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Heavy fermion compound UTe2, as a recently discovered unconventional superconductor, has received significant attention due to its potential spin-triplet superconducting pairing, high-field re-entrant superconducting phases, and unique quantum critical characteristics. However, experimental results of this system show significant changes and discrepancies, primarily due to difference in sample quality. The 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 unsolved issues hinder an in-depth understanding of the intrinsic superconducting pairing mechanism in the UTe2 system. This paper reviews recent advances in single-crystal growth methods for UTe2, 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 UTe2 studies, the samples grown by this method exhibit poor quality and significant compositional inhomogeneity, even in individual samples. Consequently, the CVT method has been progressively supplanted by the recently developed MSF method. In contrast, the MSF method and MSFLT method yield high-quality UTe2 single crystals with Tc achieving a value as high as 2.1 K and residual resistivity ratio (RRR) reaching up to 1000; however, the sample sizes are smaller than those grown by the CVT and Te-flux methods. Notably, MSF-grown samples occasionally contain magnetic impurities such as U7Te12, so careful screening is required in the sample collection process. The MSFLT combines the advantages of CVT and MSF methods to grow high-quality UTe2 single crystals while producing larger sample sizes than MSF. Our research findings highlight the importance of optimizing growth parameters such as Te/U ratio, temperature gradient, and cooling rate. For instance, lower growth temperature and precise control of the Te/U ratio can significantly enhance Tc and sample quality. Several controversies have been identified regarding high-quality MSF and MSFLT samples, including clarifying the single-component nature of the superconducting order parameter and confirming the absence of time-reversal symmetry breaking in optimized samples. This review underscores the pivotal role of advanced single-crystal growth techniques in advancing the study of UTe2. Future research should focus on utilizing these high-quality UTe2 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 the UTe2 system. -
Keywords:
- UTe2 /
- unconventional superconductivity /
- chemical vapor transport method /
- molten salt flux method
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图 1 UTe2单晶的生长方法示意图 (a) Te-flux法; (b) MSF法; (c) CVT法; (d) MSFLT法.
Figure 1. Illustration of the single crystal growth methods for UTe2: (a) Te-flux method; (b) MSF method; (c) CVT method; (d) MSFLT method.
图 3 (a)不同起始原料摩尔比MTe/U样品的电阻率测量结果[28], 并与Ran等[1]和Hayes等[13]对比; (b) Tc = 2 K样品(C6)的比热测量结果, 插图为样品照片[28]
Figure 3. (a) Resistivity of samples with different initial molar ratios MTe/U [28] and comparison with those reported by Ran et al. [1] and Hayes et al. [13]; (b) specific heat data of sample C6 with Tc = 2 K, and the inset shows a sample image [28].
图 4 (a)不同生长温度获得UTe2样品的比热数据[17]; (b) 1060—1000 ℃温度梯度下生长的UTe2样品中测得Tc附近的极化Kerr角度演化[13]. 数据来源于文献[17,13].
Figure 4. (a) Specific heat data of UTe2 samples obtained at different growth temperatures [17]; (b) polar Kerr angle evolution near Tc in a UTe2 sample grown under a temperature gradient of 1060–1000 ℃[13]. The data are taken from Ref.[17,13].
图 5 不同生长温度下获得UTe2样品的照片, 起始原料摩尔比MTe/U = 2, 标注为高温端温度 [36] (a) 810 ℃; (b) 860 ℃; (c) 930 ℃; (d) 1010 ℃; (e) 1060 ℃. 出自文献[36], 已获得授权.
Figure 5. Photos of UTe2 samples obtained at different growth temperatures, with the molar ratio of the starting materials (MTe/U) set to 2, are labeled with the high-end temperatures [36]: (a) 810 ℃; (b) 860 ℃; (c) 930 ℃; (d) 1010 ℃; (e) 1060 ℃. Reproduced with permission from Ref.[36].
表 1 CVT起始原料摩尔比MTe/U对UTe2样品实际成分和Tc的影响[28]
Table 1. The impact of the molar ratio of MTe/U in CVT starting materials on the actual composition and Tc of UTe2 samples [28].
样品分组 起始原料
摩尔比MTe/UEDX测得的
MTe/U范围电阻率
测得的TcA 1.71 1.46—1.50 1.74 K B 2.14 1.79—2.06 无超导 C 1.85 1.72—1.87 2.00 K 
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表 2 CVT生长温度对UTe2单晶的Tc和样品质量的影响[17]
Table 2. Effect of CVT growth temperature on Tc and sample quality of UTe2 single crystals [17].
样品
编号生长
温度/℃比热
测得的
Tc/KRRR γ*/γN
(γN = 121
mJ·mol–1·K–1)s1 1060—1000 1.64
1.4830—40 0.54 s2 950—860 1.68 — 0.42 s3 925—835 1.77 — 0.36 s4 875—785 1.85 55 0.34 s5 825—735 1.95 70 0.21 s6 800—710 2.00 88 0.19 s7 775—685 No SC 2 —
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表 3 MSF方法工艺参数对UTe2单晶Tc和样品质量的影响[39]
Table 3. Effect of MSF process parameters on the Tc and sample quality of UTe2 single crystals [39].
样品编号 原料比例 Tf/℃ Tc/K RRR 备注 MTe/U MSalt/U M1 2 29 650 1.7—1.8 40—60 M2 1.93 37 650 1.9—2.0 60—80 M3 1.92 36 650 1.95—2.0 30—40 M4 1.90 40 650 1.8—1.95 50—60 M5 1.90 67 650 1.9—2.05 50—60 M6 1.8 21 650 — — 产物为U7Te12 M6 a 1.8 40 650 2.0—2.1 80—130 主要产物为U7Te12 M7 1.71 60 650 2.1 170—1000 主要产物为U7Te12 H1 2.0 48 700 1.6 11—12 离心去除盐 H2 1.95 42 700 1.75—1.9 35—60 L1 1.95 38 600 1.6—1.8 20—30 L2 1.90 44 600 2.1—2.2 65—70 聚集晶体
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表 4 不同生长方法获得UTe2单晶的Tc和样品质量对比[54]
Table 4. Comparison of Tc and sample quality of UTe2 single crystals grown by different methods [54].
生长方法 原料比例MTe/U 助熔剂/输运剂 生长温度/℃ Tc/K RRR γ*/γN Te-flux 3.55 Te 1050 1.08 3.6 — CVT 2.00 I2 950—850 — 2.5 — CVT 1.50 I2 1050—990 1.65 14 0.61 CVT 1.40 I2 780—680 2.01 49 0.13 MSF 1.80 NaCl+KCl 950 ＜1.7 22 0.78 MSF 1.65 NaCl+KCl 950 2.06 220 0.046 MSFLT 1.50 NaCl+KCl 750—650 2.06 179 0.124 MSFLT 1.65 NaCl+KCl 750—650 2.09 800 0.034
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表 5 不同生长方法的工艺特点与优缺点.
Table 5. Process characteristics, advantages and disadvantages of different growth methods.
方法 最优的
工艺参数最佳的
超导样品影响因素 优点 缺点 CVT 原料比例MTe/U = 1.5;
生长温度梯度800—710 ℃.Tc = 2.0 K;
RRR = 88.原料比例MTe/U;
生长温度梯度.生长温度较低;
样品尺寸大.样品质量较差;
成分均匀性差.Te-flux 原料比例MTe/U = 3.55;
生长温度1050 ℃;
离心温度950 ℃.Tc = 1.1 K;
RRR = 4.原料比例MTe/U;
生长温度;
降温速率.工艺简单;
样品产量高;
样品尺寸大.几乎不超导;
样品质量差;
生长温度高.MSF 原料比例MTe/U = 1.71;
原料比例MSalt/U = 60;
生长温度950 ℃;
退火温度650 ℃.Tc = 2.1 K;
RRR = 1000.原料比例MTe/U;
原料比例MSalt/U;
退火温度;
降温速率;
助熔剂盐的含水量.样品质量高;
生长温度较低.产物伴随有磁性杂质U7Te12;
样品尺寸小.MSFLT 原料比例MTe/U = 1.65;
生长温度梯度750—670 ℃.Tc = 2.09 K;
RRR = 800.原料比例MTe/U;
助熔剂盐的含水量.样品质量高;
生长温度低.样品尺寸较小.
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