  UTe2 is a newly discovered unconventional superconductor, where electron
Cooper pairs combine into a spin-triplet ground state. Here we report the
specific heat C(H,T) of a high-quality single crystal of UTe2 with a single
specific heat anomaly at the superconducting transition temperature T_c
{\approx} 2 K and a small zero-field residual Sommerfeld coefficient {\gamma}_0
= C/T (T=0) = 10 mJ/mol-K^2. We applied magnetic field up to 12 T along the
three principal crystallographic axes of UTe2 to probe the nature of the
superconducting state. The evolution of the residual Sommerfeld coefficient as
a function of magnetic field, {\gamma}_0 (H), is highly anisotropic and reveals
distinct regions. In magnetic field up to 4 T applied along a, b, and c axes,
we find {\gamma}_0{\approx}{\alpha}_i {\square}H, with i=a,b,c, as expected for
an unconventional superconductor with nodes (zeros) of the superconducting
order parameter on the Fermi surface. A pronounced kink in {\gamma}_0(H),
however, is observed at roughly 4 T for field applied along both a and b axes,
whereas a smooth change from square-root to linear behaviour is observed at 4 T
for H//c. These results strongly indicate that a zero-field ground state is
stable up to 4 T and undergoes a field-induced evolution above 4 T. {\alpha_c}
> {\alpha_a} > {\alpha_b}, indicating that the nodes in the low-field state are
predominantly located in the vicinity of the a-b plane. The modification of the
order parameter is strongest when field is applied in the a-b plane, which
causes nodes to move away from the direction of the applied field. Both
d_(B_2u)+id_(B_1u) and d_(B_2u)+id_(A_u) two-component order parameters can
account for our observations, with d_(B_2u)+id_(B_1u) a more likely candidate.
In either scenario, our measurements indicate that B_2u is the primary
superconducting order parameter in UTe2.
