We report $^{ 125}$Te nuclear magnetic resonance and $^{ 181}$Ta nuclear quadrupole resonance studies on single-crystal Ta$_{ 4}$Pd$_{ 3}$Te$_{ 16}$, which has a quasi-one-dimensional structure and superconducts below $T_{ \rm c}=4.3$ K. $^{ 181}$Ta with spin $I=7/2$ is sensitive to quadrupole interactions, while $^{ 125}$Te with spin $I=1/2$ can only relax by magnetic interactions. By comparing the spin-lattice relaxation rate ( $1/T_{ 1}$) of $^{ 181}$Ta and $^{ 125}$Te, we found that electric-field-gradient (EFG) fluctuations develop below $80$ K. The EFG fluctuations are enhanced with decreasing temperature due to the fluctuations of a charge density wave that sets in at $T_{ \rm CDW}=20$ K, below which the spectra are broadened and $1/T_{ 1}T$ drops sharply. In the superconducting state, $1/T_{ 1}$ shows a Hebel-Slichter coherence peak just below $T_{ \rm c}$ for $^{ 125}$Te, indicating that Ta$_{ 4}$Pd$_{ 3}$Te$_{ 16}$ is a full-gap superconductor without nodes in the gap function. The coherence peak is absent in the $1/T_{ 1}$ of $^{ 181}$Ta due to the strong EFG fluctuations.
Categories: cond-mat.supr-con cond-mat.str-el