Molten Salt Flux Liquid Transport Method for Ultra Clean Single Crystals UTe₂

Various single-crystal growth techniques have been presented for the unconventional superconductor UTe 2 .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 UTe 2 .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 ρ 0 , followed by the Abrikosov-Gor'kov pair-breaking theory, and reaches 2.1 K for ρ 0 → 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, ∆C 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.