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

我们研究了几种铜氧化物作为低维量子自旋系统的热导率和其他物理性质，为了阐明磁振子引起的巨大热导率的机制以及热导率测量对相变的探测是否有效，我们在四臂自旋梯系统La_2Cu_3中发现了磁振子引起的巨大热导率。在一维自旋系统Sr_2CuO_3和SrCuO_2中，我们从杂质效应的研究中证明了磁振子引起的热导率很大，并证明了热导是弹道式的。结果表明，自旋之间的强反铁磁相互作用和磁振子的大色散都导致了磁振子的大热导率。此外，在Ca_2Y_2Cu_5O_3和Cu_3B_2O_6的反铁磁有序态中观察到自旋翻转相变<10>，在TlCuCl_3中观察到低温强磁场下磁振子的玻色-爱因斯坦凝聚相变，在La_ Sr_xCuO_4和La_ Ba_xCuO_4中观察到场诱导磁<2-x><2-x>有序相变。因此，我们得出结论，热导率测量是一个强有力的探针来检测和研究相变。

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.