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, kappa(magnon), in various kinds of low-dimensional quantum spin systems, such as the two-leg spin-ladder system Sr14Cu24O41, the two-dimensional spin-dimer system SrCu2(BO3)(2), the four-leg spin-ladder system La2Cu2O5, the two-dimensional spin system Cu3B2O6 and the one-dimensional spin system Ca2Y2Cu5O10, using large-sized single-crystals grown by the TSFZ method. Large kappa(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 kappa(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 kappa(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. (C) 2003 Elsevier B.V. All rights reserved.