Antiferromagnetic spin fluctuations and superconductivity in NbRh2B2 and TaRh2B2 with a chiral crystal structure

Abstract

We report the $^{11}\mathrm{B}$ nuclear magnetic resonance measurements on noncentrosymmetric superconductors $\mathrm{Nb}{\mathrm{Rh}}_{2}{\mathrm{B}}_{2}$ (superconducting transition temperature ${T}_{c}=7.8 \mathrm{K}$) and $\mathrm{Ta}{\mathrm{Rh}}_{2}{\mathrm{B}}_{2}$ $({T}_{c}=5.9 \mathrm{K})$ with a chiral crystal structure. The nuclear spin-lattice relaxation rate $1/{T}_{1}$ shows no coherence peak below ${T}_{\mathrm{c}}$, which suggests the unconventional nature of the superconductivity. In the normal state, $1/{T}_{1}T$ increases with decreasing temperature $T$ at low temperatures below $T=200 \mathrm{K}$ for $\mathrm{Ta}{\mathrm{Rh}}_{2}{\mathrm{B}}_{2}$ and $T=15 \mathrm{K}$ for $\mathrm{Nb}{\mathrm{Rh}}_{2}{\mathrm{B}}_{2}$, while the Knight shift remains constant. These results suggest the presence of antiferromagnetic spin fluctuations in both compounds. The stronger spin fluctuations in $\mathrm{Ta}{\mathrm{Rh}}_{2}{\mathrm{B}}_{2}$ compared to $\mathrm{Nb}{\mathrm{Rh}}_{2}{\mathrm{B}}_{2}$ is discussed in the context of spin-orbit coupling.