Weakly Anisotropic Noncentrosymmetric Superconductors with Radial Line Nodes and Thermodynamical Anomalies

Abstract

In noncentrosymmetric superconductors (NCSs), the inversion symmetry (IS) is most commonly broken by an antisymmetric spin-orbit coupling (SOC) removing the spin degeneracy and splitting the Fermi surface (FS) into two branches. A two component condensate is then produced with a doublet pair potential mixing an even singlet and an odd triplet. When the triplet and the singlet strengths are comparable, the pair potential can have rich nodes. The angular line nodes (ALNs) are associated with strong anisotropy and they are widely studied in the literature. When the anisotropy is not strong, they can be replaced by other types of nodes in closed or open forms affecting the low temperature properties. Here, we focus on the weakly anisotropic case and the line nodes in the superconducting plane which become circular in the limit of full isotropy. We study the topology of these radial line nodes (RLNs) and show that it is characterized by the $Z_2$ classification similar to the Quantum-Spin-Hall Insulators. From the thermodynamical perspective, the RLNs cause, even in the topological phases, an exponentially suppressed low temperature behaviour which can be mistaken by nodeless s-wave pairing, thus, providing an explanation to a number of recent experiments with contraversial pairing symmetries. In the rare case when the RLN is on the Fermi surface, the exponential suppression is replaced by a linear temperature dependence. The RLNs are difficult to detect, and for this reason, they may have escaped experimental attention. We demonstrate that Andreev conductance measurements with clean interfaces can efficiently probe the weakly anisotropic samples where the RLNs are expected to be found.