Antiferromagnetic spin fluctuations and superconductivity in NbRh$_2$B$_2$ and TaRh$_2$B$_2$ with a chiral crystal structure

TL;DR

This study uses $^{11}$B NMR to investigate the unconventional superconductivity and antiferromagnetic spin fluctuations in non-centrosymmetric NbRh$_2$B$_2$ and TaRh$_2$B$_2$ with chiral structures, revealing differences linked to spin-orbit coupling.

Contribution

It provides the first NMR evidence of antiferromagnetic spin fluctuations in these chiral superconductors and compares their magnetic properties.

Findings

No coherence peak in $1/T_1$ below $T_c$, indicating unconventional superconductivity.

Presence of antiferromagnetic spin fluctuations in both compounds.

Stronger spin fluctuations observed in TaRh$_2$B$_2$ compared to NbRh$_2$B$_2$.

Abstract

We report the $^{11}$B nuclear magnetic resonance (NMR) measurements on non-centrosymmetric superconductors NbRh$_2$B$_2$ (superconducting transition temperature $T_c$ = 7.8 K) and TaRh$_2$B$_2$ ($T_c$ = 5.9 K) with a chiral crystal structure. The nuclear spin-lattice relaxation rate $1/T_1$ shows no coherence peak below $T_{\rm c}$, which suggests unconventional nature of the superconductivity. In the normal state, $1/T_1T$ increases with decreasing temperature $T$ at low temperatures below $T$ = 200 K for TaRh$_2$B$_2$ and $T$ = 15 K for NbRh$_2$B$_2$, while the Knight shift remains constant. These results suggest the presence of antiferromagnetic spin fluctuations in both compounds. The stronger spin fluctuations in TaRh$_2$B$_2$ compared to NbRh$_2$B$_2$ is discussed in the context of spin-orbit coupling.