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 t he low-energy spin dynamics in the quasi-one-dimensional (1D) spin-1/2 antiferromagnetic (AF) cuprates such as Ca2CuO3 with single chain, Sr CuO2 at ambient pressure (AP) with zigzag chain coupled by weak ferrom agnetic exchange interaction, SrCu2O3 and Sr2Cu3O5 comprising of two- and three-leg ladder, respectively with isotropic AF exchange interact ion along and between chains. In Ca2CuO3, 1/T-1 stays constantly close to the 3D magnetic ordering temperature, T-N similar to 10 K, dominat ed 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 interact ion 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 whe re 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 a s a result, the magnetic ordering emerges around 50-60 K accompanying the divergence of 1/T-1. In the compounds composed of single and tripl e chains, it is shown that there is no analog of renormalized classica l region identified in the 2D spin-1/2 cuprate such as La2CuO4, but th e spin dynamics is in the quantum critical regime over wide T range du e to the finite size of chains, even though the 3D magnetic ordering t akes place at very low temperature. This is considered because the spi n 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 squar e lattice. For the compounds with two chains, the exchange coupling be tween chains leads to the formation of spin gap phase regardless of wh ether its sign is antiferromagnetic or ferromagnetic. A result on dopi ng into the two-leg ladder, SrCd2O3 is argued in terms of the localiza tion effect of carriers.