We report the first observation of the de Haas–van Alphen (dHvA) effect in the novel spin-triplet superconductor UTe2 using high quality single crystals with a high residual resistivity ratio (RRR)...

The molten salt flux method is applied as a synthetic route for the single crystals of the spin-triplet superconductor ${\mathrm{UTe}}_2$. The single crystals under an optimized growth condition with excess uranium exhibit a superconducting transition at $T_{\mathrm{c}}=2.1\mathrm{K}$, which is the highest $T_{\mathrm{c}}$ reported for this compound. The obtained crystals show a remarkably large residual resistivity ratio with respect to the room temperature value and a small residual electronic contribution in specific heat well below $T_{\mathrm{c}}$. These results indicate that the increase of $T_{\mathrm{c}}$ in ${\mathrm{UTe}}_2$ can be achieved by reducing the disorder associated with uranium vacancies. The excess uranium in the molten salt acts as a reducing agent, preventing tetravalent uranium from becoming pentavalent and suppressing creation of uranium vacancies. At the same time, the relatively low growth temperature can suppress Te volatilization.

Chiral spin-triplet superconductivity is a topologically nontrivial pairing state with broken time-reversal symmetry, which can host Majorana quasiparticles. The heavy-fermion superconductor UTe₂ exhibits peculiar properties of spin-triplet pairing, and the possible chiral state has been actively discussed. However, the symmetry and nodal structure of its order parameter in the bulk, which determine the Majorana surface states, remains controversial. Here we focus on the number and positions of superconducting gap nodes in the ground state of UTe₂. Our magnetic penetration depth measurements for three field orientations in three crystals all show the power-law temperature dependence with exponents close to 2, which excludes single-component spin-triplet states. The anisotropy of low-energy quasiparticle excitations indicates multiple point nodes near the k_y- and k_z-axes in momentum space. These results can be consistently explained by a chiral B_3u + iA_u non-unitary state, providing fundamentals of the topological properties in UTe₂.

The heavy fermion paramagnet UTe₂exhibits numerous characteristics of spin-triplet superconductivity. Efforts to understand the microscopic details of this exotic superconductivity have been impeded by uncertainty regarding the underlying electronic structure. Here we directly probe the Fermi surface of UTe₂by measuring magnetic quantum oscillations in pristine quality crystals. We find an angular profile of quantum oscillatory frequency and amplitude that is characteristic of a quasi-2D Fermi surface, which we find is well described by two cylindrical Fermi sheets of electron- and hole-type respectively. Additionally, we find that both cylindrical Fermi sheets possess considerable undulation but negligible small-scale corrugation, which may allow for their near-nesting and therefore promote magnetic fluctuations that enhance the triplet pairing mechanism. Importantly, we find no evidence for the presence of any 3D Fermi surface sections. Our results place strong constraints on the possible symmetry of the superconducting order parameter in UTe₂.

The recently discovered superconductor UTe ₂ is a promising candidate for spin-triplet superconductors, but the symmetry of the superconducting order parameter remains highly controversial. Here, we determine the superconducting gap structure by the thermal conductivity of ultraclean UTe ₂ single crystals. We find that the a -axis thermal conductivity divided by temperature κ/ T in zero-temperature limit is vanishingly small for both magnetic field H ‖ a and H ‖ c axes up to H / H _{c 2} ∼ 0.2, demonstrating the absence of nodes around the a axis contrary to the previous belief. The present results, combined with the reduction of nuclear magnetic resonance Knight shift, indicate that the superconducting order parameter belongs to the isotropic A _u representation with a fully gapped pairing state, analogous to the B phase of superfluid ³ He. These findings reveal that UTe ₂ is likely to be a long-sought three-dimensional strong topological superconductor, hosting helical Majorana surface states on any crystal plane.