准一维超导体Ta4Pd3Te16的核磁共振研究

NMR study of quasi-one-dimensional Ta_4Pd_3Te_(16)

Ta_4Pd_3Te_(16)是具有准一维结构的超导体,超导温度T_C=4.3 K.本文介绍利用^(125)Te核磁共振和^(181)Ta核四极矩共振研究Ta_4Pd_3Te_(16)的物性.^(181)Ta的自旋为I=7/2,对四极矩相互作用敏感;^(125)Te的自旋为I=1/2,只能感受磁相互作用.通过对比^(181)Ta与^(125)Te两种元素的自旋弛豫率(1/T_1),发现在温度低于80 K时出现电场梯度涨落,并随着降温逐渐增强,在T_(CDW)=20 K进入电荷密度波有序态.在超导态,^(125)Te的1/T_1在略低于T_C时出现Hebel-Slichter相干峰,这表明Ta_4Pd_3Te_(16)是一种无能隙节点的超导体.由于强烈电场梯度涨落,^(181)Ta的1/T_1并没有出现相干峰.

Low-dimensional systems often display rich physical phenomena, such as charge density wave（CDW）, spin density wave（SDW）, and superconductivity. Many superconductors were found at low dimensions where the superconductivity coexists with or adjoins another ordered state and has an unconventional nature. The high transition temperature（TC） superconductivity has been realized in two-dimensional copper oxides and iron pnictides, where the superconducting phase is located in the vicinity of magnetic and nematic orders. This raises an interest in finding unconventional superconductivity in low-dimensional materials. Ta4Pd3Te（16） with a quasi-one-dimensional crystal structure was discovered to be superconducting with TC-4.6 K. It consists of PdTe2 chains, TaTe3 chains, and Ta2Te4 double chains along the crystallographic b axis. Band structure calculations indicate that Ta4Pd3Te（16） is an s-wave superconductor with pairing from phonons associated with the Te-Te p bonding and a Fermi-surface associated with Te p bands. Although scanning tunneling microscopy（STM） found that the superconducting gap structure in this system is more likely anisotropic without nodes, nodal gap behaviors were claimed by thermal conductivity and specific heat measurements. STM study suggested that the system is in the vicinity of an ordered state that shows a periodic modulation. This suggestion is consistent with the observation that the magnetoresistance shows an H-linear behavior without saturation up to 50 T. In this paper, we report nuclear magnetic resonance and nuclear quadrupole resonance investigations on Ta4Pd3Te（16）. The spin-lattice relaxation rate（1/T1） divided by the temperature, 1/T1T, of -（125）Te is almost a constant and obeys the Korringa relationship as in a normal metal, but 1/T1 T of -（811）Ta increases dramatically below 80 K. These results indicate strong electric-field-gradient（EFG） fluctuations, since -（811）Ta has a nuclear spin I=7/2 with a large nuclear quadrupole moment that couples to EFG, but -（125）Te with spin I=1/2 can only relax by magnetic interactions. Upon cooling, both the full width at half maximum of the spectra and 1/T1T show a sudden change at T=20 K, indicating a CDW transition takes place at T（CDW）=20 K. This is consistent with the electronic structure calculation which implies that Ta4Pd3Te（16） prefers the CDW rather than the SDW. In the superconducting state, a Hebel-Slichter coherence peak appears in the temperature dependence of 1/T1 of -（125）Te just below TC, which indicates that Ta4Pd3Te（16） is a fully gapped superconductor. 1/T1 can be fitted by using a BCS gap with Δ（0） =1.76 k BT C. Although Ta4Pd3Te（16） is a multiband system, the main contribution to the DOS is from Te p orbitals. Therefore, the full gap derived from the -（125）Te NMR is consistent with the suggestion from the band calculation that Ta4Pd3Te（16） should be an s-wave superconductor mediated by phonons associated with the Te-Te bonding if there are no spin fluctuations. Due to coexistence of CDW and superconducting, strong EFG fluctuations eliminate the coherence peak in the temperature dependence of 1/T1 of -（811）Ta.