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

Abstract

Various single crystal growth techniques are 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). On the other hand, the Te self-flux and chemical vapor transport (CVT) method produces samples of lower quality. The MSFLT method is a hybrid approach that combines the molten salt flux (MSF) and CVT methods. One significant advantage is that the materials gradually crystallize at the relatively low temperature which is fixed during the main process. This might be crucial for preventing U deficiency and obtaining high-quality and large single crystals of UTe2. Many different single crystals obtained by different technique were characterized by resistivity, specific heat measurements. The superconducting transition temperature decreases with the residual resistivity, followed by the Abrikosov-Gor'kov pair breaking theory. The highest quality sample reaches Tc=2.1K. The residual gamma-value of specific heat for the highest quality sample is only 3 percents of the normal state gamma-value. The specific heat jump, Delta C/(gamma Tc) reaches about 2.7 for high quality samples, indicating a strong coupling superconductor. Furthermore, the magnetic susceptibility for the field along 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 muSR experiments.