Single crystal growth of superconducting UTe2 by the molten salt flux method

H. Sakai, P. Opletal , Y. Tokiwa , E. Yamamoto, Y. Tokunaga , S. Kambe, and Y. Haga
Res. Gr. for Strongly Correlated Actinide Science, JAEA

Usually, superconductivity occurs when two electrons pair up
with opposite spins to cancel out the spin component. However,there is a rare type of superconductivity called “spin-triplet”
superconductivity, in which pairs of electrons have noncanceling spins. Scientists are studying this unique type ofsuperconductivity as a potential candidate for topologicalsuperconductors. Various candidates for “spin-triplet”
superconductivity have been found in uranium compounds.
In 2019, a research team from the National Institute ofStandards and Technology (NIST) and the University of
Maryland discovered a novel superconductivity with the “spintriplet” superconducting feature in uranium ditelluride (UTe2)[1]. Since it was known that UTe2 crystals could be obtained
through the chemical vapor transport (CVT) method, scientists
were devoted to improving the existing CVT conditions. Then it
was recognized that growing single crystals withsuperconducting properties was challenging. This is due to thedifficulty of optimizing parameters like the mixing ratio ofstarting materials, temperature gradient, and amount of iodine
used as a transport agent. Numerous research groups haveencountered significant variations in the superconducting
transition temperature (Tc), and some crystals fail to exhibit
superconductivity. Scientists had assumed that regulating theamount of tellurium deficiency might be necessary to improve
the Tc. However, our previous study [2] has shown that not
tellurium but uranium deficiencies are the root cause of thisissue. Hence, addressing this issue is a crucial initial step toward
high-quality single crystals.
In this study [3], a molten-salt flux (MSF) method for growing
single crystals without uranium deficiency is developed. The
flux method is used in this development, which simplifies thesynthesis conditions. The flux method involves growing crystals
from a solution, like growing crystals of alum or salt dissolvedin water. Instead of water, molten salt, a mixture of sodium
chloride (NaCl) and potassium chloride (KCl), is used in thisstudy. The MSF method is schematically illustrated in Fig. 1(a).
A mixture of uranium and tellurium in varying ratios is packed
in a graphite crucible with a salt mixture. The crucible isvacuum sealed in a quartz tube. The sealed quartz tube is thengradually cooled from 950°C to 650°C, the melting point of thesalt mixture.
The optimization of the synthesis conditions in this method isrelatively easy. Single crystals can be extracted when the “salt”
is washed away with water from the “lump of UTe2 and moltensalt” formed after the heat treatment, as shown in Fig. 1(b).
After several growth attempts with different ratios of materials,
we have found synthesis conditions that produced single crystals
with a higher Tc of 2.1 K and good reproducibility compared tothe average Tc of 1.8 K grown by the conventional CVT method.
As shown in Fig. 1(c), the residual resistivity ratio (RRR) ofelectrical resistivity, a measure of single-crystal quality, alsoreaches an order of magnitude higher value of 1000, farsurpassing the previous maximum value reported as 88 [4]. The
extremely high RRR means the uranium vacancies areeliminated as far as possible.
The newly developed MSF method, which can obtain highquality single crystals with good reproducibility, is a significant
breakthrough in studying exotic superconductivity features inthis material. This research will accelerate fundamental researchon topological superconductivity that holds potential forFig. 1 (a) Schematic procedure of single crystal growth by the molten salt
flux method. (b) Photographs of the obtained single crystals of UTe2.
(c) Temperature dependence of normalized resistivity ρ(T)/ρ(300 K) for theCVT-grown crystals and the MSF-grown crystal of UTe2.

application in next-generation quantum computers andcontributes to developing new materials.
This study was partially supported by JSPS KAKENHI(JP20K20905) and by the JAEA Fund for Exploratory Researches (Houga fund).
References[1] S. Ran et al., Science 365, 684 (2019).
[2] Y. Haga et al., J. Phys.: Condens. Matter 34, 175601 (2022).
[3] H. Sakai et al. Phys. Rev. Materials 6, 073401 (2022).
[4] P. F. S. Rosa et al., Communications Materials 3, 33 (2022).