Spin correlation and spin gap in quasi-one-dimensional spin-1/2 cuprate oxides: ACu63NMR study

The Cu-NMR nuclear relaxation rate, ${}_{}{}^{63}{}_{}{}^{}$(1/${\mathit{T}}_1$) and Gaussian spin-echo decay rate, ${}_{}{}^{63}{}_{}{}^{}$(1/${\mathit{T}}_{2\mathit{G}}$) have been measured in order to investigate the low-energy spin dynamics in the quasi-one-dimensional (1D) spin-1/2 antiferromagnetic (AF) cuprates such as ${\mathrm{Ca}}_2$ ${\mathrm{CuO}}_3$ with single chain, ${\mathrm{SrCuO}}_2$ at ambient pressure (AP) with zigzag chain coupled by weak ferromagnetic exchange interaction, ${\mathrm{SrCu}}_2$ ${\mathrm{O}}_3$ and ${\mathrm{Sr}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_5$ comprising of two- and three-leg ladder, respectively with isotropic AF exchange interaction along and between chains. In ${\mathrm{Ca}}_2$ ${\mathrm{CuO}}_3$, 1/${\mathit{T}}_1$ stays constantly close to the 3D magnetic ordering temperature, ${\mathit{T}}_{\mathit{N}}$∼10 K, dominated by dissipation of overdamped spinons gas as shown theoretically by Sachdev. By contrast, in AP-${\mathrm{SrCuO}}_2$, 1/${\mathit{T}}_1$ below around 100 K decreases markedly down to the 3D magnetic ordering temperature, ${\mathit{T}}_{\mathit{N}}$∼3 K, providing a signature that the frustrated ferromagnetic interaction between CuO zigzag chains makes the spin gap open. In the spin-1/2 ladder compounds of ${\mathrm{SrCu}}_2$ ${\mathrm{O}}_3$ and ${\mathrm{Sr}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_5$ consisting of two- and three-leg, respectively, the former was demonstrated in the literatures to have the spin gap amounting to a half of AF exchange constant, J/2 where J=1300 K. For the latter, on the other hand, the magnetic coherence length, ξ(T) grows up according to ξ(T)∼1/(T-θ) and as a result, the magnetic ordering emerges around 50–60 K accompanying the divergence of 1/${\mathit{T}}_1$.