Nodal gap structure in the noncentrosymmetric superconductor LaNiC₂from magnetic-penetration-depth measurements

Abstract

We report measurements of the temperature dependence of the magnetic penetration depth in different quality polycrystalline samples of noncentrosymmetric LaNiC 2 down to 0.05 K. This compound has no magnetic phases and breaks timereversal symmetry. In our highest quality sample we observe a T 2 dependence below 0.4T c indicative of nodes in the energy gap. We argue that previous results suggesting conventional s-wave behavior may have been affected by magnetic impurities.

Superconductivity in non-centrosymmetric ThCoC2

“…Indeed it was shown that the specific heat of the analog LaNiC 2 compound also deviates from conventional exponential behavior with a low-temperature T 3 dependence [14] that is consistent with nodes in the energy gap, in addition to other evidence of non-conventional behavior of time reversal symmetry breaking in µSR measurements [21], and evidence for nodal gap structure with T 2 dependence in magnetic penetration depth measurements [22]. It is worth noting that earlier conflicting reports on LaNiC 2 conclude conventional BCS behavior in heat capacity [24] and Nuclear quadrupole resonance (NQR) measurements [25]; however the recent report by Bonalde et al [22] shows that is likely a result of the measurements sensitivity to magnetic Fe impurities introduced by lower quality Ni (3N) and the insufficient low temperature evaluation of the superconducting state in those studies. They correctly point out that temperatures below 0.3T c are needed to properly determine the structure of the energy gap, in which they measured magnetic penetration depth down to ∼0.017T c in LaNiC 2 .…”

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 …”

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

“…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].…”