Non-Trivial Superconductivity in the Semimetal EuAuBi

　In conventional s-wave superconductivity described by the Bardeen–Cooper–Schrieffer (BCS) theory, superconducting Cooper pairs are mediated by phonons, and these pairs (spin singlet) exhibit time-reversal and space-inversion symmetries. Therefore, magnetism and polarity that break these symmetries are generally incompatible with s-wave superconductivity. By contrast, superconductivity in systems without time-reversal and space-inversion symmetry has recently attracted attention as unconventional superconductivity. In particular, systems without space inversion symmetry exhibit a mixture of singlet and triplet pairing states. Furthermore, topological materials with surface states demonstrate spatial inversion symmetry breaking at the crystal interface, thereby providing a possibility for developing materials with unusual surface superconductivity.

　In this study, we successfully synthesized a single crystal of the topological semimetal EuAuBi, which has a layered structure comprising magnetic Eu triangle and conducting Au-Bi honeycomb layers. The synthesized crystal demonstrated antiferromagnetic and superconducting transitions at 4 and 2.4 K, respectively. The polar structure of this material is responsible for the alternating displacement of Au and Bi ions, resulting in a unique system that exhibits magnetic ordering and superconductivity with a polar structure.

　To investigate the superconducting properties of this material, changes in the superconducting transition temperature were measured down to as low as 0.1 K by applying an in-plane and an inter-plane magnetic field. Consequently, a superconducting critical field of 10 T, considerably greater than the Pauli limit expected for conventional superconductivity, was observed in the inter-plane field, while the critical field was only 3 T in the in-plane field. First-principles calculations revealed that Rashba spin splitting and a topological band structure are realized owing to the polar structure and strong spin–orbit coupling of Bi p orbitals. Therefore, the large critical field above the Pauli limit and its anisotropy may reflect a Rashba-type band structure and surface superconductivity, suggesting the realization of unconventional superconductivity.

　For the first time, we unveiled superconducting properties in the topological semimetal EuAuBi, which exhibits magnetic order and polar structure. The coexistence of large magnetization and superconductivity in this system may accelerate the development of spintronic materials that exhibit a novel external field response.

（written by H. Takahashi on behalf of all the authors. ）