Spin and charge dynamics in the hole-doped one-dimensional-chain-ladder composite material Sr14Cu24O41:: Cu NMR/NQR studies

Comprehensive Cu-63,Cu-65 NMR/NQR measurements have been performed on single crystals of Sr14Cu24O41, a hole-doped material containing alternating layers of one-dimensional CuO2 chains and Cu2O3 ladders, While the ladder sites show a unique resonance, two distinct resonance spectra are obtained for the chain sites, They are assigned to the magnetic Cu sites with spin-1/2 and the nonmagnetic Cu sites, which form the Zhang-Rice (ZR) singlet with holes on the oxygen sites. The NMR spectrum at the ZR chain sites shows sharp multipeak structure at low temperatures, indicating a long period of superstructure. The structure becomes obscure and peaks merge into a single broad line with increasing temperature due to thermally induced disorder or motion. A giant oscillation of the spin-echo intensity was observed at the magnetic chain sites as a function of the time separation between pi/2 and pi rf pulses. This is well explained if these sites form spin-singlet dimers, which interact very weakly with each other. The nuclear spin-lattice relaxation rate (1/T-1) at both chain sites shows an activated temperature dependence below T = 50 K with a gap of 125 K, corresponding to the singlet-triplet splitting of the dimers. The ZR chain sites show an anomalous increase of 1/T-1 above 200 K, The ladder Cu sites also show an activated temperature dependence of 1/T-1 with a gap of 650 K above 200 indicating a spin-gap in the ladders. However, 1/T-1 at the ladder sites measured by zero-field NQR is dominantly caused by fluctuations of the electric-field gradient (EFG) in the temperature range 30-150 K and shows apeak near T = 100 K. This is most likely caused by slow motion of doped holes and/or lattice distortion. The inverse correlation time of the EFG fluctuations is estimated using a simple model of motional effects. It shows an activated temperature dependence with a gap of 230 K, which is an order of magnitude smaller than the activation energy for the electrical conductivity (2300 K).