Thermal conductivity of the hole-doped spin ladder system Sr14-xCaxCu24O41

The thermal conductivity of the spin-1/2 ladder system Sr14-xCaxCu24O41 (x = 0, 2, and 12) has been measured both along (kappa(c)) and perpendicular t o ( kappa(a)) the ladder direction at temperatures between 5 and 300 K. Whi le the temperature dependence of K, is typical for phonon heat transport, a n unusual double-peak structure is observed for kappa(c)(T). We interpret t his: unexpected feature as a manifestation of quasi-one-dimensional magnon thermal transport mediated by spin excitations along the ladders.