Nontrivial paired states in novel topological superconductors

Abstract

We report on the superconductive paired states in TbPdBi, β-type FeSe and CaSn3 single crystals through temperature dependence of magnetic penetration depth analysis down to 0.040 K. For half-Heusler TbPdBi, in ultra-low temperature region we traced the T3 power-law behavior of penetration depth indicating the nodeless topological superconductivity with anisotropic gap structure which was confirmed with the analysis of superfluid density. That highlights the role of thermally activated quasi-particles in the pairing mechanism of TBPdBi for which the variation of Δλ(T) wanders off the renown quadratic behavior of penetration depth. In FeSe the contribution of London depth was described with a power law (n ≈ 2.4) and the superfluid density fitted well with two-band d + d model in which the values of both gaps are substantially smaller than the gap size expected by the isotropic BCS model, due to the nodeless anisotropic gap symmetry. In CaSn3, temperature variation of magnetic penetration depth exhibited linear behavior which is ascribed to the topologically nontrivial electronic states. Also, fitting the results of superfluid density exhibited features of multiband pairing allowing the nodal gap in the electronic states which is in good agreement with the expectation of London penetration depth. The complex superconducting order parameter is suggestive of two-dimensional spin singlet or mixing singlet-triplet pairing states behind the signatures of superconductivity in CaSn3. The mechanisms of pairing symmetry uncovered by the temperature dependence of penetration depth study of TbPdBi, FeSe and CaSn3 meet the requirements for topological superconductivity.