Thermal conductivity of Mg-dopedCuGeO3at very low temperatures: Heat conduction by antiferromagnetic magnons

Thermal conductivity $\kappa$ is measured at very low temperatures down to 0.28 K for pure and Mg-doped ${\mathrm{CuGeO}}_3$ single crystals. The doped samples carry a larger amount of heat than the pure sample at the lowest temperature. This is because antiferromagnetic magnons appear in the doped samples and are responsible for the additional heat conductivity, while $\kappa$ of the pure sample represents phonon conductivity at such low temperatures. The maximum energy of the magnon is estimated to be much lower than the spin-Peierls-gap energy. The result presents evidence that $\kappa$ at very low temperatures probes the magnon transport in a disorder-induced antiferromagnetic phase of spin-gap systems.