Possible unconventional superconductivity in substituted BaFe$_{2}$As$_{2}$ revealed by magnetic pair-breaking studies

TL;DR

This study investigates the pairing symmetry in BaFe$_{2}$As$_{2}$-derived superconductors using magnetic pair-breaking effects, ESR, and pressure-dependent transport, revealing unconventional superconductivity with sign-changing gap symmetry.

Contribution

It combines ESR and pressure measurements to show magnetic pair-breaking effects inconsistent with sign-preserving gaps, suggesting unconventional pairing in these materials.

Findings

ESR signals indicate localized magnetic moments in Cu and Mn compounds.

Reduction of $T_c$ cannot be explained by conventional Abrikosov-Gorkov theory.

Results suggest spin- and pressure-dependent pair-breaking effects imply unconventional pairing.

Abstract

The possible existence of a sign-changing gap symmetry in BaFe$_{2}$As$_{2}$-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe$_{1.9}M_{0.1}$As$_{2}$ ($M=$ Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for $M=$ Cu and Mn compounds, which display very low SC transition temperature ($T_{c}$) and no SC, respectively. From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of $T_{c}$ cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function. Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.