Evidence of nodal gap structure in the noncentrosymmetric superconductor Y2C3

The magnetic penetration depth λ(T) and the upper critical field μ₀H_c2(T_c) of the non-centrosymmetric superconductor Y₂C₃ have been measured using a tunnel-diode-based resonant oscillation technique. We found that the penetration depth λ(T) and its corresponding superfluid density ρ_s(T) show linear temperature dependence at very low temperatures (T≪T_c), indicating the existence of line nodes in the superconducting energy gap. Moreover, the upper critical field μ₀H_c2(T_c) presents a weak upturn at low temperatures with a rather high value of μ₀H_c2(0) ≃29 T, which slightly exceeds the weak-coupling Pauli limit. We discuss the possible origins for these nontrivial superconducting properties, and argue that the nodal gap structure in Y₂C₃ is likely attributed to the absence of inversion symmetry, which allows the admixture of spin-singlet and spin-triplet pairing states.