Two-Gap Superconductivity inLaNiGa2with Nonunitary Triplet Pairing and Even Parity Gap Symmetry

Abstract

The nature of the pairing states of superconducting LaNiC 2 and LaNiGa 2 has to date remained a puzzling question. Broken time reversal symmetry has been observed in both compounds and a group theoretical analysis implies a non-unitary triplet pairing state. However all the allowed non-unitary triplet states have nodal gap functions but most thermodynamic and NMR measurements indicate fully gapped superconductivity in LaNiC 2 . Here we probe the gap symmetry of LaNiGa 2 by measuring the London penetration depth, specific heat and upper critical field. These measurements demonstrate two-gap nodeless superconductivity in LaNiGa 2 , suggesting that this is a common feature of both compounds. These results allow us to propose a novel triplet superconducting state, where the pairing occurs between electrons of the same spin, but on different orbitals. In this case the superconducting wavefunction has a triplet spin component but isotropic even parity gap symmetry, yet the overall wavefunction remains antisymmetric under particle exchange. This model leads to a nodeless twogap superconducting state which breaks time reversal symmetry, and therefore accounts well for the seemingly contradictory experimental results. The breaking of symmetries in addition to gauge symmetry upon entering the superconducting state usually indicates an unconventional order parameter. Several materials have been found to break time reversal symmetry (TRS) in the superconducting state through the detection of spontaneous magnetic fields below T c using zero-field muon-spin relaxation (µSR [14], where it has been argued that as a result of the low symmetry of the orthorhombic crystal structures of both compounds, broken TRS necessarily implies non-unitary triplet superconductivity and all the TRS breaking states have nodes in the gap function [15]. Although evidence for nodal superconductivity was found from some measurements [16,17], recent specific heat [18,19], nuclear quadrapole relaxation [20] and penetration depth [19] measurements indicate fully gapped behavior in LaNiC 2 . In addition, evidence for two-gap superconductivity was found from the specific heat, superfluid density and upper critical field [19]. There have been fewer measurements of superconductivity in LaNiGa 2 [21], which has an orthorhombic centrosymmetric crystal structure in contrast to noncentrosymmetric LaNiC 2 , although fully gapped behavior was inferred from the specific heat [22].In this Letter, we suggest a solution to this apparent contradiction from measurements of the London penetration depth, specific heat and upper critical field, all of which consistently suggest the presence of two-gap superconductivity in LaNiGa 2 . Along with previous results of LaNiC 2 [19], we establish that nodeless, two-gap superconductivity is a common feature of these compounds. We propose that pairing between electrons with the same spins but on different orbitals gives rise to a triplet superconducting state with even parity pairing in both compounds, where the wave function re...

Quantitative theory of triplet pairing in the unconventional superconductor LaNiGa2

“…One additional complication is the multi-band nature of these systems: two [26] and five [27] bands cross the Fermi level of LaNiC 2 and LaNiGa 2 , respectively. In fact, both systems show thermodynamic properties that can be fitted with a model assuming fully-gapped, 2-band superconductivity [11,28,29]. This is inconsistent with the line nodes implied by the earlier symmetry analyses [10,25].…”

“…The known value of the critical temperature T c fixes this single parameter, allowing us to make parameter-free predictions. We obtain the electronic specific heat and find an excellent agreement with experiment [11]. We compute the spin-resolved DOS having a double-peak structure with each peak corresponding to a single spin channel.…”

“…On the other hand the 2-band model does not predict TRS breaking. To resolve the discrepancy it was proposed that only an internally-antisymmetric non-unitary triplet pairing (INT) state is compatible with the experimental observations [10,11,25]. Here we show that such a state has a very distinct experimental signature: a double-peak structure in the Density of States (DOS) which resolves in the spin channel.…”

Time-reversal symmetry breaking in the noncentrosymmetric superconductor Re6Hf : Further evidence for unconventional behavior in the α -Mn family of materials

“…In this case, the superconductivity is conventional, yet exhibits TRSB. A second explanation could be that of the mechanism governing superconductivity in LaNiC 2 and LaNiGa 2 [27].…”

“…This seems to suggest that the TRSB in LaNiC 2 occurs in spite of the fact that the crystal structure is noncentrosymmetric, and that a significant spin-singlet/-triplet admixture ground state is forbidden. Recent measurements of the gap structure in LaNiC 2 have revealed two-gap superconductivity, and it was recently proposed that the superconducting ground state is fully gapped, with nonunitary triplet pairing between electrons that exist on different orbitals [27].…”