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

We report the B11 nuclear magnetic resonance measurements on noncentrosymmetric superconductors NbRh2B2 (superconducting transition temperature Tc=7.8 K) and TaRh2B2 (Tc=5.9 K) with a chiral crystal structure. The nuclear spin-lattice relaxation rate 1/T1 shows no coherence peak below Tc, which suggests the unconventional nature of the superconductivity. In the normal state, 1/T1T increases with decreasing temperature T at low temperatures below T=200 K for TaRh2B2 and T=15 K for NbRh2B2, while the Knight shift remains constant. These results suggest the presence of antiferromagnetic spin fluctuations in both compounds. The stronger spin fluctuations in TaRh2B2 compared to NbRh2B2 is discussed in the context of spin-orbit coupling.