Nodal gap structure in the noncentrosymmetric superconductor LaNiC2 from magnetic-penetration-depth measurements

We report measurements of the temperature dependence of the magnetic penetration depth in different quality polycrystalline samples of noncentrosymmetric LaNiC2 down to 0.05 K. This compound has no magnetic phases and breaks time-reversal symmetry. In our highest quality sample we observe a T^2 dependence below 0.4Tc indicative of nodes in the energy gap. We argue that previous results suggesting conventional s-wave behavior may have been affected by magnetic impurities.

Magnetic penetration depth and gap symmetry of the noncentrosymmetric superconductors CePt3Si and LaPt3Si

We report on measurements of the temperature dependence of the magnetic penetration depth of a very high quality single crystal of nonmagnetic superconductor LaPt3Si without inversion symmetry. The results are compared with those previously reported for the isostructural antiferromagnetic superconductor CePt3Si. At low temperatures, the penetration depth follows a BCS exponential behavior that implies an isotropic energy gap in LaPt3Si, in contrast to a linear response that indicates line nodes in CePt3Si. These line nodes have been argued to be protected by symmetry or accidentally generated by parity mixing. The present results provide support for the viewpoint that parity mixing alone does not seem to lead to unconventionality in CePt3Si and that it requires the antiferromagnetic order to be included.

Unusual behaviours and Impurity Effects in the Noncentrosymmetric Superconductor CePt3Si

We report a study in which the effect of defects/impurities, growth process, off-stoichiometry, and presence of impurity phases on the superconducting properties of noncentrosymmetric CePt3Si is analysed by means of the temperature dependence of the magnetic penetration depth. We found that the linear low-temperature response of the penetration depth -indicative of line nodes in this material- is robust regarding sample quality, in contrast to what is observed in unconventional centrosymmetric superconductors with line nodes. We discuss evidence that the broadness of the superconducting transition may be intrinsic, though not implying the existence of a second superconducting transition. The superconducting transition temperature systematically occurs around 0.75 K in our measurements, in agreement with resistivity and ac magnetic susceptibility data but in conflict with specific heat, thermal conductivity and NMR data in which Tc is about 0.5 K. Random defects do not change the linear low-temperature dependence of the penetration depth in the heavy-fermion CePt3Si with line nodes, as they do in unconventional centrosymmetric superconductors with line nodes.