition, structure, and superconducting properties of single crystals of layered oxycarbonate Bi 2 Sr 4 Cu 2 CO 3 O 8 has been performed taking into consideration the growth and postgrowth annealing conditions.