Antiferromagnetic spin fluctuations and unconventional superconductivity in topological superconductor candidate YPtBi revealed by ${}^{195}$Pt-NMR

Abstract

We report ${}^{195}$Pt nuclear magnetic resonance (NMR) measurements on topological superconductor candidate YPtBi which has the broken inversion symmetry and topological non-trivial band structures due to the strong spin-orbit coupling(SOC). In the normal state, we find that Knight shift $K$ is field- and temperature-independent, suggesting that the contribution from the topological bands is very small at low temperatures. However, the spin-lattice relaxation rate 1/$T_1$ divided by temperature ($T$), 1/$T_{1}T$, increases with decreasing $T$, implying the existence of antiferromagnetic spin fluctuations. In the superconducting state, no Hebel-Slichter coherence peak is seen below $T_{\mathrm{c}}$ and 1/$T_1$ follows $T^3$ variation, indicating the unconventional superconductivity. The finite spin susceptibility at zero-temperature limit and the anomalous increase of the NMR line width below $T_{\mathrm{c}}$ point to a mixed state of spin-singlet and spin-triplet(or spin-septet) pairing.

13 pages, 5 figures, accepted for publication in Physical Review Letters