NMR/NQR Studies on Magnetism of Spin Ladder Sr_14−x A _x Cu₂₄O₄₁ (A = Ca and La)

Abstract

Hole-doped cuprates with low-dimensional Heisenberg spin systems have been investigated intensively after the discovery of high-T _c superconductivity. The predictions of spin gap and superconductivity in carrier-doped ladder systems, i.e. coupled chain systems, have stimulated a renewal of studies on quantum behavior in low-dimensional systems [1,2]. The two-leg ladder cuprate, Sr₁₄Cu₂₄O₄₁, is a unique system in which the carriers are controlled. Sr₁₄Cu₂₄O₄₁ consists of CuO₂ layers with one-dimensional (1D) chains of edge-sharing clusters and Cu₂O₃ layers with a two-leg ladder configuration, as shown in Fig. 16.1 [3]. Holes are transferred from the chain to the ladder site by the substitution of Ca for Sr and the system becomes conductive with increasing x on Sr₁₄Cu₂₄O₄₁ [4]. In this system, the Cu-O chain is regarded as a nonconductive charge reservoir. When high pressure is applied, the compounds become more conductive and superconductivity appears finally around x = 12 under a high pressure of 3 ∼ 4GPa [5]. It is important to investigate the magnetic nature without any holes or with dilute holes in the ladder system. For La³⁺-substitution for Sr²⁺, the number of holes in the system decreases. The formal valence of Cu in both the chain and the ladder is 2+ for Sr₈La₆Cu₂₄O₄₁, i.e. here are no holes in the system. Thus, the system is expected to be completely insulating with long-range magnetic ordering of the Cu spins.