Ambient-condition growth of superconducting YBa2Cu4O8 single crystals using KOH flux

YBa2Cu4O8 is a stoichiometric oxide superconductor of Tc∼80 K. Unlike YBa2Cu3O7−δ, this compound is free from oxygen vacancy or twin formation and does not have any microscopic disorder in the crystal. Doping with Ca raises its Tc to 90 K. The compound is a promising superconductor for technological application. Up to now, single crystals have not been grown without using specialized apparatus with extremely high oxygen pressure up to 3000 bar and at over 1100 °C due to the limited range of reaction kinetics of the compound. This fact has delayed the progress in the study of its physical properties and potential applications. We present here a simple growth method using KOH as flux that acts effectively for obtaining high-quality single crystals in air/oxygen at the temperature as low as 550 °C. As-grown crystals can readily be separated from the flux and exhibit a perfect orthorhombic morphology with sizes up to 0.7×0.4×0.2 mm3. Our results are reproducible and suggest that the crystals can be grown using a conventional flux method under ambient condition.