Cooking in Salt for Ultra-Clean Superconductor UTe₂

UTe₂ is among the most intriguing materials in condensed matter physics owing to its unusual superconducting properties. The significant features of UTe₂ include its immense field-reentrant superconductivity and the observation of multiple superconducting phases that have been highlighted in the study of superconducting phenomena. These experimental findings align with the spin-triplet superconductivity scenario, in which the superconducting upper critical field surpasses the limit of the critical field for conventional superconductivity. Moreover, spin and orbital degrees of freedom may contribute to the emergence of multiple superconducting phases.

To thoroughly investigate this fascinating material, precise experiments conducted under extreme conditions using high-quality single crystals are essential. This paper introduces a novel approach known as molten salt flux liquid transport (MSFLT) method for growing high-quality single crystals of UTe₂. This technique is a hybrid of the molten salt flux (MSF) and chemical vapor transport (CVT) methods . Large, high-quality single crystals were obtained by gradually synthesizing UTe₂ single crystals at relatively low temperatures that were maintained throughout the process. These single crystals were buried in the NaCl/KCl salt, akin to fish in a salt crust.

The quality of the single crystals was validated by resistivity and specific heat measurements. The superconducting transition temperature reached 2.1 K for the highest-quality sample, and the residual density of states in the superconducting state approached zero, indicating an ideal superconducting transition. The observation of quantum oscillations, known as the de Haas–van Alphen effect, under extreme conditions (low temperature and high field) clearly suggested that the high-field condition was satisfied for the cyclotron motion of the conduction electrons.

The MSFLT method, also known as the liquid transport method or horizontal flux method, holds promise for broader applications beyond UTe₂. This technique can be extended to many other materials for which high-quality single crystals are scarce, opening new avenues for the discovery of exotic superconductors.

Written by D. Aoki