Evidence for two-gap superconductivity in the non-centrosymmetric compound LaNiC₂

Abstract

We study the superconducting properties of the noncentrosymmetric compound LaNiC 2 by measuring the London penetration depth (T ), the specific heat C(T, B) and the electrical resistivity ρ(T, B). Both λ(T ) and the electronic specific heat C e (T ) exhibit behavior at low temperatures that can be described in terms of a phenomenological two-gap Bardeen-Cooper-Schrieffer (BCS) model. The residual Sommerfeld coefficient in the superconducting state, γ 0 (B), shows a rapid increase at low fields and then an eventual saturation with increasing magnetic field. A pronounced upturn curvature is observed in the upper critical field B c2 (T ) near T c . All these experimental observations support the existence of two-gap superconductivity in LaNiC 2 .

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

“…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 remains antisymmetric overall due to a sign change upon exchanging electrons between different orbitals.…”

“…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].…”

Superconductivity in non-centrosymmetric ThCoC2

“…This kind of behavior can be attributed to interband coupling that can occur within a multiband model [27,31], with the small gap contributing at low temperature. In fact, a recent study on the superconducting state of LaNiC 2 also displays positive curvature near T c and an enhancement of H c2 (0) [31]. Chen et al systematically measured the low temperature London penetration depth, heat capacity, and resistivity, and argue the superconducting state in LaNiC 2 is best described by a two-gap BCS model, which is the likely result of a moderate value of the ASOC that splits the spin degenerate bands.…”

Tuning of superconductivity by Ni substitution into noncentrosymmetric ThCo1−xNixC2

“…[27][28][29] However, more recent experiments have observed unconventional behavior, such as intriguing time reversal symmetry (TRS) breaking in muon-spin relaxation measurements, a nodal energy gap from very-low temperature magnetic penetration depth measurements, and a proposed multi-gap superconductivity due to the moderate value of the ASOC in LaNiC2. [33][34][35] The isostructural compound ThCoC2 was recently discovered to be a new NCS superconductor with a critical temperature of Tc=2.65K. 36 The superconductivity of ThCoC2 was found to exhibit unconventional behavior with a strong positive curvature in the upper critical field Hc2 …”