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Since the discovery of high critical temperature (high-T_c) superconductivity in cuprates in 1986, numerous experimental and theoretical research have been conducted to elucidate this mechanism. However, the reason for an extremely high T_c has not yet been completely understood. One approach to understanding the high-T_cmechanism is to investigate the physical properties of the material with the highest T_c and determine the essential parameters for realizing a high T_c. Among high-T_c cuprates, Hg-based cuprates with three CuO₂ layers, HgBa₂Ca₂Cu₃O_8+d (Hg-1223), have the highest T_c above 130 K at ambient pressure.

However, at present, information on the physical properties of Hg-1223 is insufficient because of the lack of high-quality large single crystals of Hg-1223. One reason for this lack may be the nature of Hg; that is, it is poisonous and has high vapor pressure. Moreover, because Hg-1223 is chemically unstable at high temperatures and difficult to obtain the phase, the range of chemical compositions and temperatures at which single crystals can be grown is extremely narrow, causing difficulty in growing Hg-1223 single crystals with good reproducibility.

Accordingly, we carefully examined earlier studies on the single crystal growth of Hg-1223 and determined the key factors for obtaining large single crystals reproducibly and safely. We employed an explosion-proof stainless-steel container to control the high pressure of the Hg vapor at high temperatures to ensure safe crystal growth.

We attempted crystal growth more than 100 times and established a method for growing Re-doped Hg-1223 ((Hg,Re)1223) single crystals using the self-flux method by optimizing the growth conditions. Under the optimal conditions, large single crystals with sizes up to 1 × 1 × 0.04 mm³were obtained, and crystals with sizes of approximately 0.8 × 0.8 mm²in the ab-plane area were obtained with good reproducibility. We expect that further research using Hg-1223 single crystals will provide clues to understand the high-T_c mechanism in cuprates.

Written by Shigeyuki Ishida on behalf of all authors.