Specific heat measurements and magnetic field-dependent pairing symmetry in UTe₂

Spin-triplet superconductivity is a promising platform for hosting Majorana zero modes, which are essential for topological qubits in quantum computation. I focus on UTe₂, a recently discovered unconventional superconductor with electron Cooper pairs forming a spin-triplet ground state. I will discuss the specific heat C(H,T) of a high-quality single crystal of UTe₂, which shows a single specific heat anomaly at the superconducting transition temperature T_c = 2.1 K and a small zero-field residual Sommerfeld coefficient γ₀ = C/T (T=0)~10 mJ/mol-K². Magnetic fields up to 12 T were applied along the three principal crystallographic axes of UTe₂ to probe the superconducting state's nature. The evolution of the residual Sommerfeld coefficient as a function of magnetic field, γ₀(H), is highly anisotropic and reveals distinct regions. In magnetic field up to 4 T applied along a, b, and c axes, I find γ₀~α_i\sqrt(H), with i=a,b,c, as expected for an unconventional superconductor with nodes (zeros) of the superconducting order parameter on the Fermi surface. However, a pronounced kink in γ₀(H) is observed at roughly 4 T for field applied along both a and b axes, while a smooth change from square-root to linear behavior occurs at 4 T for H||c. These findings strongly suggest that the zero-field ground state is stable up to 4 T and undergoes a field-induced evolution above 4 T. α_c >α_a>α_b, indicating that the nodes in the low-field state are predominantly located in the vicinity of the a – b plane. The order parameter modification is strongest when field is applied in the a – b plane, causing nodes to move away from the direction of the applied field. Both B_2u+iB_1u and B_2u+iA_u two-component order parameters can account for these observations.