Thermal Conductivity of Mg-doped CuGeO_3

The thermal conductivity $\ensuremath{\kappa}$ is measured in a series of ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{GeO}}_{3}$ single crystals in magnetic fields up to 16 T. It has turned out that heat transport by spin excitations is coherent for lightly doped samples, in which the spin-Peierls (SP) transition exists, at temperatures well above the SP transition temperature ${T}_{\mathrm{SP}}.$ Depression of this spin heat transport appears below ${T}^{*},$ at which the spin gap locally opens. ${T}^{*}$ is not modified with Mg doping and that ${T}^{*}$ for each Mg-doped sample remains as high as ${T}_{\mathrm{SP}}$ of pure ${\mathrm{CuGeO}}_{3},$ in contrast to ${T}_{\mathrm{SP}}$ which is strongly suppressed with Mg doping. The spin-gap opening enhances the phonon part of the heat transport because of reduced scattering by the spin excitations, producing an unusual peak. This peak diminishes when the spin gap is suppressed both in magnetic fields and with the Mg doping.