Evidence of nodes in the non-centrosymmetric superconductor Y$_{2}$C$_{3}$

In a non-centrosymmetric superconductor, antisymmetric spin-orbit coupling (ASOC) can admix spin-singlet and spin-triplet pairing states, leading to accidental nodes in the energy gap [1]. Y$_{2}$C$_{3}$ is such a superconductor with a T$_{c}$ of 18K. Early NMR and $\mu $SR results [2] indicated a gap structure incompatible with either BCS s-wave or typical d-wave behavior. To further elucidate its superconducting properties, we have measured the temperature dependence of the magnetic penetration depth using a tunneling-diode oscillator technique. While the high temperature penetration depth, and therefore its corresponding superfluid density, can be well described by a two-gap BCS model, as discussed in Ref. [2], the low temperature penetration depth follows a linear temperature dependence, indicating possible existence of nodes in the energy gap. Together with the large upper critical field observed in Y$_{2}$C$_{3}$ [3], the existence of nodes, we argue, might be attributed to the ASOC as a result of absent inversion symmetry even though other possibilities cannot be excluded. [1] H. Q. Yuan et al, Phys. Rev. Lett. \textbf{97}, 017006 (2006). [2] A. Harada et al, J. Phys. Soc. Jpn. \textbf{76}, 023704(2007); S. Kuroiwa et al, Phys. Rev. Lett. \textbf{100}, 097002 (2008). [3] H. Q. Yuan et al, J. Phys. Chem. Solids (in press).