Investigating the limits of superconductivity in UTe₂

Abstract

Spin-triplet bulk superconductors are a promising route to topological superconductivity, and UTe2 is a recently discovered contender. The superconducting properties of UTe2, however, vary substantially as a function of the synthetic route, and even nonsuperconducting single crystals have been reported. To understand the driving mechanism suppressing superconductivity, we investigate UTe2 single crystals grown close to the nonsuperconducting boundary (growth temperature 710 C) through a combination of thermodynamic and x-ray diffraction measurements. Specific heat measurements reveal a sharp decrease in the superconducting volume and a concomitant in- crease in the residual specific heat coefficient close to the nonsuperconducting boundary. Notably, these crystals are inhomogeneous and show an apparent double transition in specific heat measurements, similar to samples grown at much higher temperatures ( 1000 C). Our single crystal x-ray diffraction measurements reveal that there are two important tuning parameters: uranium vacancies and the atomic displacement along the c axis, which shows a twofold increase in samples with a reduced superconducting volume. Our results highlight the key role of local disorder along the uranium-uranium dimers and suggest that the apparent double superconducting transition is more likely to emerge close to the superconducting limits of UTe2.

Quasi-2D Fermi surface in the anomalous superconductor UTe2

“…Optimization of the CVT growth parameters has yielded higher quality crystals with T c = 2.0 K at a maximal reported RRR of 88 (ref. 38 ) – however, even these optimized samples exhibit uranium site vacancies of ≈ 0.7%, compared to neglibile vacancies for MSF samples 36 , 39 . Furthermore, in this study we resolve quantum oscillatory components with frequencies up to 18.5 kT, implying a mean free path of itinerant quasiparticles of at least 1900 Å (see Supplementary Information for calculation), further underlining the pristine quality of this new generation of UTe 2 samples.…”