NMR/NQR Studies on Magnetism of Spin Ladder Sr14−xAxCu24O41 (A = Ca and La)
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 . 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 . 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 . 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.
KeywordsHyperfine Field Hyperfine Coupling Constant Exchange Coupling Constant Ladder System Resonate Valence Bond
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