The characteristics of point Josephson contacts Pb0.6In0.4/KFe2As2 and KFe2As2/KFe2As2 have been studied. The temperature dependences of characteristic contact voltages VC(T) and the dependences of the first current step amplitudes in I–V characteristics on the power of 7.6-GHz electromagnetic radiation have been measured. It has been found that VC(T) curves for all contacts can be described in terms of the SIS*IS contact model (S, I, and S* stand for superconductor, insulator, and superconductor with a lower critical temperature, respectively) for superconductors with the s-symmetry of order parameter. It has been proved that the current step oscillation period as a function of microwave power can be exactly approximated with the resistive model of contact with IS = ICsin(φ). Obtained data are consistent with the normal s-symmetry of order parameter.

The characteristics of point Josephson contacts Pb 0.6 In 0.4 /KFe 2 As 2 and KFe 2 As 2 /KFe 2 As 2 have been studied. The temperature dependences of characteristic contact voltages V C ( T ) and the dependences of the first current step amplitudes in I – V characteristics on the power of 7.6-GHz electromagnetic radiation have been measured. It has been found that V C ( T ) curves for all contacts can be described in terms of the SIS*IS contact model (S, I, and S* stand for superconductor, insulator, and superconductor with a lower critical temperature, respectively) for superconductors with the s- symmetry of order parameter. It has been proved that the current step oscillation period as a function of microwave power can be exactly approximated with the resistive model of contact with I S = I C sin(φ). Obtained data are consistent with the normal s -symmetry of order parameter.

Characteristics of the Josephson effect in Pb0.6In0.4/FeSe0.4Te0.6 point contacts with the current flowing parallel to ab plane of the iron chalcogenide crystal (with critical temperature Tc ≈ 15 K) have been studied. Dependences of the critical current Ic and amplitudes In of the first steps of current (Shapiro steps) in the current–voltage (I–V) characteristics (n = 0, 1, 2; I0 = Ic) on the microwave power P (at frequency f = 7.6 GHz) and dependences of the characteristic voltage Vc(T) = IcRN (RN being the normal state resistance) on the temperature T have been measured. It is found that the characteristics of contacts (Ic, RN, Vc) obtained for initial parts of the I–V curves do not adequately describe the oscillations of current steps in the microwave radiation field. A method is proposed for determining the normalized microwave frequency Ω = 2πf(2eVc/$$\hbar $$)–1 that provides quantitative description of the current oscillation steps in the framework of a resistive contact model and allows one to check that the Josephson superconducting current Is is proportional to sin(aφ) (a = 1, 2), where φ is the phase difference of the order parameters. Comparison of the measured In($$\sqrt P $$) curves to those calculated using the resistive model showed that Is = Icsinφ, in agreement with the conventional s and s++ symmetry of the order parameter in FeSe0.4Te0.6. It is established that Vc of the contacts studied is proportional to T in a wide temperature interval.

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 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.