Yu. I. Gorina

Quantum oscillations in heavily doped bismuth chalcogenides

The observation of Shubnikov-de Haas and Hall oscillations in high-quality Bi2 − xCuxSe3 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.

Negative thermal expansion coefficient in the high-temperature superconductor Bi2Sr2CaCu2O8 + x

Powder and single-crystal samples of the high-temperature superconductor Bi2Sr2CaCu2O8 + x are studied by X-ray diffraction. Its thermal expansion coefficient is found to be negative over a wide temperature range (160–260 K). Possible causes of this effect are discussed.

Vapor-phase growth of Bi2Sr2Can−1CunOx (n= 1–3) single crystals

High-quality platelike Bi2Sr2Can−1CunOx (n= 1–3) single crystals with superconducting transition temperatures Tc=10, 85, and 110 K, respectively, are prepared by vapor-phase growth in closed cavities produced in a melt fluxed with KCl. The effect of synthesis and growth conditions on the Tc of the crystals is analyzed. The morphology of the crystals suggests that the most perfect, well-faceted crystals grow via vapor transport with constant feeding from the charge.

Dependence of superconducting properties of underdoped Bi-2201 single crystals on the charge composition and growth conditions in gas-filled cavities

High-quality Bi2 + xSr2 − yCuO6 + δ (Bi-2201) single crystals with the ratio Bi/Sr = 1.4–2.0 were grown by the free growth method in gas-filled cavities in KCl solution-melt in a range of doping levels, which provides variation in superconducting properties from insulators to optimally doped crystals. The charge composition Bi : Sr : Cu = 1.7 : 2.3 : 2.5 with excess Sr and Cu and synthesis conditions provided growth cavity formation in the KCl solution at the crystal growth stage. Lamellar single crystals and whiskers were grown under quasi-equilibrium conditions of lowered growth temperatures and partial oxygen pressure, achieved in closed gas-filled cavities.

Growth from gas cavities in a molten salt and properties of superconducting Bi2Sr2CaCu2O8+δ single crystals

Perfect single crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+δ with the superconducting transition temperature TC = 72–85 K (depending on the crystallization conditions) are obtained by the method of free growth in gas cavities formed in a KCl solution-melt. The specific features of the growth process are in the formation of an enclosed growth gas cavity in a (previously synthesized) blend of a specified phase composition dissolved in KCl and the free crystal growth in this cavity. The combination of growth and high-temperature annealing in the same process made it possible to obtain uniform (ΔTC = 1.5 K) single crystals with stable superconducting properties. Annealing of the grown single crystals in oxygen or in air in the temperature range 400–850°C confirmed that the crystals with maximum values of TC are optimally doped.

Two-band superconductivity of Sn 1– x In x Te crystals with T c = 3.6–3.8 K

The Andreev reflection spectra of high-quality superconducting Sn1–xInxTe crystals with critical temperature Tc = 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 2Δ1/kTc = 3.5 ± 0.9 and 2Δ2/kTc = 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.

Evolution of the structural and superconducting properties of Bi-HTSC monocrystals when stored in air

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.

Structural and superconducting properties of single-crystal Bi2Sr2Can − 1CunO2n + 4 + δ (n = 1–3) whiskers grown in gas-filled cavities

Using growth inside gas-filled cavities in KCl-fluxed melts, we have obtained free-standing phasepure Bi2Sr2Can − 1CunO2n + 4 + δ (n = 1–3) whiskers possessing superconducting properties in the as-grown state. Bi-2212 whiskers have been prepared in a wide range of doping levels, from underdoped (Tc = 76.5 K) to optimally doped (Tc = 84 K). The whiskers possess high structural perfection, as shown by X-ray diffraction. Annealing under optimal conditions raises the superconducting transition temperature of the underdoped Bi-2212 whiskers to Tc max = 85.5 K (ΔTc = 1.8 K), without impairing their structural perfection.

Mechanism of growth of Bi2+x Sr2−y CuO6+δ single crystals in gas cavities in a KCl melt

High-quality Bi2+xSr2−yCuO6+δ single crystals in a wide range of superconducting properties, from optimally doped to strongly underdoped (including insulators), have been obtained by free growth in gas cavities formed in a KCl flux. A model of crystal growth is proposed, in which the decisive parameter is the chemical transport in a cavity at a low partial oxygen pressure and feeding of the gaseous medium from the charge heated to a higher temperature. In this case, layer-by-layer growth through the vapor-solid mechanism is implemented. This growth, as the most ordered process, makes it possible to obtain faceted plates and whiskers with specular faces, without segregation of other phases.