Superconductivity is a remarkable quantum phenomenon in which electrons form paired bound states, known as Cooper pairs, leading to zero electrical resistance. Bardeen–Cooper–Schrieffer (BCS) theory has long provided the microscopic foundation for conventional superconductivity, describing Cooper pairs with antiparallel electron spins: spin‑singlet pairing. This theory explains a wide range of superconducting materials. By contrast, electrons may also pair with parallel spins, forming a spin‑triplet state. Spin‑triplet superconductivity can host exotic physical properties, such as unusually high upper critical fields and novel types of pairing interaction, owing to its remaining spin degrees of freedom. Because experimentally confirmed examples are extremely scarce, identifying the mechanism that stabilizes spin‑triplet superconductivity is a major challenge in contemporary condensed matter physics.

Among the promising candidates, the uranium‑based compound UTe₂ has attracted significant attention. It exhibits highly unconventional properties, including reentrant superconductivity under strong magnetic fields and exceptionally high upper critical fields, that are difficult to reconcile with spin‑singlet pairing. Since its discovery in 2018, extensive research has been devoted to clarifying its superconducting characteristics. However, early experiments were often performed on samples with relatively low transition temperatures (T_c ~ 1.6 K), where the effects of impurities and sample dependence remained a concern. A major breakthrough occurred in 2022 with the achievement of high-quality single crystals with T_c = 2.1 K, making it possible to investigate the intrinsic mechanism of its superconductivity.

A key to understanding this mechanism is determining the superconducting gap structure, which reflects the symmetry of the pairing interaction. In many unconventional superconductors, the gap vanishes at specific points or lines on the Fermi surface, known as “nodes.” These nodes induce low‑energy quasiparticle excitations, which can be sensitively detected through specific-heat measurements. In particular, when a magnetic field is applied, the quasiparticle density of states rapidly increases depending on the field direction relative to the nodes because of the Volovik effect (the Doppler energy shift). In this study, high-precision field-angle‑resolved specific-heat measurements were performed using a high-quality crystal with T_c = 2.1 K.

The most striking finding is a profound anisotropy in the field dependence of the specific heat. While the specific heat increases rapidly for fields along the a and c axes, a hallmark of nodal quasiparticle excitations, it exhibits a striking linear dependence only when the field is applied along the b axis. This suppression of the rapid increase suggests that the Fermi velocity of the nodal quasiparticles is aligned predominantly with the b axis, which minimizes the Volovik effect for this specific direction. Furthermore, although the field-angle dependence displays a complex behavior, it can be consistently explained within the b-axis node scenario by considering the characteristic anisotropy of H_c2 and the Fermi surface geometry of UTe₂. These results provide crucial constraints on the gap symmetry and advance the ongoing efforts to uncover the nature of spin-triplet superconductivity in this fascinating compound.

(written by K. Totsuka on behalf of all authors.)

Share this topic

Fields