Systematic NMR studies of high-T_c and two-leg spin-ladder systems

We review systematic NMR studies on underdoped high-T_c, cuprates and two-leg spin-ladder systems, focusing on their spin gap and dynamics. The pressure-induced superconductivity was discovered in Sr_14-xCa_xCu₂₄O₄₁ comprising the quasi-one-dimensional S = 1/2 two-leg spin-ladder Cu₂O₃ planes. Remarkably, the new parent cuprate is the Mott insulator with the spin gap in contrast to the antiferromagnetic (AF) insulator for the high-T_c cuprates. From the Cu-NMR results in the ladder Cu₂O₃ plane on single crystals Sr_14-xCa_xCu₂₄O₄₁ with x = 9 (hereafter denoted as Ca9) and x = 11.5 (Ca11.5), it is shown that the spin gap, Δ = 510 K in the Sr14 is significantly reduced upon isovalent Ca substitution, but its magnitude remains nearly constant with Δ = 270 and 280 K for the Ca9 and Ca11.5, respectively. In T range of T ≥ Δ, the spin dynamics in carrier-doped two-leg spin-ladder systems are characterized by the behaviour similar to the S = 1/2 1D Heisenberg system. By contrast, the transport property shares a common feature, i.e, due to the spin-gap opening the charge transport is confined in the 2D CuO₂ plane for underdoped high-T_c cuprates and in the quasi-1D ladder for spin-ladder compounds.