Speaker
Description
The convergence of two major research strands in modern condensed-matter physics: topological materials and unconventional superconductivity, constitutes a new field of study. Topological materials with Kramers or hourglass fermions represent a special subclass, recently realized in materials lacking inversion symmetry or with a nonsymmorphic space group. At the same time, there is a surge of interest in identifying time-reversal symmetry (TRS) breaking (a key feature of unconventional superconductivity) in this class of materials, as a new routine way to realize topological superconductivity.
By using the muon-spin rotation and relaxation technique, backed by detailed theoretical analyses, we show that TRuSi (T = Ti, Nb, Hf, and Ta) noncentrosymmetric materials represent a family of compounds encompassing all the above unique properties [1]. Their bulk normal states behave as three-dimensional Kramers nodal-line semimetals, characterized by a fairly large antisymmetric spin-orbit coupling and by glide-reflection-protected hourglass-like fermions. We also identify surface states near the Fermi level of TRuSi materials More interestingly, NbRuSi and TaRuSi undergo a superconducting transition, which spontaneously breaks TRS below $T_c$, while surprisingly showing a fully-gapped superconducting ground state. This superconducting ground state is consistent with a unitary (s + ip) pairing, i.e., with a mixture of spin-singlet and spin-triplet pairings. As such, the TRuSi family provides an ideal platform for investigating the rich interplay between the exotic properties of Kramers nodal-line/hourglass fermions and unconventional superconductivity.
[1] T. Shang, J. Z. Zhao, et al., and T. Shiroka, submitted to Sci. Adv. (2022).
