Spin correlation and spin gap in quasi-one-dimensional spin-1/2 cuprate oxides: A Cu-63 NMR study

The Cu-NMR nuclear relaxation rate, (63)(1/T-1) and Gaussian spin-echo decay rate, (63)(1/T-2G) 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 Ca2CuO3 with single chain, SrCuO2 at ambient pressure (AP) with zigzag chain coupled by weak ferromagnetic exchange interaction, SrCu2O3 and Sr2Cu3O5 comprising of two- and three-leg ladder, respectively with isotropic AF exchange interaction along and between chains. In Ca2CuO3, 1/T-1 stays constantly close to the 3D magnetic ordering temperature, T-N similar to 10 K, dominated by dissipation of overdamped spinons gas as shown theoretically by Sachdev. By contrast, in AP-SrCuO2, 1/T-1 below around 100 K decreases markedly down to the 3D magnetic ordering temperature, T-N similar to 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 SrCu2O3 and Sr2Cu3O5 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, xi(T) grows up according to xi(T)similar to 1/(T-theta) and as a result, the magnetic ordering emerges around 50-60 K accompanying the divergence of 1/T-1. In the compounds composed of single and triple chains, it is shown that there is no analog of renormalized classical region identified in the 2D spin-1/2 cuprate such as La2CuO4, but the spin dynamics is in the quantum critical regime over wide T range due to the finite size of chains, even though the 3D magnetic ordering takes place at very low temperature. This is considered because the spin correlators have a power-law decay only along the chain, but between chains is limited to the number of chains in contrast to the 2D square lattice. For the compounds with two chains, the exchange coupling between chains leads to the formation of spin gap phase regardless of whether its sign is antiferromagnetic or ferromagnetic. A result on doping into the two-leg ladder, SrCd2O3 is argued in terms of the localization effect of carriers.