Evidence of nodal gap structure in the noncentrosymmetric superconductorY2C3

Abstract

The magnetic penetration depth $\ensuremath{\lambda}(T)$ and the upper critical field ${\ensuremath{\mu}}_{0}{H}_{c2}({T}_{c})$ of the non-centrosymmetric superconductor ${\mathrm{Y}}_{2}$C${}_{3}$ have been measured using a tunnel-diode-based resonant oscillation technique. We found that the penetration depth $\ensuremath{\lambda}(T)$ and its corresponding superfluid density ${\ensuremath{\rho}}_{s}(T)$ show linear temperature dependence at very low temperatures ($T\ensuremath{\ll}{T}_{c}$), indicating the existence of line nodes in the superconducting energy gap. Moreover, the upper critical field ${\ensuremath{\mu}}_{0}{H}_{c2}({T}_{c})$ presents a weak upturn at low temperatures with a rather high value of ${\ensuremath{\mu}}_{0}{H}_{c2}(0)$ $\ensuremath{\simeq}29$ 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 ${\mathrm{Y}}_{2}$C${}_{3}$ is likely attributed to the absence of inversion symmetry, which allows the admixture of spin-singlet and spin-triplet pairing states.