Advances in Single Crystal Growth Methods for the Novel Unconventional Superconductor UTe<sub>2</sub>
Authors: Xue Ziwei  , Yuan Dengpeng  , Tan Shiyong  ,   Science and Technology on Surface Physics and Chemistry Laboratory, Mianyang 621907, China
Published: February 2025

As a recently discovered unconventional superconductor, the heavy fermion compound UTe<sub>2</sub> has garnered significant attention due to its potential spin-triplet superconducting pairing, high-field re-entrant superconducting phases, and unique quantum critical characteristics. However, experimental results on this system exhibit significant variations and contradictions, primarily due to differences in sample quality. Key unresolved issues include whether the system exhibits multi-component superconducting order parameters, whether time-reversal symmetry is spontaneously broken, and whether multiple field-induced superconducting phases share a common origin. These controversies have hindered an in-depth understanding of the intrinsic superconducting pairing mechanism in the UTe<sub>2</sub> system.<br>This paper reviews recent advances in single-crystal growth methods for UTe<sub>2</sub>, including chemical vapor transport (CVT), Te-flux, molten salt flux (MSF), and molten salt flux liquid transport (MSFLT). We systematically analyze how growth conditions influence superconductivity and crystal quality. Although the CVT method was initially employed in UTe<sub>2</sub> studies, the samples grown by this method exhibit poor quality and significant compositional inhomogeneity, even within individual samples. Consequently, the CVT method has been progressively supplanted by the recently developed MSF method. In contrast, the MSF and MSFLT methods yield high-quality UTe<sub>2</sub> single crystals with <i>T</i>c as high as 2.1 K and <i>RRR</i> reaching up to 1000; however, the sample sizes are smaller compared to those grown by the CVT and Te-flux methods. Notably, MSF-grown samples occasionally contain magnetic impurities such as U<sub>7</sub>Te<sub>12</sub>, necessitating careful screening during sample collection. MSFLT combines the advantages of both CVT and MSF methods, enabling the growth of high-quality UTe<sub>2</sub> single crystals while also producing larger sample sizes than MSF. Our findings highlight the importance of optimizing growth parameters such as the Te/U ratio, temperature gradients, and cooling rates. For instance, lower growth temperatures and precise control of the Te/U ratio significantly enhance Tc and sample quality. High-quality MSF and MSFLT samples have resolved several controversies, including clarifying the single-component nature of the superconducting order parameter and confirming the absence of time-reversal symmetry breaking in optimized samples.<br>In conclusion, this review underscores the pivotal role of advanced single-crystal growth techniques in advancing the study of UTe<sub>2</sub>. Future research should focus on utilizing these high-quality UTe<sub>2</sub> samples grown by MSF and MSFLT methods to accurately determine superconducting order parameters, elucidate mechanisms behind high-field re-entrant superconducting phases, and explore topological properties, such as potential Majorana fermions. These efforts will deepen our understanding of unconventional superconductivity, spin fluctuations, and quantum critical phenomena in UTe<sub>2</sub> system.