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

Abstract

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− x Ca x Cu 24O 41 comprising the quasi-one-dimensional S = ½ two-leg spin-ladder Cu 2O 3 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 Cu 2O 3 plane on single crystals Sr 14− x Ca x Cu 24O 41 with x = 9 (hereafter denoted as Ca9) and x = 11.5 (Ca11.5), it is shown that the spin gap, Δ = 510K in the Sr14 is significantly reduced upon isovalent Ca substitution, but its magnitude remains nearly constant with Δ = 270 and 280K 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 = ½ 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 CuO 2 plane for underdoped high- T c , cuprates and in the quasi-1 D ladder for spin-ladder compounds.