Low-temperature Magnetic Fluctuations Investigated by $^{125}$Te-NMR on the Uranium-based Superconductor UTe$_{2}$

To investigate the static and dynamic magnetic properties on the uranium-based superconductor UTe$_{2}$, we measured the NMR Knight shift $K$ and the nuclear spin-lattice relaxation rate $1/T_{1}$ in $H \parallel a$ by $^{125}$Te-NMR on a $^{125}$Te-enriched single-crystal sample. $1/T_1T$ in $H \parallel a$ is much smaller than $1/T_1T$ in $H \parallel b$ and $c$, and magnetic fluctuations along each axis are derived from the $1/T_1T$ measured in $H$ parallel to all three crystalline axes. The magnetic fluctuations are almost identical at two Te sites and isotropic at high temperatures, but become anisotropic below 40 K, where heavy-fermion state is formed. The character of magnetic fluctuations in UTe$_2$ is discussed with the comparison to its static susceptibility and the results on other U-based superconductors. It is considered that the magnetic fluctuations probed with the NMR measurements are determined by the magnetic properties inside the two-leg ladder formed by U atoms, which are dominated by the $q_a$ = 0 ferromagnetic fluctuations.