Thermodynamic evidence for the anisotropic field-induced change of the superconducting order parameter in the chiral superconductor candidate UTe₂.

UTe₂ is an unconventional superconductor widely believed to host a spin-triplet ground state. We present measurements of the specific heat C(H, T) of a high-quality single crystal sample of UTe₂ at low temperature (70 mK < T < 2 K) and with magnetic fields to 12 T applied along the three principal crystallographic axes.

Our sample of UTe₂ exhibits a single specific heat anomaly at the superconducting transition temperature T_c ≈ 2 K and a small zero-field residual Sommerfeld coefficient γ₀ = C/T (T=0) ≈ 10 mJ mol^-1K^-2. The magnetic field evolution of the residual Sommerfeld coefficient γ₀(H) is distinct for the three field directions. In magnetic fields to 4 T for fields along a, b, and c axes, we find that γ₀ ≈ α_i√H, with i=a,b,c, as expected for a nodal superconductor; however, we observe a striking change of behavior of γ₀(H) above 4 T. A well-pronounced kink in γ₀(H) is observed at 4 T for H || a,b, whereas a smooth change from square-root to linear behavior is observed around 4 T for H || c. These results strongly indicate that the zero-field superconducting ground state is stable to 4 T and undergoes a field-induced change above 4 T. At low fields, the nodes appear to be predominantly located in the ab plane. Above 4 T, the change in the superconducting order parameter is strongest when the field is applied in the ab plane, with nodes moving away from the direction of the applied field. We will discuss the implications of our results for the determination of the superconducting order parameter in UTe2 at high fields.