Superconducting Phase UTe₂ Single Crystals Grown by the Chemistry Vapor Transport Method

Recently, a uranium-based compound, UTe2, has shown remarkable superconducting properties, including an extraordinarily large upper critical field (>65 T) surpassing the Pauli limit, anomalous 125Te-NMR Knight shifts, and the presence of chiral edge states. These characteristics place UTe2 among the few strong candidates for spin-triplet superconductors. However, the underlying mechanism of superconductivity in UTe2 remains elusive and is highly sensitive to the synthesis conditions, composition, and quality of the UTe2 single crystals. To address this, we have meticulously grown UTe2 crystals using the chemical vapor transport method, systematically varying parameters such as the starting material ratio of Te/U and the temperature gradient (ΔT). Our findings reveal that the morphology of the grown UTe2 crystals transitions from bulk- to flake-like as the Te/U ratio falls below 2. Additionally, the size and quality of the crystals can be modulated by adjusting ΔT. The largest UTe2 crystal obtained had dimensions of approximately 7 × 8 × 5 cm3 and a weight of 0.6021 g. A phase diagram dependent on the initial Te/U ratio was constructed based on magnetism measurements, showing that the superconducting region lies within the range of 1.6 < Te/U < 2.0, with the highest superconducting transition temperature (Tc) found to be around 1.75 K. Our results indicate that component sensitivities and impurities pose significant challenges in achieving higher Tc in UTe2.