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

Abstract

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 (T_c) found to be around 1.75 K. Our results indicate that component sensitivities and impurities pose significant challenges in achieving higher T_c in UTe₂.