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

Abstract

Comprehensive (Formula presented)Cu NMR/NQR measurements have been performed on single crystals of Sr(Formula presented) a hole-doped material containing alternating layers of one-dimensional CuO(Formula presented) chains and Cu(Formula presented)O(Formula presented) 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 (Formula presented) and (Formula presented) 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 (Formula presented) at both chain sites shows an activated temperature dependence below (Formula presented) 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 (Formula presented) above 200 K. The ladder Cu sites also show an activated temperature dependence of (Formula presented) with a gap of 650 K above 200 K, indicating a spin-gap in the ladders. However, (Formula presented) 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 a peak near (Formula presented) 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 (2200 K). © 1998 The American Physical Society.