Controllable growth of centimetre-sized UTe2 single crystals by the chemical vapor transport method

Abstract

Recently, one of the uranium-based heavy fermion systems, UTe₂, has attracted enormous attention because of its potential spin triplet superconductivity, whose superconducting properties are closely related to their compositions and states of crystallization. In this work, the crystal growth of UTe₂ crystals was systematically studied by the CVT method. Various growth parameters, including growth temperature, the ratio of uranium to tellurium, and the type and amount of transport agents, were investigated. A lower growth temperature (710 °C) results in a smaller size of the UTe₂ crystals, and too high of a growth temperature (1060 °C) makes the nucleation rate go out of control. The ingredients with higher tellurium content (U : Te = 2 : 5) lead to larger single crystals, while a shortage of tellurium (U : Te = 2 : 3) tends to form polycrystals with smaller sizes. Among the three types of transport agents we used, iodine forms larger crystal sizes than AgBr and TeBr₄. When the amount of iodine is excess (2.45 mg cm⁻³), the crystal surface becomes rough, while not enough iodine makes the crystal thin and even bent. We have found that centimetre-sized high quality UTe₂ single crystals can be grown by using an appropriate amount of iodine as the transport agent at 860 °C. The nucleation rate of UTe₂ crystals is mainly determined by temperature and iodine partial pressure, while the growth rate is more sensitive to the tellurium partial pressure. The oxidation process of UTe₂ crystals was observed and is also discussed in this paper.