Study of giant thermal conductivity due to magnons in low-dimensional quantum spin system copper oxides

我们研究了几种作为低维量子自旋系统的氧化铜的热导率和其他物理性质，以澄清磁振子引起的巨大热导率的机理，并澄清热导率测量是否对相变检测有效。我们发现了四脚自旋阶梯体系La_2Cu_2O_5和二维自旋体系Cu_3B_2O_6中由于磁振子的存在而产生的较大导热系数。在Sr_2CuO_3和SrCuO_2的一维自旋体系中，我们从杂质效应的研究中证明了磁振子引起的热导率非常大，热导率与理论预期一致。从这些结果可以得出结论，自旋之间的大反铁磁相互作用和磁振子的大色散导致了磁振子的大热导率。此外，在以下相变中观察到导热系数的显著变化；Ca_2Y_2Cu_5O_<10>和Cu_3B_2O_6反铁磁有序态的自旋翻转跃迁，TlCuCl_3中磁子在低温强磁场下的玻色-爱因斯坦凝聚跃迁，La_<2-x>Sr_xCUO_4和La_<2-x>Ba_xCuO_4的场致磁有序。因此，我们得出结论，热导率测量是检测和研究相变的有力探针。

We have studied the thermal conductivity and other physical properties of several copper oxides regarded as low-dimensional quantum spin systems, in order to clarify the mechanism of the giant thermal conductivity due to magnons and also to clarify whether the thermal conductivity measurement is powerful for the detection of a phase transition or not.We have discovered large thermal conductivity due to magnons in the four-leg spin-ladder system La_2Cu_2O_5 and the two-dimensional spin system Cu_3B_2O_6.In one-dimensional spin systems Sr_2CuO_3 and SrCuO_2, we have proved from the study of the impurity effects that the thermal conductivity due to magnons is very large and that the thermal conduction is ballistic as theoretically expected.From these results, it has been concluded that both the large antiferromagnetic interaction between spins and the large dispersion of magnons give rise to the large thermal conductivity due to magnons.Moreover, marked changes of the thermal conductivity have been observed at the following phase transitions; the spin-flop transition in the antiferromagnetically ordered state of Ca_2Y_2Cu_5O_<10> and Cu_3B_2O_6, the Bose-Einstein condensation transition off magnons in TlCuCl_3 under high magnetic fields at low temperatures, and the field-induced magnetic order in La_<2-x>Sr_xCUO_4 and La_<2-x>Ba_xCuO_4. Accordingly, we have concluded that the thermal conductivity measurement is a powerful probe to detect and study a phase transition.