Molten salt flux liquid transport method for ultra clean single crystals UTe₂

Various single-crystal growth techniques have been presented for the unconventional superconductor UTe₂. The molten salt flux liquid transport (MSFLT) method is employed to grow high-quality and large single crystals, exhibiting a high residual resistivity ratio (RRR ∼ 200-800). However, the Te self-flux and chemical vapor transport (CVT) methods produce samples of lower quality. The MSFLT method is a hybrid approach combining the molten salt flux (MSF) and CVT methods. One significant advantage is that the materials gradually crystallize at a relatively low temperature, which is maintained throughout the main process. This may be crucial for preventing U deficiency and obtaining high-quality large single crystals of UTe₂. Many different single crystals obtained using different techniques have been characterized by resistivity and specific heat measurements. The superconducting transition temperature T_c decreases with residual resistivity ρ₀, followed by the Abrikosov-Gor'kov pair-breaking theory, and reaches 2.1 K for ρ₀ → 0. The residual γ-value of the specific heat for the highest quality sample was only 3% of the normal-state γ-value. The specific heat jump, ΔC_e/(γT_c) reached approximately 2.7 for high-quality samples, indicating a strong-coupling superconductor. Furthermore, the magnetic susceptibility for H || a-axis in a high-quality single crystal does not show an up-turn behavior on cooling, which is consistent with the results of NMR Knight shift and μSR experiments. (author)