Spin-triplet superconductivity in Weyl nodal-line semimetals

Abstract

Topological semimetals are three dimensional materials with symmetry-protected massless bulk excitations. As a special case, Weyl nodal-line semimetals are realized in materials having either no inversion or broken time-reversal symmetry and feature bulk nodal lines. The 111-family, including LaNiSi, LaPtSi and LaPtGe materials (all lacking inversion symmetry), belongs to this class. Here, by combining muon-spin rotation and relaxation with thermodynamic measurements, we find that these materials exhibit a fully-gapped superconducting ground state, while spontaneously breaking time-reversal symmetry at the superconducting transition. Since time-reversal symmetry is essential for protecting the normal-state topology, its breaking upon entering the superconducting state should remarkably result in a topological phase transition. By developing a minimal model for the normal-state band structure and assuming a purely spin-triplet pairing, we show that the superconducting properties across this family can be described accurately. Our results demonstrate that the 111 materials reported here provide an ideal test-bed for investigating the rich interplay between the exotic properties of Weyl nodal-line fermions and unconventional superconductivity.

Quantum conductivity in the topological surface state in the SbV3S5 kagome lattice

“…Weyl3 and Weyl4 clearly reveal itinerant spin electron transport via small polarons due to the two valence states of the Sb atom differing by 2e (on-site pairing), which can serve as a phenomenological description of unconventional superconductivity. 23 Weyl5 and Weyl6 exhibit similar electronic transport behaviour. In addition, one can see (Fig.…”

Unconventional superconductivity in topological Kramers nodal-line semimetals

“…To date, only a handful of weakly correlated NCSC families have been shown to exhibit TRS breaking in the superconducting state. These include LaNiC 2 ( 22 ), La 7 (Rh,Ir) 3 ( 23 , 62 ), Zr 3 Ir ( 27 ), Re T ( 24 – 26 ), CaPtAs ( 29 ), and, very recently, LaPt 3 P ( 30 ) and La T (Si,Ge) ( 28 ). Among the TRS-breaking superconductors, CaPtAs and LaPt 3 P represent a rare case, where broken TRS and nodal SC were observed to occur simultaneously below T c ( 29 , 30 ).…”

“…Such INT pairing was originally proposed to explain the TRS breaking and nodeless SC in LaNiGa 2 ( 65 , 66 ) and LaNiC 2 ( 22 , 67 ), both characterized by a relatively weak SOC. Recently, this model was applied also to La T (Si,Ge) and (Nb,Ta)OsSi superconductors ( 28 , 68 ). In this scenario, microscopic supercurrent loops with a uniform onsite and intra-orbital singlet pairing form spontaneously within the unit cell.…”

“…In parallel, current research has shown that several noncentrosymmetric materials exhibit SC at low temperatures (19)(20)(21) and, in view of their structure, are known as noncentrosymmetric superconductors (NCSCs). Time-reversal symmetry (TRS) breaking has been shown to occur in the superconducting state of several NCSCs (22)(23)(24)(25)(26)(27)(28)(29)(30). Broken TRS in the superconducting state, one of the possible realizations of unconventional SC, is here manifested as the spontaneous appearance of magnetic fields below the onset of SC.…”

La4TX (T = Ru, Rh, Ir; X = Al, In): A family of noncentrosymmetric superconductors with tunable antisymmetric spin-orbit coupling

“…These findings stimulated interest in weakly correlated NCS where there is strong band-splitting by the ASOC due to the presence of heavy elements [1,[14][15][16][17]. On the other hand, the properties of many reported weakly correlated NCSs are consistent with fully gapped superconductivity, including La(Ni, Pt)Si [18][19][20][21], La(Rh, Pt, Pd, Ir)Si 3 [22][23][24][25], (Rh, Ir) 2 Ga 9 [26,27], Re-T (T = transition metal) [28][29][30][31][32], and BiPd [33,34], despite a number of these systems exhibiting an appreciable band splitting due to the ASOC [1]. As such, the relationship between the ASOC and nature of the superconducting order parameter, as well as the necessary conditions for significant singlet-triplet mixing, still remains to be determined, requiring the study of additional classes of NCS where the ASOC can be tuned.…”