Magnon thermal conductivity in the spin-gap state and the antiferromagnetically ordered state of low-dimensional copper oxides

We have looked for large thermal-conductivity due to magnons, k _magnon, in various kinds of low-dimensional quantum spin systems, such as the two-leg spin-ladder system Sr₁₄Cu₂₄O ₄₁, the two-dimensional spin-dimer system SrCu₂(BO ₃)₂, the four-leg spin-ladder system La₂Cu ₂O₅, the two-dimensional spin system Cu₃B ₂O₆ and the one-dimensional spin system Ca ₂Y₂Cu₅O₁₀, using large-sized single-crystals grown by the TSFZ method. Large k_magnon has been found in the spin-gap state of Sr₁₄Cu₂₄O₄₁ whose bandwidth of the triplet excitation is large, owing to the enhancement of the mean free path of magnons. In gapless systems, it has been found that K _magnon increases with increasing magnon-velocity, which increases with increasing exchange-interaction between Cu²⁺ spins and is larger in antiferromagnetic spin-correlation systems than in ferromagnetic spin-correlation systems. Therefore, it is concluded that essential factors of large k_magnon are a large bandwidth of the triplet excitation in a spin-gap system, a large exchange-interaction and antiferromagnetic spin-correlation rather than ferromagnetic spin-correlation.