Investigating the crystal field ground state and the limits of superconductivity in UTe₂

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 ^oC) through a combination of thermodynamic and x-ray diffraction measurements. Specific heat measurements reveal a sharp decrease in the superconducting volume and a concomitant increase in the residual specific heat coefficient near 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 ^oC). 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. We will discuss the relationship between these changes in superconducting properties and the crystalline electric field ground state of UTe₂.