title: Two-Gap Superconductivity in LaNiGa_{2} with 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 nonunitary triplet pairing state. However, all the allowed nonunitary 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 wave function has a triplet spin component but isotropic even parity gap symmetry, yet the overall wave function remains antisymmetric under particle exchange. This model leads to a nodeless two-gap superconducting state which breaks time reversal symmetry, and therefore accounts well for the seemingly contradictory experimental results.
authors.affiliation: Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China.
authors.forename: Z F
authors.initials: ZF
authors.lastname: Weng
authors.affiliation: Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China.
authors.forename: J L
authors.initials: JL
authors.lastname: Zhang
authors.affiliation: Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China.
authors.forename: M
authors.initials: M
authors.lastname: Smidman
authors.affiliation: Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China.
authors.forename: T
authors.initials: T
authors.lastname: Shang
authors.affiliation: SEPnet and Hubbard Theory Consortium, University of Kent, Canterbury CT2 7NH, United Kingdom.
authors.forename: J
authors.initials: J
authors.lastname: Quintanilla
authors.affiliation: H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United Kingdom.
authors.forename: J F
authors.initials: JF
authors.lastname: Annett
authors.affiliation: Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Nicklas
authors.affiliation: Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China.
authors.forename: G M
authors.initials: GM
authors.lastname: Pang
authors.affiliation: Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China.
authors.forename: L
authors.initials: L
authors.lastname: Jiao
authors.affiliation: Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China.
authors.forename: W B
authors.initials: WB
authors.lastname: Jiang
authors.affiliation: Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China.
authors.forename: Y
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China.|Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Steglich
authors.affiliation: Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China.|Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China.
authors.forename: H Q
authors.initials: HQ
authors.lastname: Yuan