Abstract
Superconductors with broken time-reversal symmetry represent arguably one of the most promising venues for realizing highly sought-after topological superconductivity that is vital to fault-tolerant quantum computation. Here, by using extensive muon-spin relaxation and rotation measurements, we report that the isostructural silicide superconductors (Ta, Nb)OsSi spontaneously break time-reversal symmetry at the superconducting transition while surprisingly showing a fully-gapped superconductivity characteristic of conventional superconductors. The first-principles calculations show that (Ta, Nb)OsSi are three-dimensional Dirac semimetals protected by nonsymmorphic symmetries. Taking advantage of the exceptional low symmetry crystal structure of these materials, we have performed detailed theoretical calculations to establish that the superconducting ground state for both (Ta, Nb)OsSi is most likely a nonunitary triplet state.
Highlights
Dirac or Weyl semimetals have attracted significant research interest due to their exceptional physical properties arising from the topologically protected gapless electronic excitations [1,2]
(Ta, Nb)OsSi crystallize in a TiNiSi-type centrosymmetric orthorhombic crystal structure and have similar physical and chemical properties [15]
ZF-μSR: ZF-μSR measurements were performed in search for spontaneous magnetic fields that can appear in the superconducting state leading to breaking of timereversal symmetry (TRS)
Summary
Dirac or Weyl semimetals have attracted significant research interest due to their exceptional physical properties arising from the topologically protected gapless electronic excitations [1,2]. It is usually difficult to unambiguously establish the structure of the superconducting order parameters for the TRS-breaking superconductors mainly due to two reasons: (a) lack of sufficient knowledge of the electron pairing mechanism and (b) highly symmetric crystal structures leading to many possibilities with similar low-temperature properties. The point group D4h of Sr2RuO4 allows for 20 possibilities with weak spin-orbit coupling (SOC) and 2 possibilities with strong SOC, of TRS-breaking superconducting instabilities [5,6] In this regard, the superconductors LaNiC2 [7,8], LaNiGa2 [9–12], and UTe2 [13] are exceptions due to their very low-symmetry crystal structure that leads only to a few symmetry-allowed superconducting order parameters. By means of symmetry analysis and model calculations, our observations are found to be consistent with a nonunitary triplet superconducting ground state, the verification of which shall stimulate further study, both experimentally and theoretically
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