Charge fluctuations and nodeless superconductivity in quasi-one-dimensional Ta4 Pd3 Te16 revealed by Te 125 -NMR and Ta 181 -NQR

We report Te125 nuclear magnetic resonance and Ta181 nuclear quadrupole resonance studies on single-crystal Ta4Pd3Te16, which has a quasi-one-dimensional structure and superconducts below Tc=4.3 K. Ta181 with spin I=7/2 is sensitive to quadrupole interactions, while Te125 with spin I=1/2 can only relax by magnetic interactions. By comparing the spin-lattice relaxation rate (1/T1) of Ta181 and Te125, 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 TCDW=20 K, below which the spectra are broadened and 1/T1T drops sharply. In the superconducting state, 1/T1 shows a Hebel-Slichter coherence peak just below Tc for Te125, indicating that Ta4Pd3Te16 is a full-gap superconductor without nodes in the gap function. The coherence peak is absent in the 1/T1 of Ta181 due to the strong EFG fluctuations.