Superconductors without an inversion center of symmetry: The s-wave state

TL;DR

This paper investigates the superconducting order parameter in non-centrosymmetric materials, focusing on the s-wave state, its response to disorder, and the potential for line nodes in the gap, with implications for understanding experimental data.

Contribution

It analyzes the structure and disorder response of the s-wave superconducting state in non-centrosymmetric materials, exemplified by CePt₃Si, and explores the possibility of line nodes in the gap.

Findings

Superconducting order parameter can be a mixture of singlet and triplet states.

Impurity concentration affects the superconducting instability.

Presence of accidental line nodes could explain low-temperature experimental data.

Abstract

In materials without an inversion center of symmetry the spin degeneracy of the conducting band is lifted by an antisymmetric spin orbit coupling (ASOC). Under such circumstances, spin and parity cannot be separately used to classify the Cooper pairing states. Consequently, the superconducting order parameter is generally a mixture of spin singlet and triplet pairing states. In this paper we investigate the structure of the order parameter and its response to disorder for the most symmetric pairing state ($A_1$). Using the example of the heavy Fermion superconductor CePt$_3$Si, we determine characteristic properties of the superconducting instability as a function of (non-magnetic) impurity concentrations. Moreover, we explore the possibility of the presence of accidental line nodes in the quasiparticle gap. Such nodes would be essential to explain recent low-temperature data of thermodynamic quantities such as the NMR-$T_1^{-1}$, London penetration depth, and heat conductance.