Topological triplet-superconductivity in spin-1 semimetal

Abstract

Spin-triplet superconductors are expected to host topological excitations, which makes them potentially useful materials for future quantum technologies. Here, the authors theoretically report a triple point semimetal that, through triple point fermions, stabilizes an s-wave spin-triplet pairing distinct from conventional BCS and other multi-band superconductors. Superconductivity in topological semimetals gives a new paradigm of unconventional superconductors. Their exotic gap structures and topological properties have fascinated searching for material realizations and applications. In this work, we focus on a triple point semimetal where quasiparticle excitations, triple point fermions, carry the effective integer spin-1 in two distinct valleys. Our work demonstrates that the triple point fermion stabilizes inter-valley s-wave spin-triplet pairing. This is due to Fermi statistics, which strictly forbids the formation of inter-valley s-wave spin-singlet pairings. This feature is clearly distinct from the BCS and other multi-band superconductors. We find that two distinct inter-valley s-wave spin-triplet superconductors are allowed which in principle can be controlled by tuning the chemical potential: time-reversal symmetric (s(z)) state with topologically protected nodal lines and time-reversal broken (s(x) + is(y)) state with topologically protected Bogoliubov Fermi surfaces. Our study provides guidance in searching for spin-triplet superconductivity.