Superconductivity

With the aim to shed light on the symmetry of the gap functions of novel superconductors, we carry out precise (resolution lower than 1 Å) measurements of the temperature dependence of the magnetic penetration depth utilizing a tunnel-diode oscillator in the range 10-20 MHz down to 40 mK and in ac magnetic field of the order of 1 mOe. This last condition assures that the sample is kept in the Meissner state.

The temperature behavior of the penetration depth depends basically on the symmetry and structure of the energy gap. In the local limit of the electrodynamics the penetration depth can be transformed into the superfluid density, which allows for a better comparison with theory.

Recently we have focused on noncentrosymmetric superconductors. The presence of an antisymmetric spin-orbit coupling in these compounds induces the mixing of spin-singlet and spin-triplet states. We have studied several of these superconductors. In CePt₃Si (T_c=0.75 K), the most emblematic of the noncentrosymmetric materials, we found support for line nodes in the energy gap. This was one of the first two works that gave evidence for unconventional superconductivity in noncentrosymmetric materials. In Mg₁₀Ir₁₉B₁₆ (T_c=5.7 K) we obtained indication for a two-gap behavior, possibly the first time that such a response was observed in a superconductor without inversion symmetry. We also studied KOs₂O₆ (T_c=9.6 K), a compound with a spin-frustrated lattice structure. Our results suggest that this material is a strong-coupling superconductor with a fully isotropic energy gap. More recently, we have measured LaPt₃Si (T_c=0.6 K), BaPtSi₃ (T_c=2.4 K), and La₂NiC (T_c=2.7 K).

We are now extending our penetration depth measurements to the high-pressure range. This will allow us to study pressure-induced superconductors and pressure effects on novel superconductors.