Using Optical Zone Melting for Growing Single Crystals of Superconductors

Abstract

The reliability of fundamental studies of superconductivity depends on the quality of the materials under study. Optical zone melting yields high-quality single crystals without impurities, which can be difficult for other technologies. The paper describes the growth procedure for single crystals of several families of superconductors: bismuth high-temperature superconductors Bi₂Sr₂CaCu₂O_{8 + δ} and Bi₂Sr_2–xLa_xCuO_{6 + δ} and a superconductor with an assumed p symmetry of the superconducting order parameter Sr₂RuO₄. We discuss the search criteria for synthesizing high-temperature yttrium superconductors YBa₂Cu₃O_{7 + δ} by optical zone melting, which do not lead to the formation of single crystals. The procedure for obtaining single crystals includes several stages. The first is to anneal a mixture of powders of the required oxides and carbonates, taken in specific proportions, at temperatures up to 850°C. A solid-phase reaction takes place, resulting in the desired polycrystalline complex oxide; rods with a length of ~5–10 cm are obtained from this oxide using a hydraulic press. The second stage is annealing of the rods in air at temperatures up to 940°C and, if necessary, melting in an optical-zone-melting unit using lamps with a rated power of 500 W at an adjustable power from 20 to 95% with a drawing speed of 20–30 mm/h. The third stage is the growth of a single crystal at 20–95% power at a rate of 0.1–20 mm/h. The result is a mixture that disintegrates upon cracking into single crystals up to several millimeters in size. Measurements of the temperature dependence of the dynamic magnetic susceptibility of the synthesized single crystals at a frequency of 100 kHz are carried out, which makes it possible to determine the temperature of the superconducting transition and its width.