Charge fluctuations and nodeless superconductivity in quasi-one-dimensionalTa4Pd3Te16revealed byTe125-NMR andTa181-NQR

We report $¹²⁵\mathrm{Te}$ nuclear magnetic resonance and $¹⁸¹\mathrm{Ta}$ nuclear quadrupole resonance studies on single-crystal ${\mathrm{Ta}}₄{\mathrm{Pd}}₃{\mathrm{Te}}₁₆$, which has a quasi-one-dimensional structure and superconducts below ${T}_c=4.3$ K. $¹⁸¹\mathrm{Ta}$ with spin $I=7/2$ is sensitive to quadrupole interactions, while $¹²⁵\mathrm{Te}$ with spin $I=1/2$ can only relax by magnetic interactions. By comparing the spin-lattice relaxation rate ($1/{T}₁$) of $¹⁸¹\mathrm{Ta}$ and $¹²⁵\mathrm{Te}$, 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 ${T}_{\mathrm{CDW}}=20$ K, below which the spectra are broadened and $1/{T}₁T$ drops sharply. In the superconducting state, $1/{T}₁$ shows a Hebel-Slichter coherence peak just below ${T}_c$ for $¹²⁵\mathrm{Te}$, indicating that ${\mathrm{Ta}}₄{\mathrm{Pd}}₃{\mathrm{Te}}₁₆$ is a full-gap superconductor without nodes in the gap function. The coherence peak is absent in the $1/{T}₁$ of $¹⁸¹\mathrm{Ta}$ due to the strong EFG fluctuations.