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

Abstract

We have looked for large thermal-conductivity due to magnons, κ magnon, in various kinds of low-dimensional quantum spin systems, such as the two-leg spin-ladder system Sr 14Cu 24O 41, the two-dimensional spin-dimer system SrCu 2(BO 3) 2, the four-leg spin-ladder system La 2Cu 2O 5, the two-dimensional spin system Cu 3B 2O 6 and the one-dimensional spin system Ca 2Y 2Cu 5O 10, using large-sized single-crystals grown by the TSFZ method. Large κ magnon has been found in the spin-gap state of Sr 14Cu 24O 41 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 κ magnon increases with increasing magnon-velocity, which increases with increasing exchange-interaction between Cu 2+ spins and is larger in antiferromagnetic spin-correlation systems than in ferromagnetic spin-correlation systems. Therefore, it is concluded that essential factors of large κ 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.